Pinnipedimorph Evolutionary Biogeography

32

Chapter 3

Pinnipedimorph Evolutionary Biogeography

THOMAS A. DEMERE,1 ANNALISA BERTA,2 AND PETER J. ADAM2,3

ABSTRACT

Previous hypotheses for the origin and diversification of pinnipeds have followed a narrativeapproach based mostly on dispersalist (i.e., center of origin) explanations. Using an analyticalapproach, we present a testable hypothesis to explain the evolutionary biogeography of pin-nipedimorphs (fur seals, sea lions, walruses, seals, and their fossil relatives) based on bothdispersal and vicariant events in the context of a species-level phylogenetic framework. Thisintegrated hypothesis considers many lines of evidence, including physical and ecologic factorscontrolling modern pinniped distributions, past geologic events related to opening and closingof seaways, paleoceanographic models, the improving pinniped fossil record, and pinnipedphylogenetic analyses based on both morphologic and molecular data sets. Oceanic biogeo-graphic regions and faunal provinces are defined and oceanic circulation patterns discussedwith reference to the distribution of extant and fossil species. Paleobiogeographic hypothesesfor each of the major pinniped lineages are presented using area cladograms and paleogeo-graphic maps showing oceanographic and tectonic changes during successive intervals of theCenozoic.

Our biogeographic hypothesis supports an eastern North Pacific origin for pinnipedimorphsduring the late Oligocene coincident with initiation of glaciation in Antarctica. During theearly Miocene, pinnipedimorphs remained restricted to the eastern North Pacific, where theybegan to diversify. Otariids (fur seals and sea lions) are first known from the late Miocene inthe North Pacific, where they remained restricted until the late Pliocene. A transequatorialdispersal into the western South Pacific at this time preceded the rapid diversification of thisgroup that occurred during the Pleistocene in the Southern Ocean. Odobenids (walruses)evolved in the North Pacific during the late early Miocene and underwent dramatic diversi-fication in the late Miocene with later members of the odobenine lineage dispersing into theNorth Atlantic, most likely via an Arctic route. Extinct archaic phocoids, the desmatophocids,known only from the early to late Miocene, were confined to the eastern and western NorthPacific. Phocids, although postulated here to have a North Pacific origin, are first known asfossils from the middle Miocene in the eastern and western North Atlantic region, as well asthe Paratethys. Both monachine and phocine seals are distinct lineages beginning in the middleMiocene in the eastern and western provinces of the North Atlantic. During the late Miocene,phocids underwent a dramatic diversification. The early biogeographic history of phocine sealsis centered in the Arctic and North Atlantic. Subsequent dispersal of phocines into the Para-tethys and Pacific occurred during the Pleistocene. In contrast, monachine seals have a southernhemisphere center of diversity, especially the lobodontines of the Southern Ocean. Southerndispersal of this clade most likely occurred through the Neogene Central American Seawayprior to its closure in the mid-Pliocene. The pagophilic nature of extant phocine and lobodon-tine seals is largely a function of Pleistocene glacioeustatic events.

1 Curator of Paleontology, Department of Paleontology, San Diego Natural History Museum, P.O. Box 121390,San Diego, CA 92112.

2 Professor of Biology, Department of Biology, San Diego State University, San Diego, CA 92182-4614.3 Ph.D. Candidate, Department of Organismal Biology, Ecology, and Evolution, University of California, Los Angeles,

CA 90095-1606.

2003 33DEMERE ET AL.: PINNIPEDIMORPH EVOLUTIONARY BIOGEOGRAPHY

INTRODUCTION

Pinnipedimorphs are a monophyleticgroup of neritic marine arctoid carnivoransconfined largely to the continental shelf areasof the world’s oceans. They have a fossil re-cord extending back at least to the late Oli-gocene (27–25 Ma). This record now in-cludes specimens from all ocean basins ex-cept the Indian Ocean. Pinnipedimorphs in-clude 33–36 living species (Rice, 1998) andat least 51 named fossil species. Basal pin-nipedimorphs include species of the extincttaxon Enaliarctos and its relatives knownfrom the North Pacific (Mitchell and Ted-ford, 1973). All other pinnipedimorphs canbe placed in the more exclusive Pinnipediaclade, which includes three major taxa: (1)the Otariidae (fur seals, sea lions, and theirextinct relatives), (2) the Odobenidae (wal-ruses and their extinct relatives), and (3) thePhocoidea (Phocidae [true or earless seals]plus the extinct desmatophocids).

Proposals for the geographic origin and di-versification of pinnipeds have largely beenwritten as narrative accounts rather than asanalytical models. This is a consequence ofthe fact that attempts to explain pinniped bio-geographic distributions have been predom-inantly based on analysis of historical factors(i.e., temporal and geographic occurrence inthe fossil record) and/or geologic and cli-matic factors (i.e., tectonic events, glacial/in-terglacial oscillations, oceanic currents, andformation of migration/dispersal routes and/or barriers) without regard to explicit phy-logenetic hypotheses as the basis for con-straining and testing historic biogeographichypotheses. However, with recent advancesin pinniped systematics, including well-sup-ported phylogenies for fossil taxa based onmorphology and for extant taxa using bothmorphologic and molecular data, it is nowpossible to offer a more comprehensive viewof pinniped historical biogeography. Such aview is also enhanced by new discoveries offossil pinnipedimorphs including discoveriesin the eastern and western North Pacific,eastern South Pacific, and Paratethys regions.Detailed studies of present ocean circulationpatterns, including both wind-driven circu-lation and thermohaline circulation, provide

a model for analyzing the relationship be-tween pinniped distributions and ancientocean circulation patterns. In addition, recentadvances in plate tectonic theory have pro-vided more refined ways of viewing eventsrelated to the opening and closing of oceanicbasins, the development of possible dispersalcorridors or barriers, and the possible frag-mentation of species ranges. The applicationof cladistic methodology to analyzing bio-geographic questions has been a major ad-vance that provides an empirical approachfor evaluating the role of both dispersal andvicariance in pinnipedimorph evolutionaryhistory.

In this paper, we reexamine the historicalbiogeography of pinnipedimorphs in light ofan improved resolution of their phylogeneticrelationships, recent fossil discoveries pro-viding revised distributional data, and newinformation regarding the nature and timingof paleogeographic and paleooceanographicevents. We propose a framework for inter-preting the historical biogeography of pin-nipeds that is based on current ecologic, geo-logic, climatic, and historical factors, as wellas hypotheses of pinniped phylogenetic re-lationships at a higher level of resolution(species level) than has been previously pos-sible. We intend this model to stimulate oth-ers to compare pinniped historical biogeo-graphic patterns with those from other ma-rine organisms to develop broader, largerscale integrative and testable biogeographichypotheses for the marine biota of the worldocean.

PREVIOUS WORK

The earliest attempts to explain pinnipedbiogeography were narrative accounts. Scla-ter (1897) proposed marine zoogeographicregions based on the distributions of marinemammals and generated lists of species char-acteristic of each region. He suggested thatotariids and certain phocids originated in theSouthern Ocean and dispersed into theNorthern Hemisphere. Von Boetticher (1934)reexamined Sclater’s regions, specificallywith regard to the distribution of pinnipeds.Davies (1958a) presented the first thoroughreview of pinniped zoogeography, relying on

34 NO. 279BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY

fossils, geology, and pinniped distributions toformulate an hypothesis of an Arctic Oceancenter of origin. Scheffer (1958), in a mono-graphic work on pinniped biology and sys-tematics, included discussions on evolutionand biogeography and also proposed aNorthern Hemisphere origin for the group.These early studies assumed a static conti-nental configuration and relied on the peri-odic opening and closing of hypotheticalland bridges and marine straits to explain thepresent geographic distribution of pinnipeds.In more recent decades, an understanding ofcontinental drift in the context of plate tec-tonic theory challenged the dominant viewheld early in the 20th century that the posi-tion of continents and ocean basins was fixedand that phantom land bridges were respon-sible for limiting dispersal. We now realizethat just as continents have separated, collid-ed, or drifted further apart, so too have oceanbasins and their constituent marine biotas. Infact, an inverse relationship of interchange,termed complementarity by Hallam (1974),often exists between terrestrial and marineenvironments and their biotas. A classic caseof complementarity occurred during forma-tion of the Isthmus of Panama in the mid-Pliocene. Elevation of the isthmus created acorridor for terrestrial interchange betweenNorth and South America, but it also createda barrier to marine interchange between thetropical eastern Pacific Ocean and the Carib-bean Sea. The relevance of complementarityto pinniped biogeography is obvious and willbe discussed later.

Historical biogeography examines generalpatterns of occurrence of species and ex-plains them in terms of evolutionary andgeologic history. The role of historical bio-geography in understanding the distributionof organisms has involved both dispersal andvicariant models. In the context of pinnipeddistributions, dispersal biogeographers havetraditionally been interested in where pinni-peds originated (center of origin) and howand when they dispersed from that center. Onthe other hand, vicariant biogeographershave emphasized that the geographical pat-tern of pinnipeds is the result of common,past events (geologic and/or climatic) inearth history, with changes in configurationof ocean basins, land bridges, and straits

fragmenting or juxtapositioning species rang-es. This latter approach also offers a meansfor testing biogeographic hypotheses usingthe premise that the same processes thatcaused a particular pinniped distribution alsocaused a similar distribution in other marineorganisms.

As expected, much of the previous bio-geographic history of pinnipeds has been dis-cussed within a dispersalist framework. Da-vies (1958a) proposed a center of origin forpinnipeds in the Arctic Basin based, in part,on fossil evidence at that time for the earliestpinnipeds and their assumed primitive phys-iological adaptations to cold water. He wenton to propose that the ancestors of otariidsand walruses dispersed south into the NorthPacific and were isolated there by emergenceof an Aleutian land bridge, while phocids di-versified in the Arctic and gradually dis-persed south into the North Atlantic, follow-ing an advancing cold water boundary. Thisidea was further explored by him (Davies,1958b), especially the effects that the expan-sion and contraction of sea ice during thePleistocene had on the present day distribu-tion of northern pinnipeds. In a classic treat-ment of pinniped biogeography based on animproved knowledge of fossil pinnipeds, Re-penning et al. (1979) suggested that ‘‘otar-ioid’’ pinnipeds (a paraphyletic grouping ofsea lions, fur seals, walruses, their fossil rel-atives, and the extinct desmatophocids)evolved in Neogene temperate waters of theNorth Pacific from an arctoid (ursid) stockand that phocid pinnipeds (true seals)evolved in the North Atlantic from a differ-ent arctoid (mustelid) ancestry. This hypoth-esis, which assumes a diphyletic Pinnipedia,is based in large part on the published bio-geographic pattern of fossil pinnipeds at thattime, with phocids only known from depositsin the Atlantic Ocean basin and Paratethysregion until the late Miocene (eastern SouthPacific) and Pleistocene (North Pacific andArctic Ocean), and otariids only known fromthe North Pacific basin until the late Pliocene(eastern South Pacific) and Pleistocene(South Atlantic and Southern Ocean). Thisdiphyletic view is still advocated by someworkers to interpret past patterns of geo-graphical distribution (e.g., Bonner, 1990;Knox, 1994). In contrast, few attempts have


been made to explain the biogeographic dis-tribution of various pinniped lineages basedupon well-corroborated hypotheses of phy-logenetic relationships; these include Muizon(1982) and Bininda-Emonds and Russell(1996) for phocids, Kohno et al. (1995a) forodobenids, and Berta and Demere (1986) forotariids. See further comments about thesephylogenetic hypotheses in the section Phy-logeny and Biogeography.

BIOGEOGRAPHY OF EXTANTPINNIPEDS

In describing the distribution of extant pin-nipeds, it is useful to recognize nine oceanicbiogeographic regions (fig. 3.1) including the(1) Arctic Ocean region, (2) North AtlanticOcean region, (3) Mid-Atlantic Ocean re-gion, (4) South Atlantic Ocean region, (5)North Pacific Ocean region, (6) Mid-PacificOcean region, (7) South Pacific Ocean re-gion, (8) Indian Ocean region, and (9) South-ern Ocean region. We have subdivided eachregion into provinces (e.g., eastern North Pa-cific province, Paratethys province) for a to-tal of 26 provinces summarized in table 3.1and illustrated in figure 3.1. The followingdiscussion describes the pinniped assemblag-es characteristic of each region (based pri-marily on King, 1983b; Riedman, 1990; andRice, 1998) and concludes with an overviewof general biogeographic patterns. The haul-out distributions of species are emphasized.Some pinnipeds (i.e., Mirounga spp., Callor-hinus ursinus) have more pelagic habits andare often found outside the ranges describedhere. Figure 3.2 summarizes the major oce-anic currents and areas of coastal upwellingthat are discussed.

The Arctic Ocean region covers the entireArctic Basin and includes the Beaufort Sea/Chukchi Sea province (1a) adjacent to west-ern Canada and Alaska, the East SiberianSea/Laptev Sea province (1b) adjacent toeastern Russia, and the Greenland Sea/Ba-rents Sea/Kara Sea province (1c) adjacent towestern Russia, Greenland, and Elsmere Is-land. This region roughly corresponds to theextreme northern portions of the Atlantic–Arctic and Pacific–Arctic ocean zones ofScheffer (1958) and the northern portion ofthe Arctatlantis pinniped region of Davies

(1958a; modified from Sclater, 1897). Thegreatest flow of marine water into the ArcticOcean comes from the Atlantic via the Nor-wegian Current (a northeastern extension ofthe Gulf Stream; fig. 3.2), which splits intothe North Cape and the Spitsbergen Currents.Marine water also enters from the BeringStrait and crosses the Arctic as the Trans-Arctic Current. The anticyclonic BeaufortGyre system circulates surface waters in thewestern Canadian Arctic. The East Green-land Current carries the major flow of marinewater out of the Arctic and into the NorthAtlantic.

Permanent sea ice in the Arctic Ocean andits fringe of seasonally changing pack ice andfast ice is a major factor controlling the dis-tribution of pinnipeds in the region. The Arc-tic Ocean supports a pinniped assemblageconsisting of six phocids and one odobenid.Otariids are, and have always been, entirelyabsent from the region. Two subspecies ofOdobenus occur in the Arctic Ocean and dis-play a disjunct distribution. Odobenus ros-marus rosmarus occurs in the GreenlandSea/Barents Sea/Kara Sea province and isdistinct and apparently reproductively isolat-ed from O. r. divergens from the BeaufortSea/Chukchi Sea and East Siberian Sea/Lap-tev Sea provinces. Populations of Erignathusbarbatus have a similar disjunct, but morecircum-Arctic, distribution and some workers(King, 1983b) have assigned these to sub-species (E. b. barbatus in the Greenland Sea/Barents Sea/Kara Sea province and E. b.nauticus in the Beaufort Sea/Chukchi Seaand East Siberian Sea/Laptev Sea provinces).Pusa hispida is probably the most character-istic pinniped of the Arctic Ocean and isfound throughout the region wherever thereis open water. This taxon appears to be un-dergoing allopatric speciation with severaldistinct populations recognized and assignedby some workers to subspecies rank (Rice,1998). The majority of these purported sub-species occur in the subarctic portions of theNorth Pacific and North Atlantic regions. Pa-gophilus groenlandica occurs in the open seaof the Greenland Sea/Barents Sea/Kara Seaprovince and extends its range into the sub-arctic North Atlantic (Hudson Bay/BaffinBay province). Histriophoca fasciata is re-stricted to the Beaufort Sea/Chukchi Sea


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TABLE 3.1Oceanic Regions and Provinces (see also fig. 3.1)

province of the Arctic and the adjacent sub-arctic North Pacific (Bering Sea and OkhotskSea). Cystophora cristata occurs in the west-ern portion of the Greenland Sea/BarentsSea/Kara Sea province and the adjacent Hud-son Bay/Baffin Bay province of the subarcticNorth Atlantic. Although primarily a NorthAtlantic species, Halichoerus grypus followsthe Norwegian Current into the GreenlandSea/Barents Sea/Kara Sea province.

The North Atlantic Ocean region includesthe northern portion of the Atlantic fromabout 708N to about 358N in the westernprovince (2a) and to about 408N in the east-ern province (2b; includes the North Sea and

Baltic Sea). The subarctic portion of this re-gion is divided between the Hudson Bay/Baffin Bay province (2c) and the DenmarkStrait/Norwegian Sea province (2d). This re-gion roughly corresponds to the southernportion of the Atlantic–Arctic ocean zone ofScheffer (1958) and the southern portion ofthe Arctatlantis pinniped region of Davies(1958a; modified from Sclater, 1897), whoused the 208C SST (sea surface temperature)isotherm as the southern regional boundary.The western boundary current is the warmGulf Stream and its northeastern extension,the Norwegian Current, while the CanaryCurrent is the eastern boundary current.Cold, south-flowing countercurrents includethe Labrador Current in the western NorthAtlantic and the East Greenland Current inthe central North Atlantic.

This region supports a diverse assemblageof pinnipeds, including five phocines and oneodobenid. Otariids are entirely absent fromthe region. Odobenus rosmarus rosmarus oc-curs throughout the Hudson Bay/Baffin Bayprovince, as well as the western portion ofthe Denmark Strait/Norwegian Sea province.A trans-Arctic, allopatric distribution occursin subspecies of Phoca vitulina, which canbe further divided into eastern and westernsubspecies. P. v. concolor occupies the west-ern North Atlantic and Hudson Bay/BaffinBay provinces, while P. v. vitulina occurs inthe eastern North Atlantic and DenmarkStrait/Norwegian Sea provinces. Halichoerusgrypus also has two distinct North Atlanticpopulations, one resident in the westernNorth Atlantic and Hudson Bay/Baffin Bayprovinces and another in the eastern NorthAtlantic and Denmark Strait/Norwegian Seaprovinces. As mentioned, Erignathus bar-batus and Pusa hispida, although primarilyresidents of the Arctic Ocean, do extend theirranges into the Hudson Bay/Baffin Bay andDenmark Strait/Norwegian Sea provinces.Disjunct populations of P. hispida also occurin the Baltic Sea and adjacent freshwaterlakes in Finland (Lake Simaa) and Russia(Lake Ladoga). Pagophilus groenlandica ischaracteristic of the Hudson Bay/Baffin Bay,Denmark Strait/Norwegian Sea, and Green-land Sea/Barents Sea/Kara Sea provinces,but extends its range into the western North


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Atlantic province along the Labrador Cur-rent.

The Mid-Atlantic Ocean Region encom-passes the tropical and subtropical portionsof the North and South Atlantic and includesthe Caribbean Sea and Gulf of Mexico in thewest (western Mid-Atlantic province; 3a) andthe West African and Iberian coastlines in theeast (eastern Mid-Atlantic province; 3b), aswell as the Mediterranean Sea (3c) and rem-nants of the Paratethys Sea (3d; Black, Cas-pian, and Aral seas). This region correspondswith the Mesatlantis pinniped region of Da-vies (1958a; modified from Sclater, 1897),who placed its northern and southern bound-aries at the 208C SST isotherm. Ocean cir-culation in this region is dominated by theNorth and South Atlantic gyres (fig. 3.2).The South Equatorial Current (a northernsegment of the South Atlantic Gyre) joinswith the North Equatorial Current in thewestern equatorial Atlantic and flows west-ward before splitting into the Antilles Cur-rent that flows north of the West Indies andthe Caribbean Current that flows westthrough the Yucatan Channel into the Gulfof Mexico. The Florida Current is the west-ern boundary current that flows north alongthe east coast of North America to eventuallybecome the Gulf Stream as it moves out tosea at Cape Hatteras.

Pinnipeds are rare in this warm water re-gion and only include species of Monachus.The former range of the historically extinctMonachus tropicalis included the Gulf ofMexico and the Caribbean Sea between theYucatan Peninsula and the Bahama Islandsand has recently been extended south to theFrench Antilles (Debrot, 2000). Populationsof Zalophus californianus in the westernMid-Atlantic province are the result of hu-man introduction, and are not consideredhere (Rice, 1998). M. monachus is charac-teristic of the Mediterranean Sea, but is alsoknown from the Azores, Canary Islands, andMoroccan coast in the eastern mid-Atlanticprovince and from the Black Sea in the Par-atethys province. This province also supportsthe lacustrine endemic phocine Pusa caspica,which is only found in the Caspian Sea. Pusasibirica is another lacustrine endemic knownonly from Lake Baikal in eastern Russia (notpart of the Paratethys). It is interesting to

note that the region of coastal upwelling offWest Africa caused by the Northeast TradeWinds (A in fig. 3.2A) is not associated witha pinniped fauna.

The South Atlantic Ocean region includesthe southern portion of the Atlantic fromabout 608S to about 358S in the western prov-ince (4a) and from about 458S to about 208Sin the eastern province (4b). This region isgenerally equivalent to the Temperate SouthAtlantic ocean zone of Scheffer (1958). Itsnorthern boundary roughly corresponds withthe 208C SST isotherm and, as such, it in-cludes part of the northern portion of the No-topelagia pinniped region of Davies (1958a;modified from Sclater, 1897) who used the208C isotherm as the southern boundary. Thewestern boundary current is the warm BrazilCurrent, which eventually merges with theWest Wind Drift that moves eastward acrossthe South Atlantic. The eastern boundarycurrent is the cold Benguela Current. In thewestern South Atlantic, an important coun-tercurrent is the cold Falkland Current, whichflows north along the coast of Argentina toalmost 308N. Southeast Trade Winds blowingacross southwestern Africa create an area ofstrong upwelling off the coast of Namibia (Bin fig. 3.2A). The western province (4a) sup-ports a pinniped assemblage that includestwo otariids and one phocid, while the east-ern province supports only a single otariid.The otariine Otaria byronia ranges in thewest from Cape Horn to Uruguay and fromTierra del Fuego to the Falkland Islands. Al-though not as widespread, the range of thearctocephaline Arctocephalus australis isgenerally sympatric with Otaria. The south-ernmost part of this region is also occupiedby the monachine phocid Mirounga leonina,which occurs at Tierra del Fuego, the Falk-land Islands, and Gough Island. Arctoce-phalus pusillus pusillus is the only residentpinniped in the eastern South Atlantic prov-ince and ranges from South Africa (Port Eliz-abeth) northward along the coast to Namibia.

The North Pacific Ocean region includesthe northern portion of the Pacific from about608N to about 308N in the western province(5a; includes the Sea of Okhotsk) and toabout 258N in the eastern province (5b; in-cludes the Gulf of Alaska). The Bering Seaconstitutes the subarctic portion of this re-


gion (5c). This region roughly correspondsto the Temperate North Pacific oceanic zoneof Scheffer (1958) and the Arctirenia pinni-ped province of Davies (1958a, modifiedfrom Sclater, 1897) who used the 208C iso-therm as the southern boundary. The westernboundary current is the warm Kuroshio Cur-rent, which becomes the North Pacific Cur-rent as it flows eastward south of the Aleu-tian Archipelago. The eastern boundary cur-rent is the cold California Current, whicheventually merges with the North EquatorialCurrent to complete the North Pacific Gyre.The Oyashio Current and Alaskan Currentare important south-flowing cold currents inthe western and eastern North Pacific, re-spectively. Northeast Trade Winds blowingacross western North America create an areaof strong upwelling off the coast of Califor-nia and Baja California (D in fig. 3.2).

This region supports a varied pinniped as-semblage of four otariids, one odobenid, andtwo phocids. Callorhinus ursinus is probablythe most abundant otariid of this region. Thismigratory species ranges from the BeringSea (Pribilof Islands) south along both shoresof the North Pacific to at least Honshu (308N)in the western Pacific and southern Califor-nia (328N) in the eastern Pacific. The otariineEumetopias jubata is generally sympatricwith C. ursinus, the two species often sharingbreeding sites in the Pribilof Islands andAleutian Archipelago. Zalophus californi-anus is a common species in the temperateportion of this region, with three disjunctsubspecies recognized. Z. c. californianusrepresents the largest population and occu-pies the eastern North Pacific from BritishColumbia (498N) to Mexico (238N). Popu-lations in the western North Pacific areknown from the Sea of Japan. This subspe-cies, Z. c. japonicus, may be extinct. Thethird subspecies, Z. c. wollebacki does notoccur in this region, but is confined to theGalapagos Islands in the eastern South Pa-cific (7b). Arctocephalus townsendi, the onlyarctocephaline otariid in this region, is con-fined to the southern subtropical portion ofthe eastern North Pacific province. Althoughthis species is typically found at Isla Gua-dalupe (298N), its breeding range appears tobe extending north to include the CaliforniaChannel Islands (348N). Odobenus rosmarus

divergens occurs in the Bering Sea, primarilyin winter as individuals move south at thefront of the advancing pack ice. As discussedalready, the common phocine Phoca vitulinahas disjunct subspecies occurring in thenorthern hemisphere. P. v. richardsi rangesfrom the Bering Sea south along the NorthAmerican coast to Isla Cedros in Baja Cali-fornia (288N). P. v. stejnegeri ranges fromthe Commander Islands (588N) to Hokaido(48N). The monachine Mirounga angustiros-tris only occurs in the eastern North Pacificprovince between about San Francisco, Cal-ifornia (388N) and Bahia Magdalena, BajaCalifornia Sur (248N).

The Mid-Pacific Ocean region encompass-es the tropical and equatorial Pacific includ-ing the Philippine Sea and Coral Sea in thewest (6a) and the Hawaiian Islands, and westcoast of Mexico and Central America in theeast (6b). Because of a robust eastern bound-ary current (Peru Current) bringing cold wa-ter almost to the Equator coupled with strongcoastal upwelling off Peru and Ecuador (E infig. 3.2), the Galapagos Islands are not in-cluded in this region and are instead placedin the South Pacific Ocean region. The west-ern boundary current is the East AustralianCurrent, which brings warm surface water asfar south as New Zealand (358S). The NorthEquatorial Current and South EquatorialCurrent transport equatorial surface water tothe west, while the intervening EquatorialCounter Current carries surface water to theeast.

Monachus schauinslandi is the only pho-cid species that occurs in this warm-waterregion. Its range is restricted to the HawaiianIslands (308 to 208N), mainly the northwest-ern islands in the archipelago. It is notewor-thy that the western Mid-Atlantic phocid M.tropicalis is not known from the western sideof the Panamanian Isthmus. The otariinePhocarctos hookeri is historically knownfrom New Zealand’s North Island in thewestern province of the Mid-Pacific region,but is now known only from the westernSouth Pacific province. The arctocephalineArctocephalus forsteri is also known fromthe shores of the North Island of New Zea-land.

The South Pacific Ocean region includesthe southern portion of the Pacific from


about 508S to about 308S in the western prov-ince (7a; includes the Tasman Sea) and fromabout 608S to about 68S in the eastern prov-ince (7b). The Antarctic Convergence is tak-en as the region’s southern boundary. Thisregion roughly corresponds to the TemperateSouth Pacific oceanic zone of Scheffer(1958) and the eastern South Pacific portionof the Notopelagia pinniped province of Da-vies (1958a, modified from Sclater, 1897).The western boundary current is the warmEast Australian Current, which eventuallymerges with the West Wind Drift that moveseastward across the South Pacific. The east-ern boundary current is the cold Peru Currentthat carries surface waters north along thewest coast of South America. The SoutheastTrade Winds blowing across western SouthAmerica create an area of strong coastal up-welling off the coast of Peru and Ecuador (Ein fig. 3.2A).

This region supports a diverse pinniped as-semblage of eight otariids and one phocid.As discussed above, the Galapagos Islandsare included in this region because of north-ward displacement of the 208C isotherm bythe Peru Current and strong coastal upwell-ing off the west coast of Ecuador and Peru.

The Galapagos islands are home to the en-demic otariine Zalophus californianus wol-lebacki and the arctocephaline Arctocephalusgalapagoensis. The otariine Otaria byroniais very common along the west coast ofSouth America from Cape Horn and Tierradel Fuego (558S) to Isla Lobos de Tierra offPeru (68S). The arctocephaline Arctocephal-us australis is broadly sympatric with Otariaand is joined by the insular endemic A. phi-lippi in the Isla Juan Fernandez group (338S)and Isla San Felix group (268S) off Peru.Three other otariids have very restrictedranges in the western South Pacific province.These include the otariine Phocarctos hook-eri in the seas south of New Zealand aroundCampbell Island and the Auckland Islands(known archaeologically from the North Is-land; 6a); Arctocephalus forsteri is broadlysympatric with P. hookeri, but also occursalong the shores of New Zealand (North andSouth Islands; 6a, 7a), with a disjunct pop-ulation also occurring off the south coast ofWestern and South Australia. Arctocephaluspusillus doriferus inhabits the shores of New

South Wales and Victoria and occursthroughout the Bass Strait between Tasmaniaand Australia. The circum-Antarctic Miroun-ga leonina is found in the subantarctic is-lands south of New Zealand (Campbell Is-land and the Auckland Islands).

The Indian Ocean region is divided into anorthern province (8a) extending north fromabout 158S to the Red Sea, Arabian Sea, andBay of Bengal, a western province (8b) ex-tending along the eastern shore of Africafrom about 508S to 158S, and an easternprovince (8c) extending along the westernshore of Australia from about 508S to 158S.The southern boundary of this region isplaced at the Antarctic Convergence. Thewestern boundary current is the warm Agul-has Current that flows south along easternAfrica west of Madagascar. The easternboundary current is the cold West AustralianCurrent. The Southeast Trade Winds blowingoff the Australian continent create strong up-welling off the north coast of Western Aus-tralia (C in fig. 3.2A). Neophoca cinerea oc-curs in this area of coastal upwelling, ex-tending its range south and east along thesouthern coast of Western and South Austra-lia, where it is broadly sympatric with Arc-tocephalus forsteri. Arctocephalus tropicalisoccurs in the extreme southern portion of thisregion in the seas around the subantarctic is-lands of Prince Edward Island, Crozet Island,Marion Island, Amsterdam Island, and St.Paul Island (9b). Although Arctocephalusgazella is confined primarily to the SouthernOcean south of the Antarctic Convergence,it maintains breeding colonies on Prince Ed-ward Island just north of the convergence inthe subantarctic southern Indian Ocean.

The Southern Ocean region encompassesthe southern portions of the Atlantic (9a), In-dian (9b), and Pacific (9c) oceans. As usedhere, the Southern Ocean includes areas onlysouth of the Antarctic Convergence. This re-gion roughly corresponds to the Antarcticocean zone of Scheffer (1958) and the south-ern portion of the Notopelagia pinnipedprovince of Davies (1958a, modified fromSclater, 1897). The West Wind Drift (the sur-face portion of the Antarctic CircumpolarCurrent) carries surface water in an easterlydirection in the Southern Ocean. This surfacecurrent carries the southerly flow of the


South Atlantic, South Pacific, and South In-dian Ocean gyres and gives rise to the coldnorth-flowing eastern boundary currents ineach ocean (Benguela, Peru, and West Aus-tralian currents, respectively). The AntarcticConvergence is a major oceanographicboundary where the colder and saltier Ant-arctic surface waters sink beneath the north-erly, warmer subantarctic surface waters.South of the Antarctic Convergence is theAntarctic Divergence, an area of upwellingand high primary productivity at the bound-ary between the east-flowing West WindDrift and the west-flowing East Wind Drift.

Pinnipeds of the Southern Ocean includea diverse assemblage of monachine phocidsand a single species of arctocephaline otariid.The range of Arctocephalus gazella, the soleotariid, includes remote subantarctic islandsin the Atlantic Southern Ocean province(South Georgia, South Sandwich, South Ork-ney, South Shetland, and Bouvet islands) andIndian Southern Ocean province (Marion,McDonald, Heard, and Kerguelen islands).Leptonychotes weddelli, the most southerlyranging pinniped, is circumpolar south of theAntarctic Convergence and remains closelyassociated with fast ice near the shore of theAntarctic continent. Breeding colonies alsooccur on subantarctic islands including SouthShetlands, South Georgia, and South Orkneyislands. Ommatophoca rossi is also probablycircumpolar, but is most often associatedwith pack ice in the Pacific Southern Oceanprovince (Ross Sea) and Atlantic SouthernOcean province (King Haakon VII Sea). Thecircumpolar Lobodon carcinophagus is themost abundant pinniped in the world. It ispelagic in habit and found throughout theyear associated with pack ice. The largestpopulations are recorded from the PacificSouthern Ocean province (Amundsen Seaand Ross Sea). Hydrurga leptonyx is a cir-cumpolar lobodontine that occurs in associ-ation with the outer fringes of the pack ice.This species is reported to migrate northwardin winter, reaching the subantarctic islands ofSouth Sandwich and South Orkney south ofthe Antarctic Convergence, as well as islandsnorth of the convergence (Auckland, Mac-quarie, Campbell, Heard, and St. Paul is-lands). Mirounga leonina occurs on bothsides of the Antarctic Convergence and is

known to reach the Antarctic Continent dur-ing the austral summer. The subantarctic is-lands in the Atlantic Southern Ocean prov-ince (South Georgia, South Orkney, andSouth Sandwich islands) support populationsof this circumpolar species.

Having reviewed the modern distributionof pinnipeds, it is now possible to offer somegeneral comments on the emerging biogeo-graphic patterns. Pinnipeds generally occupycool temperate to polar regions in the conti-nental shelf areas of the world’s oceans. Spe-cies of Monachus are a notable exception,and are confined to the tropics (3a and 6b)and Mediterranean Sea (3c). Unlike ceta-ceans, there are no cosmopolitan pinnipedspecies. Distributional patterns include insu-lar endemism (e.g., Arctocephalus philippii,A. galapagoensis, Phocarctos hookeri, Mon-achus schauinslandi), coastal endemism(e.g., A. pusillus pusillus, Neophoca cinerea),lacustrine endemism (e.g., Pusa caspica andP. sibirica), sympatry (e.g., A. australis andOtaria bryonia, Erignathus barbatus andOdobenus rosmarus, Callorhinus ursinusand Eumetopias jubata, Neophoca cinerea,and A. forsteri), allopatry (e.g., M. mona-chus, A. pusillus pusillus), and antitropicality(e.g., Mirounga leonina and M. angustiros-tris, A. townsendi and Arctocephalus spp.).Within an ocean basin, pinnipeds generallyhave an asymmetric distribution extending tolower latitudes on the eastern shores of theoceans than on the western shores. This pat-tern is the combined result of the polar originof eastern boundary currents (e.g., CaliforniaCurrent, Peru Current, Benguela Current)and the trade wind-induced location of areasof strong coastal upwelling (e.g., westernNorth America, South America, Africa). Pin-nipeds are entirely missing from the northernand central Indian Ocean and the entire Ma-laysian Archipelago as well as most Poly-nesian islands (A. forsteri is known from theCook Islands). Otariids, although entirely ab-sent from the North Atlantic, typically occurat high latitudes and, as a group, display agross pattern of antitropicality. The otariineEumetopias occurs around the margins of theNorth Pacific (5a–c), while Zalophus occursin the temperate western North Pacific (5a)and temperate eastern North Pacific (5b), aswell as in the Galapagos Archipelago in the


tropical eastern Pacific (7b). The remainingotarines are confined to the southern hemi-sphere and include Otaria, with a coastalSouth American distribution (4a and 7b),Neophoca, with a coastal Australian distri-bution (8c), and Phocarctos, with a coastalNew Zealand distribution (6a and 7a). Pho-cine phocids are confined to the NorthernHemisphere and except for a few species, arerestricted to the Arctic and subarctic portionsof this region. In contrast, monachine pho-cids (except Mirounga angustirostris andspecies of Monachus) are confined to theSouthern Hemisphere, with the majority ofspecies restricted to the Southern Oceansouth of the Antarctic Convergence.

With few exceptions (e.g., Callorhinus,Mirounga spp., Hydrurga, Ommatophoca,Lobodon), pinnipeds have limited migra-tions. All, however, are amphibious andshow a distinct pattern of marine feeding andterrestrial/sea ice breeding. Many specieshave elaborate breeding behaviors that in-volve establishment and defense of terrestrialterritories by males, and generally extremesexual dimorphism.

PHYSICAL AND ECOLOGIC FACTORSAFFECTING DISTRIBUTIONS—PAST

AND PRESENT

Factors influencing the distribution of pin-nipeds may be grouped as follows: (1) phys-ical, including habitat and the type of haul-out substrate (e.g., ice), temperature, salinity,ocean current patterns, and water depth; and(2) ecologic, including the distribution andabundance of prey, predators, and competi-tors. Although one or more factors may exertgreater influence on a particular distribution,usually a combination of factors influencesobserved distribution patterns.

Since all phocid seals must give birth totheir pups on land or ice, their distributionsduring the breeding seasons are determinedby the availability of suitable habitats. Forice-breeding seals, especially those inhabit-ing pack ice (e.g., Hydrurga, Lobodon, Cys-tophora, Pagophilus, and Histriophoca), theinstability of the drifting ice floes from oneseason to the next affects their distribution.For land-breeding species, such as the SouthFarallon Island populations of Mirounga an-

gustirostris and central California’s popula-tions of Phoca vitulina, it has been suggestedthat the availability of, and access to, highquality breeding habitat may limit their dis-tribution (Sydeman and Allen, 1999). Withthreats on neonates by terrestrial predators animportant limiting factor, many pinnipedschoose isolated islands and/or rugged rockymainland shores for birthing sites. The geo-graphic distribution of such sites plays a di-rect role in determining the distribution ofmany species of pinnipeds, especially thoseof the Southern Ocean (e.g., Arctocephalustropicalis, A. gazella, Phocarctos hookeri,and Mirounga leonina). In another study thatexamined the role of competition as an influ-ence, the hypothesis that haul-out space is acontested resource was confirmed by com-paring the number of agonistic interactionsamong harbor seals in northern Californiawhen this resource was limited (Neumann,1999).

An important ecologic variable influencingthe distribution of pinnipeds is food. Theglobal distribution patterns of pinnipeds re-veal that species diversity is higher in areasof coastal upwelling near continental marginswhere ocean circulation patterns bring nutri-ent-rich bottom water to the surface. Al-though several fur seals live in tropical orsubtropical latitudes, ocean waters in theseareas are often cold and rich in nutrients be-cause of upwelling (Riedman, 1990). Thediet of pinnipeds ranges from zooplankton(e.g., amphipods and euphausids) to cepha-lopods (e.g., squid and octopus) and fromschooling fish (e.g., sardines and anchovies)to penguins and (occasionally) other pinni-peds. Zooplankton such as krill, but alsocephalopods (particularly squid), are typical-ly concentrated in well-defined layers duringdaylight. These ‘‘deep scattering layers’’ mi-grate vertically to shallower areas at night intemperate areas and during the warmermonths in high latitudes, and a correspon-dence has been found between pinniped dis-tributions and that of krill and squid. It hasbeen further suggested that the change in dai-ly behavior of many common deep scatteringlayer species is likely the basis for markeddifferences in maximum depths between dayand night dives seen in Mirounga angusti-rostris (Stewart and De Long, 1993), Arcto-


cephalus gazella (Croxall et al., 1994), andLobodon (Lowry et al., 1988). In addition tothe effects of water depth on prey availabil-ity, there are seasonal and regional changesin the distribution of pinnipeds and theirprey. For example, both Halichoerus grypusand Phoca vitulina exhibit seasonal dispersalfrom breeding and moulting sites. These sea-sonal changes in movement patterns oftenappear related to changes in foraging areas(see references cited in Bowen and Siniff,1999). Furthermore, there is good evidencethat the foraging ranges of males and femalesdiffer among species, as evidenced in speciesof Mirounga (Stewart and De Long, 1993;Slip et al., 1994).

Changes in pinniped distributions havealso been correlated with periods of warmsea surface temperatures associated with ElNino conditions that cause a 3–128C increasein ocean temperature in the eastern Pacific.The 1982–1983 El Nino was a time of foodshortage and increased rate of mortality, es-pecially for temperate and tropical pinnipeds(e.g., Arctocephalus galapagoensis, Mona-chus sp., Ano Nuevo Island population ofMirounga angustirostris; Trillmich et al.,1991). Some pinniped populations, however,increased during the 1983 and 1992 (and1997–1998) El Ninos due to a general north-ward migration from southern Californiabreeding grounds, the latter associated withpoor food availability (references cited in Sy-deman and Allen, 1999).

In addition to short term climatic fluctua-tions such as El Nino events, long-term cli-matic changes such as decreased ice-associ-ated habitat, and changes in prey availabilityaffect pinnipeds—particularly those living inthe Arctic (e.g., Histriophoca, Pagophilus,Cystophora, Erignathus, and Odobenus;Tynan and De Master, 1997; Bowen and Sin-iff, 1999). For example, declines in Pusa his-pida density have been linked with the se-verity of ice conditions in the Beaufort Seain 1974–1975 and 1982–1985 (Stirling et al.,1977; Harwood and Stirling, 1992). For pin-nipeds inhabiting the Arctic, climatic changeis likely to affect their prey availability. Forexample, the distribution of Arctic cod, animportant prey item for Pagophilus, Cysto-phora, Erignathus, and Pusa hispida, varies

with ice conditions (references cited in Tyn-an and De Master, 1997).

The role of predation in influencing thedistribution of pinnipeds has been examinedin some pinniped populations, most notablyMirounga angustirostris. For example, pre-dation by white sharks on juveniles and sub-adults has increased with time and may belimiting the size of the South Farallon Islandpopulation (Pyle et al., 1996). The long-termeffect of this interaction may be one of lim-iting recruitment.

Since physical and ecologic factors affectthe distribution of extant pinnipeds, it is likelythat these factors also affected the distributionof pinnipeds in the past. It has been proposedthat a key factor in the initial radiation of pin-nipeds (and other marine mammals) was theonset of glaciation in the late Oligocene andthe resulting development of a thermally strat-ified world ocean with cold bottom water,strong surface circulation gyres, and long-range thermohaline circulation. This restruc-tured oceanic condition coupled with the de-velopment of extensive areas of coastal up-welling changed the location of primary ma-rine productivity in the world’s oceans fromthe equatorial region to high latitudes (For-dyce, 1980). It has also been postulated thatthe apparent decline in marine mammal di-versity at various intervals during the Tertiaryreflects oscillations in the strength of oceanicthermal gradients, with decreases in the ther-mal gradient correlated with decreased up-welling and primary productivity and thuslimited dispersal and speciation in marinemammals (Lipps and Mitchell, 1976). The ap-plicability of this general hypothesis as an ex-planation for fluctuations in marine mammaldiversity through time is currently being re-examined (by PJA). The distribution of pin-nipeds, however, cannot be explained solelyin terms of changing food resources. Severallarge-scale physical factors (e.g., opening andclosing of seaways, glacial and interglacialclimatic events, and changes in ocean circu-lation patterns) have played an important rolein previous explanations of fossil and modernpinniped biogeographic distributions. Belowwe review the major seaways that have beeninvoked as dispersal corridors used by pinni-peds and other marine mammals.

The Central American Seaway separating


North and South America allowed free ex-change of marine waters between the equa-torial Pacific and Atlantic oceans for most ofthe Cenozoic (see below). By the early lateMiocene (11 Ma), however, tectonic activityin the region had resulted in progressive up-lift of the Panama Sill and restriction oftrans-oceanic circulation (Hallam, 1994).Duque-Caro (1990) further suggests that astrong eastern boundary current in the NorthPacific (California Current) was bringingcool surface waters to the region and increas-ing the circulation barrier through the sea-way. Such a cool current so close to theEquator, and possibly converging on a strongnorth-flowing eastern boundary current in theSouth Pacific (Peru Current), would havealso provided a path for north–south crossingof the tropics by temperate water organisms,including pinnipeds. Although in the latestMiocene (6.3 Ma), free east–west trans-oce-anic circulation was reestablished for awhile, this was lost by the mid-Pliocene(3.7–3.1 Ma) with final emergence of theIsthmus of Panama (Duque-Caro, 1990). Thefinal closure of the seaway is correlated withstrengthening of the western boundary cur-rent in the North Atlantic (Florida Currentand Gulf Stream) and warming of surfacewaters in the region (Dowsett and Cronin,1990). The importance of the Central Amer-ican Seaway to pinniped paleobiogeographyinvolves the potential use of this seaway asan east–west dispersal corridor between thePacific and the Atlantic. For example, it hasbeen suggested that odobenine walruses andmonachine seals followed this route (Repen-ning et al., 1979), the latter dispersing fromeast to west through the portal and the formerdispersing from west to east. The impact ofclosure of the seaway on pinniped distribu-tions has already been mentioned with regardto the use of strong eastern boundary cur-rents as cool water pathways for crossing thetropics. However, an additional impact fromclosure of the seaway is the possible frag-mentation of species ranges and the resultingdivergence/speciation of taxa (e.g., Mona-chus tropicalis and M. schauinslandi). Cur-rently, there is no good fossil evidence fromthe tropical eastern Pacific or Caribbean ar-eas to unequivocally support or refute oreven provide a temporal constraint on the hy-

pothesized dispersal of pinnipeds through theCentral American Seaway. Those fossils thatare available occur outside the Panamanianregion and provide only a minimum age forconstraining possible dispersal events throughthe seaway. These include the important fossilmonachine phocid assemblage from the PiscoFormation in coastal Peru (Muizon, 1981) andthe diverse fossil pinniped (phocid and odob-enid) assemblage from the Yorktown Forma-tion and correlative deposits in coastal NorthCarolina and Florida (Ray, 1976; Morgan,1994).

The history of the Mediterranean Sea rep-resents another important marine geologicfactor for pinniped evolution. The modernMediterranean Sea is a remnant of the Me-sozoic and early Cenozoic Tethys Sea thatprovided a direct connection between the Pa-leogene North Atlantic and Indian oceans.During the early Miocene, tectonic events re-lated to the collision of the African and Eur-asian lithospheric plates resulted in severingthe marine connection between the Mediter-ranean Sea and the Indian Ocean, and theestablishment of a north–south land corridorbetween the two continents in the MiddleEast. This tectonic event also resulted in iso-lation of a northern arm of the Tethys to formthe Paratethys, a region of present-day East-ern Europe extending from Austria to Uz-bekistan and occupied by the modern Black,Caspian, and Aral seas. The initial isolationof Paratethys was followed by a short-livedmarine transgression during the middle Mio-cene (Rogl and Steiniger, 1984) and eventu-ally by renewed isolation and brackish con-ditions in the late middle/early late Miocene(Sarmatian). A large-scale drying of theMediterranean region occurred in the lateMiocene (Messinian Salinity Crisis), result-ing in hypersaline conditions in the Mediter-ranean basin and formation of a series of hy-posaline Paratethys lakes. The connectionbetween the Mediterranean basin and the At-lantic was reestablished by the early Plio-cene, while Paratethys remained isolated.The Mediterranean reconnection with the At-lantic provided a potential corridor for dis-persal of the endemic phocid assemblage outof the region. The full impact of the contin-ued isolation of the Paratethys on phocineevolution and biogeography (e.g., Pusa),


however, is controversial, with some workerssuggesting a Paratethyan origin for this lin-eage followed by dispersal into the Arctic ba-sin and other workers proposing an Arcticorgin followed by dispersal into Paratethys(see below).

The establishment of the Bering Strait,which provides a direct polar connection be-tween the oceans of the Northern Hemi-sphere (North Pacific, Arctic, and North At-lantic), represents a third major geologicevent having a significant influence on pin-niped evolution. The Bering Strait opened asthe result of plate tectonic interactions duringthe latest Miocene to earliest Pliocene (5.5–4.8 Ma; Marincovich, 2000). This openingwas associated with an initial phase of trans-Arctic biotic interchange in which waterflowing north to south through the strait fa-cilitated dispersal of Atlantic and Arctic mol-luscs into the North Pacific region. Pinni-peds, particularly odobenine walruses, couldhave also participated in this interchange (seelater discussion) but there is no direct fossilevidence in the Arctic basin to support thishypothesis. The flow of surface marine wa-ters through the Bering Strait reversed duringthe mid-Pliocene (3.6 Ma), an event corre-lated with the elevation of the Isthmus ofPanama, closure of the Central AmericanSeaway, and reorganization of NorthernHemisphere ocean circulation (Haug andTiedemann, 1998). The resulting south-to-north flow established the modern ArcticOcean circulation pattern (Trans-Arctic Cur-rent) and facilitated the dispersal of Pacificmolluscs into the Arctic and North Atlantic(Marincovich, 2000). Subsequent glacioeus-tatic oscillations during the late Pliocene andPleistocene caused the cyclic exposure andsubmergence of the Bering Land Bridge, theclosing and opening of the Bering Strait, andthe periodic isolation of the North Pacificfrom the Arctic Ocean and North Atlantic.The importance of these glacioeustatic eventsto Pleistocene phocid and odobenid bioge-ography (see below) was discussed exten-sively by Davies (1958b).

The Southern Ocean, a circumpolar sea-way connecting the southern portions of thePacific, Atlantic, and Indian oceans, was animportant marine corridor for east–west dis-persal of fur seals, sea lions, and lobodontine

phocids. Unlike the preceding seaways, it hasremained open throughout the Neogene. Pri-or to the mid-Oligocene, remnants of theGondwana supercontinent prevented the freecircumpolar circulation of ocean water.Opening of the Southern Ocean via north-ward continental drift of South America, Af-rica, and Australia in the mid-Oligocene iscorrelated with establishment of a circum-polar ocean current (West Wind Drift) andinitiation of glaciation in east Antarctica.This event had a global impact on restruc-turing circulation systems of the world oceanand initiation of the glacial conditions of thePleistocene. Although today the SouthernOcean is a major area of pinniped diversity,the fossil record from this region is too poor-ly known at present to provide any direct ev-idence of the biological events leading up tothat diversity.

Having reviewed the physical, historical,and ecologic factors that have certainlyplayed major roles in establishing the bio-geographic pattern of modern pinnipeds, wenow turn to the value of phylogeny in con-straining and testing competing hypothesesproposed to explain this pattern.

HISTORICAL REVIEW OFPINNIPEDIMORPH HIGHER LEVEL

PHYLOGENY

As mentioned previously, the approachtaken here differs from previous narrative ac-counts in that it explicitly employs a phylo-genetic framework as a context from whichto examine the history of pinnipedimorph di-versification. This method forces the bioge-ographer to ask several questions. Is thegroup monophyletic? If so, what and whereis its sister group? On both morphologic andmolecular grounds pinnipedimorphs consti-tute a monophyletic group (Wyss, 1987;Flynn et al., 1988; Berta and Wyss, 1994;Vrana et al., 1993; Bininda-Emonds and Rus-sell, 1996; Flynn and Nedbal, 1998). Al-though we have depicted a hypothesis thatsupports ursids as the sister group of pinni-pedimorphs, it is acknowledged that the var-ious arctoid carnivorans (including mustelidsand procyonids in addition to ursids) are dif-ficult to separate at their point of divergence.Other hypotheses support pinnipedimorphs


as either allied with mustelids (e.g., Bininda-Emonds and Russell, 1996) or as having anunresolved arctoid ancestry (e.g., Arnasonand Widegren, 1986; Arnason and Ledje,1993). Another as yet unresolved issue is theinterrelationships among major pinnipedi-morph groups. Most morphologic data sup-ports a link between phocids and odobenids(Wyss, 1987; Wyss and Flynn, 1993; Bertaand Wyss, 1994), whereas molecular data fa-vors a more traditional link between odob-enids and otariids (Vrana et al., 1993; Lentoet al., 1995; Arnason et al., 1995). Resolutionof this conflict will likely benefit most frommore detailed exploration of morphologicand molecular data sets that offer both ad-ditional (e.g., fossil) taxa and characters. Wefollow here the morphologic data that sup-port odobenid 1 phocid alliance (Phocomor-pha clade) and phocids in turn allied with theextinct desmatophocids (Phocoidea clade).

METHODOLOGY

A complete species-level phylogeny forpinnipeds including fossil and extant taxa is,as yet, unavailable. We here use a compositetree (fig. 3.3) based on the basic topology ofthe generic-level phylogeny of Berta andWyss (1994) but with the following substi-tutions: (1) we replaced Otariidae with thephylogeny of Bininda-Emonds et al. (1999)and our own work based on morphologic andmolecular data, (2) we replaced phocid sub-families with the consensus phylogeny ob-tained by Bininda-Emonds et al. (1999) andadded fossil taxa based on Berta and Wyss(1994) and Muizon (1982), (3) we replacedOdobenidae with the phylogeny of Demereand Berta (2001), and (4) we replaced Des-matophocidae with the phylogney of Demereand Berta (2002). These major clades arewidely accepted as monophyletic and theircombination into a single tree is supportedby the robustness of basal nodes in the com-posite tree (an exception is the Phocoideaclade with 55% support). We next examineinterrelationships of the various pinnipedi-morph groups and use temporal and geologicevidence (figs. 3.3–3.7, table 3.2) to supportor refute previous hypotheses of their origin,diversification, and dispersal.

PHYLOGENY AND BIOGEOGRAPHY:AN INTEGRATION

BASAL PINNIPEDIMORPHS: Basal pinnipedi-morphs were amphibious carnivorans withshearing teeth, flexible spines, and flipper-like fore and hind limbs. They were likelycoastal dwellers capable of more efficient ter-restrial locomotion than extant pinnipeds(Berta and Ray, 1990). The basal taxon En-aliarctos is known from five species (Mitch-ell and Tedford, 1973; Berta, 1991) from thelate Oligocene and early Miocene of Cali-fornia (27–25 Ma; figs. 3.3, 3.7). Anotherbasal taxon is Pinnarctidion with two speciesdescribed from the early Miocene, P. bishopifrom California and P. rayi from Oregon. Al-though Pinnarctidion has previously been re-garded as a member of the Phocoidea clade(Berta, 1994b; Berta and Wyss, 1994) thisalliance is no longer supported (Demere andBerta, 2002). Pinnarctidion is more likely abasal pinnipedimorph as proposed by Barnes(1979). A later diverging lineage, moreclosely allied with pinnipeds than with En-aliarctos, is represented by Pteronarctos andPacificotaria from the early and middle Mio-cene (19–15 Ma) of coastal Oregon (Berta,1994a; fig. 3.3).

When the hypothesis of pinnipedimorphmonophyly is used to constrain the historicalbiogeography of the group, it is most parsi-monious to consider that the common ances-tor of all pinnipeds, including basal pinni-pedimorphs, originated in the North Pacificrather than hypothesizing two separate areasof origin, one for otariids and odobenids inthe North Pacific and another for phocids inthe North Atlantic (Repenning et al., 1979).Among those supporting a single-origin hy-pothesis, Davies (1958a) has proposed an or-igin of pinnipeds in the Arctic basin followedby separate dispersals and subsequent diver-sification of phocids in the North Atlanticand otariids and odobenids in the North Pa-cific. The fossil record, however, does notprovide any evidence for an Arctic origin,but instead favors a single origin in the NorthPacific basin. The oldest known record(OKR; Walsh, 1998) for pinnipedimorphs iscurrently established by Enaliarctos tedfordiand E. barnesi (table 3.2) from the late Oli-gocene (Chattian) Yaquina Formation of


coastal Oregon (eastern North Pacific, 5b).Until an older OKR is discovered, theOregon record provides both a minimum agefor the origin of pinnipedimorphs and a like-ly region of origin. It should be mentionedthat two partial femora assigned to the Pho-cidae by Koretsky and Sanders (1997) werepurportedly collected from upper Oligocenedeposits in South Carolina (western NorthAtlantic, 2a). However, these fragmentaryspecimens have not been formally describedand their stratigraphic provenience may be inquestion. It would be of major significanceto find late Oligocene phocid remains in thisregion.

PINNIPEDIA CLADE: The Pinnipedia in-cludes three major monophyletic taxa: (1) theOtariidae, (2) the Odobenidae, and (3) thePhocoidea (Phocidae plus the extinct des-matophocids; Berta and Wyss, 1994).

Pinnipeds include animals that lost thewell-developed carnassial teeth of basal pin-nipedimorphs and evolved important middleear aquatic adaptations (Repenning, 1972,1976). The OKR of Pinnipedia is currentlyestablished by Desmatophoca brachycephalafrom the early Miocene (Aquitanian) AstoriaFormation of Washington State (easternNorth Pacific, 5b) suggesting that divergenceof pinnipeds from basal pinnipedimorphslikely occurred sometime before 18 Ma,probably in the North Pacific Ocean Basin.

OTARIIDAE—FUR SEALS AND SEA LIONS:The monophyletic Otariidae or eared seals(Berta and Wyss, 1994; figs. 3.3, 3.4) in-cludes animals with broad supraorbital pro-cesses of the frontals and generally homo-dont dentitions. Otariids swim with forelimbpropulsion and have retained the ability torotate the hind feet forward for quadrapedal‘‘walking’’ during terrestrial locomotion. Ex-tant otariids are often divided into two sub-groupings, the Otariinae (sea lions) and theArctocephalinae (fur seals). Although priorsystematic work based on morphology (Bertaand Demere, 1986) suggested that only theotariines were monophyletic, a more recentanalysis (Bininda-Emonds et al., 1999) sug-gests that both subgroups are monophyletic(but see Lento et al., 1995, 1997 for a dif-ferent view).

The most basal otariid is the poorly knownPithanotaria starri from the late Miocene

(Tortonian) Sisquoc Formation of California(eastern North Pacific, 5b; figs. 3.4, 3.7).This record also establishes the otariid OKR.Other basal otariids include ‘‘Thalassoleon’’mcnallyae and ‘‘Thalassoleon’’ inouei fromthe late Miocene of California and early Pli-ocene of Japan, respectively (Repenning andTedford, 1977; Kohno, 1992). The next di-verging otariid lineage includes ‘‘Thalasso-leon’’ mexicanus and the Callorhinus cladefrom the late Miocene and late Pliocene toRecent, respectively, of the western NorthPacific (5b; Repenning and Tedford, 1977;Berta and Demere, 1986) and Hydrarctos lo-masiensis from the late Pliocene or earlyPleistocene of the eastern South Pacific (7b;Muizon, 1978; Muizon and de Vries, 1985).The latter record establishes the OKR forsouthern hemisphere otariids.

The otariid crown clades, Otariinae andArctocephalinae, share a rather recent com-mon ancestry and do not contain any extinctgenera. The OKR for this pairing is problem-atical, with all pre-Pleistocene records of du-bious validity (see discussions in Miyazakiet al., 1995). Until more taxonomically di-agnostic fossil material is found and de-scribed, only the late Pleistocene occurrencesfrom Brazil (Otaria byronia; Drehmer andRibeiro, 1998) and New Zealand (Neophocapalatina; King, 1983a) can be considered re-liable. Within the extant Otariinae, fivemonotypic genera and species are recognized(table 3.2). Relationships among sea lionssuggest that Zalophus is the basal taxon withEumetopias, Neophoca, Phocarctos, andOtaria as sister taxa (Bininda-Emonds et al.,1999). It is noteworthy that the two basalotariines represent North Pacific Ocean en-demics. The Arctocephalus clade consists ofeight extant species whose interrelationships(fig. 3.4) suggest a division into three sub-groups: (1) the ‘‘pusillus’’ species group con-sisting of A. pusillus pusillus and A. pusillusdoriferus, (2) the ‘‘gazella’’ species groupthat includes A. gazella and A. tropicalis, and(3) the ‘‘galapagoensis’’ species group thatincludes A. townsendi as sister taxon to anunresolved clade containing A. galapagoen-sis, A. australis, A. forsteri, and A. philippii(Bininda-Emonds et al., 1999). Interestingly,the two basal arctocephaline groups representSouthern Hemisphere endemics.


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TABLE 3.2Chronostratigraphy of Pinnipedimorphs

Numbers preceding taxonomic names refer to localities shown in figure 3.7.


TABLE 3.2(Continued)



By combining phylogenetic and strati-graphic data, it seems clear that otariidsevolved in the eastern North Pacific some-time before 11 Ma with basal taxa occurringin the eastern and western parts of that regionby the late Miocene. Using the phylogeny, itis most parsimonious to assume that diver-gence of Thalassoleon 1 Callorhinus andotariine 1 arctocephaline clades also oc-curred in the North Pacific. Stratigraphic data(i.e., OKR for Thalassoleon mexicanus) sug-gest that this divergence occurred prior to 6Ma (late Miocene). The divergence of theotariine and arctocephaline clades is poorlyconstrained by stratigraphic data. Species ofthe arctocephaline clade are known in thefossil record only from the Pleistocene, withpoorly documented records from the easternSouth Atlantic (4b; A. pusillus) and the east-ern North Pacific (5b; A. townsendi; see Re-

penning and Tedford, 1977). It seems likelythat initial divergence of this clade involveda single transequatorial dispersal event fromthe North Pacific into the Southern Hemi-sphere crossing into the eastern South Pacific(7b; fig. 3.4) via eastern boundary currents(i.e., California and Peru currents). Hydrarc-tos lomasiensis from the late Pliocene/earlyPleistocene of Peru (7b) provides a possibleminimum age for this Southern Hemispheredispersal event, since it predates all describedfossil occurrences of Arctocephalus. Once inthe Southern Hemisphere, speciation of theArctocephalus clade occurred, with six of thecurrently recognized species diversifying inhigh latitudes. High species diversity of thisclade likely reflects rapid speciation duringthe Pleistocene. Dispersal of A. pusillus pus-illus in the eastern South Atlantic (4b) andA. pusillus doriferus in the western South Pa-


Fig. 3.4. Area cladogram of otariids and summary of principal dispersal/vicariant events hypothesizedto explain their distribution around the world. (I) Earliest otariids in North Pacific before 11 Ma; Thalassoleon1 Callorhinus and divergence of arctocephaline 1 otariine clades and establishment of circum-North Pacificrange by early Pliocene; (II) transequatorial dispersal from North Pacific into the Southern Hemispherecrossing the eastern South Pacific; earliest fossil otariid in Southern Hemisphere 2–3 Ma; (III) speciation ofArctocephalus clade and dispersal into eastern South Atlantic; (IV) continued diversification and dispersalinto southern Indian Ocean and western South Pacific following West Wind Drift; (V) Pleistocene reinvasionof the Northern Hemisphere by dispersal or vicariance (glacial-interglacial oscillation).


cific (7a) followed the West Wind Drift. Alater divergence following this same dispers-al route is hypothesized to account for thepresent day distributions of A. tropicalis inthe Atlantic and Indian Southern Ocean(9a,b), A. gazella in the Atlantic and IndianSouthern Ocean (9a,b), and A. forsteri in thewaters off New Zealand and southern Aus-tralia (7a). Evolution of the A. australis–A.phillippi–A. galapagoensis clade likely oc-curred in the eastern South Pacific (7b), withexpansion of the A. australis range along theeastern shore of South America (4a) possiblyfollowing the cold Falkland Current. Themodern geographic range of A. townsendi inthe temperate eastern North Pacific off BajaCalifornia (5b) is anomalous relative to otherspecies of Arctocephalus, and suggests asouth-to-north transequatorial dispersal eventduring a Pleistocene glacial period whenstrong eastern boundary currents and coastalupwelling would provide an oceanographic‘‘corridor’’ for crossing the tropics. However,the hypothesized phylogenetic position of A.townsendi outside the A. australis–A. phillip-pi–A. galapagoensis clade does not provideany supporting evidence for a possible di-vergence from known eastern South Pacificarctocephaline taxa. The phylogeny, howev-er, does provide possible evidence for south-to-north divergence of A. galapagoensisfrom an A. australis–A. phillippi clade. Thisdivergence could have been either by south-to-north dispersal to the Galapagos Archi-pelago or by a vicariant event related toPleistocene glacial-interglacial oscillations,fragmentation of an ancestral home range,and allopatric speciation in the GalapagosArchipelago.

The historical biogeography of sea lions issimilar to that of fur seals and also suffersfrom a limited fossil record. As mentioned,basal sea lions Zalophus and Eumetopias areknown from the Pleistocene (and possiblyPliocene) of the western and eastern NorthPacific (Miyazaki et al., 1995). Except formodern populations of Zalophus in waters ofthe Galapagos Archipelago (7b), otariines arepresently, and have probably always been,confined to temperate, high latitudes. Wepropose that otariines reached the SouthernHemisphere via a single transequatorial dis-persal event in the eastern Pacific Ocean,

perhaps coincident with the arctocephalinedispersal event discussed previously. Speci-ation in the Neophoca–Phocarctos–Otariaclade most likely occurred during the Pleis-tocene in the Southern Hemisphere via theWest Wind Drift, either preceding or follow-ing the Southern Ocean dispersal of otariines(i.e., speciation then dispersal or range ex-pansion then allopatric speciation). Theseevents were likely synchronous with fur sealdispersal events. In support of this claim, wenote the broadly sympatric distributions ofOtaria bryonia and Arctocephalus australisin the eastern South Pacific and westernSouth Atlantic, of Neophoca cinerea andArctocephalus forsteri in waters off southernAustralia, and of Phocarctos hookeri andArctocephalus forsteri in waters south ofNew Zealand.

PHOCOMORPHA CLADE: Phocomorpha in-clude two major monoplyletic taxa: (1) theOdobenidae and (2) the Phocoidea (Phocidaeplus the extinct Desmatophocidae; Berta andWyss, 1994). Phocomorphs include animalswith enlarged auditory ossicles (Berta andWyss, 1994). The OKR of Phocomorpha iscurrently established by Desmatophoca bra-chycephala from the early Miocene (Aqui-tanian) Astoria Formation of WashingtonState (eastern North Pacific, 5b), suggestingthat evolution of phocomorphs likely oc-curred sometime prior to 18 Ma, probably inthe North Pacific Ocean Basin (fig. 3.5, table3.2).

ODOBENIDAE—WALRUSES: The monophy-letic Odobenidae or walrus clade (Demere,1994b; Kohno et al., 1995a; figs. 3.3, 3.5)includes a single extant species Odobenusrosmarus, and at least 20 fossil species ar-ranged in 14 genera. Living Odobenus swimsprimarily with hindlimb propulsion and, likeall odobenids, has retained the ability to ro-tate the hind feet forward for quadrapedal‘‘walking’’ during terrestrial locomotion.

The most basal odobenid taxa comprisethe Prototaria–Proneotherium clade fromlate early and/or early middle Miocene de-posits of the western and eastern North Pa-cific (5a, 5b; Kohno et al., 1995a). The odob-enid OKR is currently established by Pro-neotherium repenningi from the late earlyMiocene (Burdigalian) Astoria Formation ofthe eastern North Pacific (5b). Although


Fig. 3.5. Area cladogram of odobenids and summary of principal dispersal/vicariant events hypoth-esized to explain their distribution around the world. (I) Earliest odobenids in North Pacific before 18Ma; evolution and divergence of dusignathines and odobenines also in North Pacific; dusignathinesNorth Pacific endemics; (II) diversification and dispersal of odobenines following a northern ArcticOcean route (IIa) through the open Bering Strait 4–5 Ma from the Pacific into the Atlantic (IIb);continued diversification and dispersal of odobenines to both shores of the North Atlantic (III).


Kohno et al. (1995a) assign this taxon to theearly middle Miocene, the geologic age ofProneotherium is probably older because ofits apparent occurrence near the base of thelong-ranging Astoria Formation (Demere andBerta, 2001). Other basal odobenids includeNeotherium mirum (middle Miocene; easternNorth Pacific) and Imagotaria downsii (lateMiocene; eastern North Pacific) as succes-sive sister taxa. Later diverging walruses canbe grouped into two monophyletic groups(figs. 3.3, 3.5). The Dusignathinae includesthe extinct genera Dusignathus, Gomphotar-ia, and Pontolis. The dusignathine OKR isestablished by Dusignathus santacruzensis,Gomphotaria pugnax, and Pontolis magnusfrom late late Miocene (Messinian) depositsof the eastern North Pacific (5b). The Odob-eninae includes, in addition to the modernwalrus Odobenus, the extinct genera Aivikus,Protodobenus, Pliopedia, Alachtherium,Prorosmarus, and Valenictus (Demere,1994b; figs. 3.3, 3.5). The latter five taxacomprise the Odobenini crown clade, theOKR of which is established by Protodob-enus japonicus from the early Pliocene (Zan-clian) Tamugigwa Formation of Japan (5a;western North Pacific).

Phylogenetic and stratigraphic data sug-gest that odobenids first evolved in the NorthPacific region sometime before 18 Ma (lateearly Miocene) with basal taxa being con-fined to the eastern and western parts of thisregion during the middle Miocene. The fossilrecord reveals that basal odobenids ranged asfar south as northern Baja California, Mexicoin the eastern North Pacific region during themiddle Miocene (based on undescribed ma-terial aff. Neotherium sp.; Barnes, 1998; fig.3.5). Using the phylogeny it is most parsi-monious to assume that divergence of the du-signathine and odobenine clades also oc-curred in the eastern North Pacific. Strati-graphic data (i.e., OKRs of Gomphotariapugnax and Aivukus cedrosensis) suggestthat this divergence occurred prior to 6 Ma(late Miocene). The fossil record further sug-gests that dusignathines remained endemic tothe eastern North Pacific region throughouttheir evolutionary history. In contrast, odob-enines underwent dramatic diversificationduring the late Miocene with later membersof this lineage dispersing from the North Pa-

cific into the eastern and western North At-lantic by early Pliocene time (fig. 3.5). It hasbeen proposed that members of the odoben-ine lineage first entered the Caribbean fromthe Pacific via the Central American Seawaybetween 5 and 8 Ma (Repenning et al., 1979)and then dispersed northward into the NorthAtlantic, where the Odobenini clade is pos-tulated to have evolved. According to thishypothesis, odobenids became extirpatedfrom the Pacific in the Pliocene, and then lessthat one million years ago the modern genusOdobenus returned to the North Pacificthrough the Arctic Ocean (Repenning andTedford, 1977; Repenning et al., 1979). Alate Miocene southern dispersal route is sup-ported by the occurrence of the basal odob-enine Aivukus from Baja California, Mexico.However, a new and improving record offossil walruses from Japan and Californiasupports an alternative hypothesis involvinga northern, east-to-west dispersal route dur-ing the early Pliocene. Rather than being ex-tirpated from the North Pacific during thePliocene, odobenids continued to diversify inthat region into the Pleistocene (Tomida,1989; Kohno et al., 1995b). In fact, using thephylogeny (fig. 3.5), it is most parsimoniousto assume that evolution of the Odobeniniclade actually occurred in the North Pacific.The OKR of Protodobenus japonicus furthersuggests that this divergence happened priorto 4.5 Ma (early Pliocene). The discovery ofAlachtherium in the early Pliocene of Japan(Kohno et al., 1998) supports the hypothesis.This taxon was formerly considered to be aNorth Atlantic endemic odobenine from thelate Pliocene and Pleistocene of western Eu-rope (Ray, 1976; Demere, 1994b). Its earlieroccurrence in the North Pacific, however,suggests that Alachtherium more likelyevolved in that ocean region before dispers-ing into the North Atlantic during the earlyPliocene. The timing of this event is impor-tant because the potential for biotic exchangethrough the Central American Seaway hadbeen dramatically reduced by the early Pli-ocene (Duque-Caro, 1990) making it lesslikely that odobenines would use this warmwater route. At the same time, however, geo-logic events in the Arctic Ocean had estab-lished a wide open Bering Strait and highglobal sea levels combined with ice-free,


trans-Arctic circulation, which in turn, pro-vided a suitable northern dispersal route be-tween the North Pacific and North Atlanticregions (Marincovich, 2000). The timing forthis event is also coincident with dispersal ofPacific mollusks (e.g., Mya arenaria) into theAtlantic and their appearance in such odob-enine-producing deposits as the Scaldisiansands of Belgium (2b; eastern North Atlan-tic) and the Yorktown Formation of NorthCarolina, USA (2a; western North Atlantic;Vermij, 1991).

The historical biogeography of the moderngenus Odobenus is more difficult to deter-mine because of a limited fossil record andimprecise biochronologic control. The pau-city of complete skulls and dentitions makesit impossible to resolve phylogenetic rela-tionships at this time among the various pro-posed fossil species of this genus (fig. 3.5),and the strong possibility exists that severalof these taxa are conspecific. Kohno et al.(1995b) concluded on the basis of strati-graphic and geographic occurrences of Pleis-tocene Odobenus that the genus evolved inthe North Pacific, dispersed into the ArcticOcean, and eventually the North Atlantic,probably during one of the early interglacialevents of the latest Pliocene or Pleistocene.During the Pleistocene, there was possibly asingle circum-arctic species of Odobenus thatextended its range into near temperate lati-tudes in both the Pacific and Atlantic duringinterglacial periods. The two extant subspe-cies of Odobenus rosmarus may representthe product of range fragmentation and al-lopatric speciation during a glacial period ofmaximum Arctic sea ice volume.

PHOCOIDEA CLADE: Phocoidea includestwo monoplyletic taxa: (1) the extinct Des-matophocidae and (2) the extant Phocidae(true or earless seals; Berta and Wyss, 1994).Phocoids include animals with a mortisedsquamosal/jugal contact and posterior termi-nation of nasals posterior to contact betweenthe frontal and maxilla bones (Berta andWyss, 1994). The OKR of Phocoidea is cur-rently established by Desmatophoca brachy-cephala from the early Miocene (Aquitanian)Astoria Formation of Washington State (east-ern North Pacific, 5b), suggesting that diver-gence of phocoids from odobenids likely oc-

curred sometime before 18 Ma, probably inthe North Pacific Ocean Basin.

DESMATOPHOCIDS—EXTINCT PHOCID RELA-TIVES: An extinct family of archaic phocoids,the desmatophocids were animals with en-larged paroccipital processes and bulbouscrowned postcanine teeth (Demere and Ber-ta, 2002). Desmatophocids probably swamwith forelimb propulsion and retained theability to rotate the pes forward for quadra-pedal ‘‘walking’’ during terrestrial locomo-tion. The Desmatophocidae includes twogenera, Desmatophoca and Allodesmus,which together contain at least six describedspecies from the early and middle Miocene(fig. 3.3). The OKR of Desmatophocidae iscurrently established by Desmatophoca bra-chycephala from the early Miocene (Aqui-tanian) Astoria Formation of Washington,USA (eastern North Pacific, 5b), suggestingthat divergence of desmatophocids from pho-cids likely occurred sometime before 18 Ma,probably in the North Pacific Ocean Basin.The OKR of Desmatophocidae is the sameage as for the Phocoidea clade. The youngestknown record (YKR; Walsh, 1998) of Des-matophocidae is based on unpublished spec-imens of Allodesmus from the late Miocene(Tortonian) Montesano Formation of Wash-ington, USA (Bigelow, 1994).

The basal taxon Desmatophoca evolved inthe eastern North Pacific region in the earlyMiocene and, based on current paleontolog-ical evidence, was endemic to that region.However, its sister taxon, Allodesmus, under-went an important diversification during themiddle Miocene that eventually involved dis-persal to the western North Pacific (Barnesand Hirota, 1995). In the eastern North Pa-cific, Allodesmus extended its range duringthe late middle Miocene southward into BajaCalifornia, Mexico (Barnes, 1998).

PHOCIDAE—SEALS: The monophyletic Pho-cidae (Wyss, 1988) includes seals with in-flated auditory bullae, no alisphenoid canal,and derived tarsal morphology (Muizon,1982). Phocids swim with hind limb propul-sion and have lost the ability to rotate the pesforward. Consequently, terrestrial locomo-tion for phocids does not involve quadrupe-dal ‘‘walking’’. Traditionally, extant phocidshave been divided into two to four major sub-groupings; monachines (monk seals), lobo-


dontines (Antarctic seals), cystophorines(hooded and elephant seals), and phocines(remaining Northern Hemisphere seals). Theconsensus phylogeny for extant phocids(Bininda-Emonds et al., 1999) supportsmonophyly of the Monachinae, Lobodontini,and Phocinae (figs. 3.3, 3.6). The Cysto-phorinae, however, is not recognized as amonophyletic group (King, 1966) and itsmembers are instead divided among Mona-chinae and Phocinae. The Monachinae cladeincludes Monachus (monk seals), Mirounga(elephant seals), and the Lobodontini (Ant-arctic seals). Bininda-Emonds et al. (1999)support the Monachus species group asmonophyletic (contrary to Wyss, 1988) andposition M. monachus as the basal taxon(figs. 3.3, 3.6). The next diverging monachi-ne lineage includes Mirounga and the Lo-bodontini clade, which in turn includes themonotypic taxa Leptonychotes, Ommatopho-ca, Lobodon, and Hydrurga (Wyss, 1988;Bininda-Emonds et al., 1999). The relation-ships of several fossil monachines to extanttaxa were discussed by Muizon (1982) butare poorly supported (as shown by alternatetopologies in figs. 3.3 and 3.6). The positionsof other fossil taxa are, as yet, unresolved.

The Phocinae clade includes sister taxaErignathus, Cystophora, and the tribe Pho-cini (Wyss, 1988). The Phocini (fig. 3.3) isfurther divided into Histriophoca 1 Pago-philus, Halichoerus, and Pusa 1 Phoca(Bininda-Emonds et al., 1999). The lattercrown clade includes two sister groups: (1)Pusa caspica 1 Pusa siberica 1 Pusa his-pida and (2) Phoca largha 1 Phoca vitulina.The relationships of fossil phocines to oneanother and to extant taxa are, as yet, unre-solved, as indicated by their position as a po-lytomy at the base of the phocine clade (figs.3.3, 3.6). The need for detailed taxonomicstudies and rigorous phylogenetic hypothesesof relationships for these fossil taxa is criticalto a better understanding of the historicalbiogeography of the phocine clade.

The OKR of phocids is currently estab-lished by Leptophoca lenis from the middleMiocene (Langhian) Calvert Formation ofMaryland, USA (western North Atlantic; 2a).As noted by Ray (1977), the Calvert phocidsare assigned to the Phocinae and Monachi-nae, suggesting that divergence of the two

clades occurred sometime prior to the middleMiocene. An older, poorly documented re-cord of phocids from the late Oligocene(Chattian) of the western North Atlantic(South Carolina) has been proposed by Ko-retsky and Sanders (1997) but awaits de-scription and scientific scrutiny. Overall, ourknowledge of fossil phocids is severely lim-ited due to a paucity of detailed morphologicdescriptions of taxa and rigorous analyses ofcharacter evolution and phylogenetic rela-tionships. The studies of Muizon (1981,1982) and Muizon and Hendey (1980) arenotable exceptions.

On the basis of phylogenetic data, it ismost parsimonious to assume that phocidsevolved in the North Pacific during the earlyMiocene (fig. 3.6), with divergence of thebasal phocoid taxa (i.e., phocids and des-matophocids) occurring sometime before 18Ma. Phylogenetic and stratigraphic data,however, also suggest that divergence of thephocine and monachine clades likely oc-curred in the North Atlantic sometime before16 Ma. These two closely timed events re-quire long-distance dispersal of some cur-rently unknown basal phocid from the NorthPacific into the North Atlantic, either north-ward through the Arctic Ocean or southwardthrough the Central American Seaway. Asdiscussed by Costa (1993) and Bininda-Emonds and Russell (1996), a southern routeis more likely because of the lack of an earlyMiocene marine corridor through Beringiaand the presence of a wide-open CentralAmerican Seaway at the same time. It is ad-mitted that the absence of basal phocid fos-sils in the North Pacific and Caribbean is aweakness of this hypothesis.

Combining the phylogenies of Bininda-Emonds et al. (1999) for extant monachinesand Muizon (1982) for fossil monachinessuggests that it is most parsimonious to as-sume that monachines evolved in the NorthAtlantic, possibly in the western part of thatocean basin based on the monachine OKR ascurrently established by Monotherium? wy-mani from the middle Miocene (Langhian)Calvert Formation (Ray, 1976). Other earlymonachine fossil taxa, including Monother-ium aberratum from the late Miocene (Tor-tonian) of the eastern and western North At-lantic (Ray, 1976), Pristiphoca vetusta from


Fig. 3.6. Area cladogram of phocids and summary of principal dispersal/vicariant events hypothe-sized to explain their distribution around the world. Rough outline of Paratethys shaded gray. (I) Earliestphocids in North Pacific with divergence of basal phocoids (phocids and desmatophocids) before 18Ma; (II) following dispersal of presently unknown basal phocid from the North Pacific into NorthAtlantic via a southern route (i.e., Central American Seaway) divergence of monachines and phocinesin the North Atlantic; earliest record of both clades 15 Ma; (III) dispersal of monachine relativessouthward into the eastern South Pacific and/or South Atlantic in the early middle Miocene; (IV)


←

diversification and dispersal of lobodontines southward into colder waters of the Southern Ocean; (V)dispersal of Monachus from the Mediterranean into the Caribbean and continuing on to the Hawaiianislands through the Central American seaway; (VI) dispersal of basal phocines from North Atlantic intothe Arctic; Erignathus extends range into the Bering Sea; (VII) diversification of phocines; (VIII) rangeexpansion of Histriophoca and Pagophilus clade followed by range fragmentation (i.e., glacial/intergla-cial oscillations) and allopatric speciation; Histriophoca enters the North Pacific and Pagophilus entersthe North Atlantic; Halichoerus disperses southward from the Arctic into the North Atlantic; (IX)vicariance during the Pleistocene (i.e., glacial/interglacial oscillations) resulting in speciation with Pusahispida entering the circum-Arctic, Pusa caspica entering the Caspian Sea, and Pusa siberica enteringLake Baikal; (X) vicariance and divergence of Phoca vitulina and Phoca largha in the North Pacific.

the middle Miocene (Badenian 5 Serravali-an) of Central Paratethys (Muizon, 1982),and Pontophoca sarmatica from the lateMiocene (Sarmatian 5 Tortonian) of EasternParatethys (Grigorescu, 1977), suggest atrans-Atlantic dispersal event in the middleand late Miocene that also involved invasionof the Paratethys. A separate invasion (per-haps post-Messinian) of the MediterraneanTethys is documented by the occurrence of‘‘Pristiphoca’’ occitana in the early Plioceneof southern France and Pliophoca etrusca inthe late Pliocene of northern Italy. Muizon(1982) proposed that these Pliocene Medi-terranean monachines were probably ances-tral to the Monachus species clade. Muizonfurther suggests that Monachus evolved inthe Mediterranean, expanded its range to in-clude Mauritania in the eastern North Atlan-tic, and then dispersed east to west across theequatorial Atlantic via the North EquatorialCurrent to reach the Caribbean. Presumably,allopatric speciation in the Caribbean result-ed in the evolution of the modern M. tropi-calis. There is no consensus on the timing ofthe subsequent dispersal of Monachus fromthe Caribbean through the Central AmericanSeaway and into the central North Pacific(M. schauinslandi), but it must have occurredprior to the mid-Pliocene closure of this cor-ridor. Repenning and Ray (1977) suggest thatthe divergence and dispersal of M. schauins-landi likely occurred prior to 14.5 Ma, whileMuizon (1982) suggested that these eventsoccurred just prior to 4.0–3.5 Ma. Regardlessof when the divergence and east-to-west dis-persal occurred, the fact that they did occuris supported by Monachus phylogeny, whichplaces the Mediterranean Monachus mona-

chus in a basal position to the M. tropicalis1 M. schauinslandi clade.

The monachine crown clade of Miroungaplus Lobodontini also has a North Atlantic or-igin based on the occurrence of early diverg-ing fossil members from the eastern and west-ern shores of this ocean basin (Ray, 1976).The OKR for this clade is established byMonotherium aberratum, M. affine, and M.delognei from the late Miocene (Tortonian)Diest Formation and St. Mary’s Formation ofBelgium and Maryland, respectively (Ray,1976), suggesting that the divergence of theMirounga and lobodontine clades occurredsometime before 11 Ma in the North Atlantic.Muizon (1982) proposes that species of Mon-otherium are allied with the lobodontine seals,and that this group evolved in the North At-lantic prior to its dispersal into the SouthernHemisphere. Whether this dispersal occurredsolely through the Central American Seawayand then south along the Pacific coast ofSouth America, or involved at least some pe-riod of migration south along the Atlanticcoast of Africa via the eastern boundary cur-rents is controversial. Clearly, there was atleast some dispersal of phocids through theCentral American Seaway, as is evidenced bythe well-studied monachine fauna from thelate Miocene (Messinian) to early Pliocene(Zanclian) Pisco Formation of Peru (Muizon,1981). The Pliocene fauna includes two well-documented monachine phocids, Acrophocalongirostris and Piscophoca pacifica. The for-mer taxon Muizon (1982) allies with the ex-tant Lobodon–Hydrurga clade. The OKR forsouthern lobodontines is established by un-described fossils from the late Miocene por-tion of the Pisco Formation (Muizon and de


Vries, 1985) suggesting that dispersal oc-curred sometime before 7 Ma. From Peru itis suggested that lobodontines radiated intohigh southern latitudes, initially along the Pa-cific shore of South America and thenthroughout the Southern Ocean via the WestWind Drift. This Southern Ocean diversifica-tion and dispersal included the evolution andestablishment of Homiphoca capensis knownfrom the early Pliocene (Zanclian) VarswaterFormation of South Africa (Muizon and Hen-dey, 1980). An alternative hypothesis holdsthat while one group of ancestral lobodontinesdispersed through the Central American Sea-way, a second group dispersed south along theeastern shore of Africa to become H. capen-sis. The reported, but undocumented occur-rence of Homiphoca in the early Pliocene(Zanclian) Yorktown Formation of North Car-olina (Ray, 1976) complicates the historicalbiogeography of this taxon, but may be ex-plained by a hypothesis involving a south-to-north return of Homiphoca via the Benguelaeastern boundary current, followed by aneast-to-west crossing of the Atlantic via theSouth Equatorial Current, and finally a south-to-north dispersal via the Florida or GulfStream western boundary current. This elab-orate ad hoc hypothesis underscores the crit-ical need for the collection and detailed studyof the fossil phocid faunas of the Atlantic,Mediterranean, and Paratethyan Neogene.

The historical biogeography of the Mir-ounga clade is similar to that of lobodontinesand also suffers from a limited and poorlydocumented fossil record. The OKR for thisclade is established by Callophoca obscurafrom the early Pliocene (Zanclian) YorktownFormation of North Carolina (western NorthAtlantic; 2a). The trans-Atlantic distributionof this taxon as discussed by Ray (1976) andMuizon (1982) suggests that C. obscura mayactually have evolved in the eastern NorthAtlantic sometime before 4.5 Ma and dis-persed east to west across the equatorial At-lantic via the North Equatorial Current, con-tinuing northward along the eastern shore ofNorth America. Some members of this dis-persing lineage could have passed throughthe Central American Seaway at this time(late Miocene or earliest Pliocene) to estab-lish the lineage in the eastern South Pacific.Subsequent speciation in the Pacific resulted

in the evolution of Mirounga (M. leonina)and its dispersal in the subantarctic SouthernOcean via the West Wind Drift. The occur-rence of Mirounga (M. angustirostris) in theeastern North Pacific suggests a transequa-torial dispersal event and allopatric specia-tion, possibly coincident with the Pleistocenedispersal and speciation events proposed forthe Arctocephalus townsendi lineage. Obvi-ous weaknesses of this hypothesis includethe lack of Callophoca or related fossil taxain the pinniped-rich Neogene deposits of thePisco Formation of Peru and the lack of fos-sils of Mirounga in rocks older than the latePleistocene.

Although both the monachine and phocineseals had their origin in the relatively warmNeogene North Atlantic, only monachineswere diverse enough to include one thermo-philic lineage that remained in relativelywarm Quaternary seas and at least two morethermophobic lineages that dispersed to thesouth and have their greatest modern diver-sity in the high latitudes of the SouthernHemisphere. In contrast, phocines remainedin the Northern Hemisphere, moving pro-gressively farther north as Pleistocene cli-mates cooled so that today they have theirgreatest diversity in arctic and subarctic re-gions. As discussed earlier, the phocine OKRis currently established by Leptophoca lenisfrom the middle Miocene (Langhian) CalvertFormation of Maryland, USA (western NorthAtlantic region; 2a). At about this same time,however, fossil seals are also present in theCentral Paratethys (Koretsky and Holec,2002) documenting a trans-Atlantic phocinedistribution and posing the question of awestern versus eastern North Atlantic originfor the group. Species closely related to thewestern North Atlantic phocines are recordedfrom the late middle Miocene (Serravallian)Berchem Formation of Belgium (easternNorth Atlantic; 2b), while the poorly docu-mented but coeval (early Sarmatian) ‘‘Pho-ca’’ vindobonensis shows continued occu-pation of the Central Paratethys (Grigorescu,1977). The latest well-documented Parateth-yan phocines, Cryptophoca maeotica and‘‘Pusa’’ pontica, are known from the lateMiocene (middle Sarmatian 5 Tortonian) ofthe Eastern Paratethys and coincide with themajor period of isolation and hyposalinity


proposed for this inland waterway (Koretskyand Ray, 1994). As described by Repenninget al. (1979), the Pliocene phocine record ismore diverse than that for the late Mioceneand consists of a somewhat cosmopolitantrans-Atlantic fauna (table 3.2) known fromthe Yorktown and Upper Bone Valley for-mations of the western North Atlantic (2a)and the Kattendijk and Lillo formations ofthe eastern North Atlantic (2b). These au-thors suggest that an apparent asymmetry inPliocene phocid dominance documented inthese two regions (i.e., monachine domi-nance in the western North Atlantic and pho-cine dominance in the eastern North Atlan-tic) may be an initial indication of the mod-ern temperature preferences of these twogroups, with the center of monachine diver-sification in the warmer western North At-lantic and the center of phocine diversifica-tion in the cooler eastern North Atlantic. Thetiming of this apparent temperature dichoto-my is synchronous with closure of the Cen-tral American Seaway and the proposednorthward deflection of the North EquatorialCurrent and strengthening of the warm GulfStream. Unfortunately, rigorous phylogeneticanalyses of Miocene and Pliocene fossil pho-cines are lacking, making it impossible to re-liably employ evolutionary relationships inevaluating historical biogeographic patternsat this time. Phylogenies for extant phocines,however, are available (Bininda-Emonds etal., 1999), and provide a means for constrain-ing discussion of higher phocine biogeogra-phy.

It has been proposed that the biogeographyof modern phocine seals is the product oftwo rather recent dispersal events, one in-volving dispersal of the Pusa–Phoca lineageinto the Arctic from a Paratethyan refugiumand another involving the northward dispers-al of the remaining phocine lineages into theArctic from a North Atlantic center of originfollowed by glacioeustatic-forced diversifi-cation in the Arctic and invasion of the NorthPacific (see Repenning et al., 1979). Thisdual dispersal hypothesis is countered by asingle dispersal hypothesis that involves thedispersal of phocines into the Arctic from aNorth Atlantic center of origin followed byglacioeustatic-forced diversification in theArctic and radial invasion of the North Pa-

cific, Caspian Sea, and Lake Baikal (see Da-vies, 1958b). Unfortunately, the fossil recordof higher phocine seals is limited to Pleis-tocene occurrences and does not provide anyuseful evidence for testing these competinghypotheses except to show that phocids en-tered the North Pacific sometime before theearly Pleistocene (Barnes and Mitchell,1975). The phylogeny, however, does pro-vide a means for testing these biogeographichypotheses (e.g., see Hoberg and Adams,1992) and lends strong support to the singledispersal hypothesis. Basal taxa such as Er-ignathus and Cystophora appear to haveoriginated in the Arctic during the Pleisto-cene, with Erignathus extending its rangeinto the Bering Sea (5c) and North Atlantic(2c) and Cystophora dispersing into theNorth Atlantic (2c,d). This pattern could alsobe explained by a North Atlantic origin fol-lowed by dispersal and diversification in theArctic and, for Erignathus, invasion of theBering Sea. Histriophoca fasciata and its sis-ter taxon Pagophilus groenlandica apparent-ly represent the products of glacioeustatic-forced allopatric speciation during the Pleis-tocene with the evolution of Histriophoca inthe North Pacific (5a,c) and Pagophilus inthe North Atlantic (2c,d). Halichoerus rep-resents southward dispersal from the Arctic(1c) into the temperate North Atlantic (2a,b).Phylogenetic and biogeographic evidencesuggests that divergence of the Pusa–Phocalineage from Halichoerus likely occurred inthe Greenland Sea/Barents Sea portion of theArctic, as did the subsequent divergence ofPusa from Phoca. Following divergence, thesubarctic Phoca vitulina expanded its rangeduring Pleistocene interglacial periods to in-clude the circum-arctic shoreline and theNorth Pacific. Succeeding glacial/interglacialoscillations resulted in reduced gene flow be-tween isolated populations and the formationof distinct subspecies. The closely relatedPhoca largha may be the descendant of oneof the earliest Phoca populations to be iso-lated by glacial conditions. A similar hypoth-esis is proposed for evolution of the Pusalineage. Following initial divergence, theringed seal Pusa hispida expanded its rangeto include nearly the entire Arctic region andthe subarctic portions of the Atlantic and Pa-cific. The Caspian seal Pusa caspica, found


Fig. 3.7. Paleogeographic maps showing the distribution of pinnipedimorphs during time intervals:A, late Oligocene; B, early Miocene; C, middle Miocene; D, late Miocene; E, Pliocene; F, Pleistocene(see table 3.4; base maps from Smith et al., 1994).

only in the inland Caspian Sea far from otherphocids, may represent the product of allo-patric speciation in a peripheral population ofPusa hispida. As briefly discussed already,Davies (1958a) has suggested that Pusa en-tered the Paratethys from the Arctic duringthe Pleistocene by way of an ancestral VolgaRiver drainage and became trapped in thatdrainage. Another landlocked species, Pusasibirica, found only in Lake Baikal of easternSiberia, is proposed to have had a similarhistory. In support of this view, Davies(1958a) noted the modern occurrence of iso-lated populations of Pusa hispida in fresh-water lakes in Russia (Lake Ladoga) and Fin-land (Lake Simaa). The alternative hypothe-sis for the evolution of the Pusa lineage fromMiocene Paratethyan phocines such as‘‘Pusa’’ pontica proposes that the occurrenceof Pusa and Phoca in the Arctic basin is theresult of a south-to-north dispersal eventfrom this landlocked refugium (McLaren,

1960; Grigorescu, 1977; and Repenning etal., 1979). There are several problems withthis hypothesis, not the least of which is thepresumed thermophilic nature of MioceneParatethyan seals and the observed pago-philic nature of modern Pusa caspica and itssister taxa P. sibirica and P. hispida. Anotherweakness is the current lack of convincinganatomical evidence for the close phyloge-netic relationship of Paratethyan fossil sealsand modern Pusa.

DIVERSITY THROUGH TIME

In this final section we summarize pinni-pedimorph diversity through time using pa-leogeographic maps (fig. 3.7) and a chron-ostratigraphic framework (table 3.2). Theearliest pinnipedimorphs (Enaliarctos spp.)are known from latest Oligocene (Chattian)age sublittoral marine deposits in the easternNorth Pacific region of Oregon and Wash-


ington (Berta, 1991; fig. 3.7A, table 3.2).Globally, during the Oligocene, northwardmovement of Australia and India resulted ininitial opening of the Southern Ocean, estab-lishment of a Circumantarctic Current, anddevelopment of steepening thermal gradientsbetween subantarctic waters and those attemperate latitudes (Kennett, 1980). Progres-sive intensification of this current systemproduced a decoupling of the warm subtrop-ical oceanic gyres from the cooling SouthernOcean circulation, and this led to initiationof glaciation in Antarctica. With establish-ment of the Circumantarctic Current (WestWind Drift), phytoplankton productivity in-creased, especially in the New Zealand–Campbell Plateau region of the SouthernOcean. This increased productivity has beencorrelated with the initial evolution of mys-ticete cetaceans in the early Oligocene of thesouthern hemisphere (Fordyce, 1977). Theimpact of this reorganization of ocean cir-culation on the late Oligocene evolution ofpinnipeds in the Northern Hemisphere, al-though not precisely known, is likely to havebeen one of increased coastal food resources.In addition, a major global sea level rise inthe late Oligocene (Haq et al., 1987) is likelyto have greatly increased the area of neritichabitats in continental shelf regions.

During the early Miocene, pinnipedi-morphs remained restricted to the easternNorth Pacific, where they began to diversify(fig. 3.7B, table 3.2). This diversification in-cluded additional species of Enaliarctos, thebasal pinnipedimorph Pinnarctidion, and theearliest desmatophocid (Desmatophoca). Theearliest odobenids (Proneotherium) mayhave evolved in the late early Miocene (Bur-digalian), but are clearly present by the earlymiddle Miocene (Langhian). Basal phocidsare likely to have diverged at this time anddispersed into the Atlantic via the wide openCentral American Seaway. During the earlyMiocene, the Drake Passage opened, result-ing in a stronger Circumantarctic Current andestablishment of the general oceanic watermasses of modern oceans (Wise et al., 1985).Oceanic productivity remained greatest in theequatorial regions in spite of the breakup ofthe Tethys Sea by collision of India and Af-rica with Eurasia and closure of the connec-tion between the eastern Mediterranean and

the Indian Ocean. Paratethys began to be iso-lated from the Mediterranean Sea at thistime, beginning its history as a series of sep-arate hyposaline basins.

The earliest confirmed phocids are knownfrom the early middle Miocene (Langhian) ofthe western North Atlantic and had estab-lished a circum-North Atlantic distributionby the late middle Miocene (Serravalian) thatalso included the Mediterranean Sea and Par-atethys (fig. 3.7C, table 3.2). In the NorthPacific, species of the basal pinniped taxaPteronarctos and Pacificotaria were com-mon in the early middle Miocene. Desma-tophoca persisted during this period, whileits sister taxon, Allodesmus, underwent animpressive radiation in the late middle Mio-cene with at least three species known fromCalifornia and one from Japan (Repenning,1976; Repenning et al., 1979; Barnes andHirota, 1995). Species of the early divergingodobenid Prototaria are recorded from thewestern North Pacific in the early middleMiocene and probably overlapped temporal-ly with Proneotherium (sister taxon) in theeastern North Pacific. The later divergingodobenid Neotherium is found in the easternNorth Pacific in the late middle Miocene ofCalifornia and Baja California. The basalpinnipedimorphs of the early Miocene, En-aliarctos and Pinnarctidion, were extinct bylate middle Miocene time.

During the early middle Miocene, forma-tion of a permanent ice cap in East Antarc-tica resulted in a sharp decline in bottom wa-ter temperatures, steepened latitudinal ther-mal gradients, and increased levels of up-welling and plankton productivity alongcontinental margins (Woodruff and Savin,1989). Ongoing tectonic activity and uplift ofthe Panama Sill resulted in intensified gyralcirculation of surface waters, especially inthe North Pacific. The Central American Sea-way remained a dispersal corridor; howeverit was becoming more of a filter. Coastal up-welling intensified off the Peruvian coast atthis time (Ibaraki, 1992), establishing themodern pattern of low latitude cold water inthe northeastern South Pacific.

During the late Miocene, phocid andodobenid pinnipeds underwent a dramatic di-versification (fig. 3.7D, table 3.2). Monachi-ne phocids became common in the western


and eastern North Atlantic, while phocinesradiated in Paratethys prior to the MessinianSalinity Crisis. Lobodontine monachines dis-persed from the western North Atlanticthrough the Central American Seaway andsouth into the eastern South Pacific. There iscurrently no evidence that this dispersal ex-tended into the North Pacific.

In the North Pacific, species of the gen-eralized odobenid Imagotaria are found inboth the western and eastern North Pacific.A major radiation produced two crown odob-enid clades, the dusignathines and the odob-enines, with at least two genera each (Dusig-nathus and Gomphotaria, Aivukus and Plio-pedia, respectively). These taxa apparentlyremained in the eastern North Pacific. Otariidfur seals first appeared in the early late Mio-cene of the eastern North Pacific (Pithano-taria) and dispersed to the western North Pa-cific by the late late Miocene (Thalassoleon).

An ice cap formed in West Antarctica dur-ing the late Miocene and intensified a declinein bottom water temperatures and a steep-ening of latitudinal thermal gradients. TheSouthern Ocean experienced a major coolingevent and worldwide sea levels were low(Warheit, 1992). Late Miocene collision ofAfrica with the Iberian Peninsula, closure ofthe western Mediterranean portal, and isola-tion of the Paratethys as a brackish inland seamark the time of the Messinian Salinity Cri-sis. The flow of warm water from the Med-iterranean to the North Atlantic was shut off,which intensified the cooling of the NorthAtlantic. A steeper thermal gradient in theupper part of the water column caused anincrease in the speed of the anticyclonicNorth Atlantic Gyre. Steeper latitudinal andwater column thermal gradients were proba-bly associated with increased levels of up-welling, evidence for which is supplied bythe rich Miocene and Pliocene phosphate de-posits of eastern North America (Whitmore,1994). Increased upwelling is in turn corre-lated with high phytoplankton productivityand higher productivity with cetacean diver-sification. A similar chain of events is pos-tulated for the North Pacific except that herethe increased productivity is reflected in theextensive siliceous (diatoms) deposits of Cal-ifornia.

During the Pliocene, odobenines had a cir-

cum-North Pacific distribution that includedValenictus in the eastern North Pacific andProtodobenus and Alachtherium in the west-ern North Pacific. Dispersal of this clade intothe North Atlantic via the Arctic Ocean wascomplete at this time, having begun duringthe preceding late Miocene. The last of thedusignathine walruses is known from the latePliocene of the eastern North Pacific. Plio-cene otariids remained confined to the NorthPacific and included Thalassoleon (westernNorth Pacific) and Callorhinus (easternNorth Pacific).

The center of monachine diversity hadshifted to the western North Atlantic by theearly Pliocene, while phocines became thedominant phocids in the eastern North Atlan-tic. The majority of these phocines appear torepresent circum-North Atlantic endemics(Ray, 1976). In the Mediterranean, vicariantevents in place during the Messinian SalinityCrisis drove speciation of the resident mon-achine lineages, which remained in completeisolation until at least the late Pliocene. Thefirst monachine seals to reach the easternSouth Atlantic (early Pliocene, South Africa)dispersed either directly from the North At-lantic or indirectly by way of the CentralAmerican Seaway and the eastern South Pa-cific. In the eastern South Pacific, lobodon-tine seals that had probably arrived duringthe late Miocene speciated to produce severalgenera and endemic species.

During the Pliocene between 3.7 and 3.1Ma, the Central American Seaway finallyclosed with elevation of the Isthmus of Pan-ama (Duque-Caro, 1990). Prior to this time,water flow through the seaway had been re-duced substantially (Benson et al., 1991) andthe North Atlantic Gyre had probably be-come well developed. With the seawayclosed the Gulf Stream was strengthened andsurface water temperatures in the westernNorth Atlantic increased (Dowsett andPoore, 1991). In the North Pacific, closure ofthe seaway had a similar effect on strength-ening of the anticyclonic circulation gyre, al-though in the North Pacific water tempera-tures decreased. Onset of Northern Hemi-sphere glaciation began 2.5 Ma (Barron andBaldauf, 1989) and was followed by a periodof rapid global sea level fluctuations. Thesefluctuations extended through the Pleistocene


and were associated with intensified tectonicactivity, polar glaciation, and decreasing wa-ter temperatures. This established the strongnorth–south thermal gradient of modernoceans. The late Pliocene and Pleistocene cli-matic deterioration was effective in creatingbarriers to marine mammal dispersal. Highlatitude taxa could not disperse across theequatorial region during interglacial periodsand this resulted in the development of an-titropical distributions for certain cetaceansand pinnipeds (Gaskin, 1982; fig. 3.7E, table3.2).

During the early Pleistocene, fossil speciesof odobenine walruses occurred in the east-ern North Atlantic and probably also in thewestern North Atlantic (fig. 3.7F, table 3.2).Their distribution was more southern thanlate Pleistocene occurrences of Odobenusrosmarus from the North Atlantic and NorthPacific and probably reflected glacial maxi-ma. Pleistocene otariids from the North andSouth Pacific are all assigned to modern gen-era, although their diversity is lower than forthe modern fur seal and sea lion fauna. Re-mains of phocid seals are reported for thefirst time from the eastern North Pacific inthe early Pleistocene and are referred to themodern taxon, Phoca vitulina. This dispersalevent is postulated to have occurred via theArctic Ocean (Repenning et al., 1979). NorthPacific phocid diversity increased in the latePleistocene to include additional phocinesand a monachine seal. In the North Atlantic,phocine diversity began to approach its mod-ern level, especially in late Pleistocene LakeChamplain of eastern Canada (Ray et al.,1982; Harington, 1988).

The Pleistocene continued warm and coldclimatic fluctuations that began in the Plio-cene. The global refrigeration of glacial pe-riods established effective barriers to the dis-persal of ‘‘relict Tethyan’’ marine mammaltaxa such as Monachus (Gaskin, 1982).These taxa became confined to the tropics.Phocine seals at this time apparently adaptedto the colder northern waters and began theirimpressive high-latitude adaptive radiation.This radiation was probably driven by the‘‘speciation pump’’ of glacioeustatic oscilla-tions and the related increase and decrease ofsea ice volume and the cyclic expansion andfragmentation of species home ranges and re-

duced gene flow (Davies, 1958b). This pho-cine radiation is still very much underway(Ray, 1976). In the Southern Hemisphere,monachines diversified (in the absence ofphocines) in the colder waters to produce theAntarctic lobodontine seal fauna of today.The antitropical distribution of species ofArctocephalus and Mirounga may be the re-sult of Pleistocene fragmentation of formerlymore cosmopolitan species ranges.

ACKNOWLEDGMENTS

Long term study of fossil pinnipeds bytwo of us (AB and TAD) has been supportedby various grants from the National ScienceFoundation including BSR 8607061, BSR9006535, DEB 9419802, and INT 9417088to AB and DEB 9419803 to TAD. AB wasfirst introduced to fossil carnivorans by Rich-ard H. Tedford and is pleased to offer hercontribution to this volume in his honor.

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Pinnipedimorph Evolutionary Biogeography

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