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FLASHING A MOUTHFUL of 15-centimeter-long teeth like serrated knives, Tyran-nosaurus rex ripped flesh from bone—and not just at mealtimes. In between entrées, the brute very likely battled its fellow tyrannosaurs over territory andmates. Ichthyosaurs, those fish-shaped lizards with sinuous bodies that mea-sured twice the height of a human, chased lesser monsters through the openoceans. Some of the birds could have given Alfred Hitchcock fresh nightmares:two-meter-tall phorusrhacoids sprinted at 70 kilometers an hour and snappedtheir massive beaks on victims, beat them senseless against the ground and then

swallowed them whole.Yes, from a purely self-interested

standpoint, it’s good that the dinosaursand their ancient ilk are dead. Yet theylive on in our imaginations and our intel-lectual pursuits, where they retain thepower to puzzle, fascinate and startle us.How did they hunt (and what huntedthem)? Were they orphans from birth,surviving on instinct and appetite alone,or did parents nurture them? Over mil-lennia, how did their species evolve?

Studying the mineralized remains ofprehistoric beasts from the comfortabledistance of a few eons, scientists havelearned a great deal about how these awe-some creatures stalked and swam through

the long-ago world. For instance, a mother lode of fossils—many of them near-ly complete skeletons—baking in the Gobi Desert is giving paleontologists abroader and more vivid picture of Central Asia between 100 million and 75million years ago. From what is now Australia, specimens with huge eyes andother adaptations reveal how dinosaurs endured months of darkness duringfrigid polar winters 100 million years ago. Rocks in Madagascar are divulgingpreviously unknown assemblages of animals that foraged together 230 millionyears ago. Teeming bodies resembling pill bugs plucked from Canada’s 600-million-year-old Burgess Shale illustrate the quirky, punctuated nature of evo-lutionary change.

This special edition from Scientific American presents articles about thoseand other discoveries in the field of paleontology, written by the experts whoare leading the investigations. We invite you, in the pages that follow, to takean armchair safari into prehistory, to spend some quality time with the terrorsof Earth’s distant past.

Fierce CreaturesE D I T O R I N C H I E F : John Rennie E X E C U T I V E E D I T O R : Mariette DiChristina I S S U E E D I T O R : Mark Fischetti

E D I T O R I A L D I R E C T O R , O N L I N E : Kate Wong A S S O C I A T E E D I T O R , O N L I N E : Sarah Graham

A R T D I R E C T O R : Edward Bell I S S U E D E S I G N E R : Jessie Nathans P H O T O G R A P H Y E D I T O R : Bridget Gerety P R O D U C T I O N E D I T O R : Richard Hunt

C O P Y D I R E C T O R : Maria-Christina Keller C O P Y C H I E F : Molly K. Frances C O P Y A N D R E S E A R C H : Daniel C. Schlenoff, Rina Bander, Michael Battaglia, Emily Harrison, David Labrador

E D I T O R I A L A D M I N I S T R A T O R : Jacob Lasky S E N I O R S E C R E T A R Y : Maya Harty

A S S O C I A T E P U B L I S H E R , P R O D U C T I O N : William Sherman M A N U F A C T U R I N G M A N A G E R : Janet Cermak A D V E R T I S I N G P R O D U C T I O N M A N A G E R : Carl Cherebin P R E P R E S S A N D Q U A L I T Y M A N A G E R : Silvia Di Placido P R I N T P R O D U C T I O N M A N A G E R : Georgina Franco P R O D U C T I O N M A N A G E R : Christina Hippeli C U S T O M P U B L I S H I N G M A N A G E R : Madelyn Keyes-Milch

A S S O C I A T E P U B L I S H E R / V I C E P R E S I D E N T , C I R C U L A T I O N :Lorraine Leib Terlecki C I R C U L A T I O N D I R E C T O R : Katherine Corvino F U L F I L L M E N T A N D D I S T R I B U T I O N M A N A G E R : Rosa Davis

V I C E P R E S I D E N T A N D P U B L I S H E R : Bruce Brandfon W E S T E R N S A L E S M A N A G E R : Debra Silver S A L E S D E V E L O P M E N T M A N A G E R : David Tirpack W E S T E R N S A L E S D E V E L O P M E N T M A N A G E R : Valerie Bantner S A L E S R E P R E S E N T A T I V E S : Stephen Dudley, Hunter Millington, Stan Schmidt

A S S O C I A T E P U B L I S H E R , S T R A T E G I C P L A N N I N G : Laura Salant P R O M O T I O N M A N A G E R : Diane Schube R E S E A R C H M A N A G E R : Aida Dadurian P R O M O T I O N D E S I G N M A N A G E R : Nancy Mongelli G E N E R A L M A N A G E R : Michael Florek B U S I N E S S M A N A G E R : Marie Maher M A N A G E R , A D V E R T I S I N G A C C O U N T I N G A N D C O O R D I N A T I O N : Constance Holmes

D I R E C T O R , S P E C I A L P R O J E C T S : Barth David Schwartz

M A N A G I N G D I R E C T O R , O N L I N E : Mina C. Lux S A L E S R E P R E S E N T A T I V E , O N L I N E : Gary BronsonW E B D E S I G N M A N A G E R : Ryan Reid

D I R E C T O R , A N C I L L A R Y P R O D U C T S : Diane McGarvey P E R M I S S I O N S M A N A G E R : Linda Hertz M A N A G E R O F C U S T O M P U B L I S H I N G : Jeremy A. Abbate

C H A I R M A N E M E R I T U S : John J. Hanley C H A I R M A N : Rolf Grisebach P R E S I D E N T A N D C H I E F E X E C U T I V E O F F I C E R : Gretchen G. Teichgraeber V I C E P R E S I D E N T A N D M A N A G I N G D I R E C T O R , I N T E R N A T I O N A L : Dean Sanderson V I C E P R E S I D E N T : Frances Newburg AL

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THE GOOD OLD DAYS? Allosaurus and itskin are best appreciated in retrospect.

w w w . s c i a m . c o m D I N O S A U R S A N D O T H E R M O N S T E R S 1

Dinosaurs and Other Monsters is published by the staff of Scientific American, with project management by:

John RennieEditor in Chief

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SCIENTIFIC AMERICAN Digital

DINOSAURSand other Monsters

contentsVolume 14, Number 22004

1 Letter from the Editor

2 S C I E N T I F I C A M E R I C A N D I N O S A U R S A N D O T H E R M O N S T E R S

22 Breathing Life intoTyrannosaurus rexBY GREGORY M. ERICKSON

By analyzing previously overlooked fossils andby taking a second look at some old finds,paleontologists are providing the first glimpsesof the actual behavior of the tyrannosaurs.

30 Madagascar’s Mesozoic SecretsBY JOHN J. FLYNN AND ANDRÉ R. WYSS

The world’s fourth-largest island divulgesfossils that could revolutionize scientific viewson the origins of dinosaurs and mammals.

40 Dinosaurs of the AntarcticBY PATRICIA VICKERS-RICH AND THOMAS HEWITT RICH

Their excellent night vision and apparent warm blood raise a question: Could they havesurvived icehouse conditions at the end of the Cretaceous period?

48 Killer Kangaroos and Other Murderous MarsupialsBY STEPHEN WROE

Australian mammals were not all as cute as koalas. Some were as ferocious as they were bizarre.

4 Rulers of the Jurassic SeasBY RYOSUKE MOTANI

Fish-shaped reptiles called ichthyosaursreigned over the oceans for as long asdinosaurs roamed the land, but only recentlyhave paleontologists discovered why these creatures were so successful.

12 The Mammals ThatConquered the SeasBY KATE WONG

New fossils and DNA analyses elucidate theremarkable evolutionary history of whales.

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Scientific American Special (ISSN 1048-0943), Volume 14, Number 2, 2004, published by Scientific American, Inc.,415 Madison Avenue, New York, NY 10017-1111. Copyright © 2004 by Scientific American, Inc. All rights reserved. Nopart of this issue may be reproduced by any mechanical, photographic or electronic process, or in the form of aphonographic recording, nor may it be stored in a retrieval system, transmitted or otherwise copied for public or privateuse without written permission of the publisher. Canadian BN No. 127387652RT; QST No. Q1015332537. To purchaseadditional quantities: U.S., $10.95 each; elsewhere, $13.95 each. Send payment to ScientificAmerican, Dept. SAPB, 415 Madison Avenue, New York, NY 10017-1111. Inquiries: fax 212-355-0408or telephone 212-451-8890. Printed in U.S.A.

92 The Evolution of Life on EarthBY STEPHEN JAY GOULD

The history of life is not necessarilyprogressive; it is certainly not predictable. The earth’s creatures have evolved through a series of contingent and fortuitous events.

Cover illustration by Alfred T. Kamajian.

56 Fossils of the Flaming CliffsBY MICHAEL J. NOVACEK, MARK NORELL,MALCOLM C. MCKENNA AND JAMES CLARK

Mongolia’s Gobi Desert contains one of therichest assemblages of dinosaur remains everfound. Paleontologists are uncovering much ofthe region’s history.

64 Captured in AmberBY DAVID A. GRIMALDI

The exquisitely preserved tissues of insects in amber reveal some geneticsecrets of evolution.

72 Which Came First, theFeather or the Bird?BY RICHARD O. PRUM AND ALAN H. BRUSH

Feathers originated and diversified indinosaurs, before birds or flight evolved.

82 The Terror Birds of South AmericaBY LARRY G. MARSHALL

These huge, swift creatures were the dominant carnivores of the continent for millions of years, until competitors drove them into extinction.

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Fish-shaped reptiles called ichthyosaurs reigned over the oceans for as long

as dinosaurs roamed the land, but only recently have paleontologists

discovered why these creatures were so successful

icture a late autumn evening some 160 million years ago, during the Jurassic time period, when dinosaurs inhabited the continents. The setting sun hardly penetrates the shimmering surface of a vast

blue-green ocean, where a shadow glides silently among the dark crags of asubmerged volcanic ridge. When the animal comes up for a gulp of evening air, it calls to mind a small whale—but it cannot be. The first whale will not evolve foranother 100 million years. The shadow turns suddenly and now stretches more thantwice the height of a human being. That realization becomes particularly chillingwhen its long, tooth-filled snout tears through a school of squidlike creatures.

The remarkable animal is Ophthalmosaurus, one of more than 80 species now known to have constituted a group of sea monsters called the ichthyosaurs,

Rulers of the Jurassic Seas

By Ryosuke Motani

P


ICHTHYOSAURS patrolled the world’s oceans for 155 million years.


or fish-lizards. The smallest of these an-imals was no longer than a human arm;the largest exceeded 15 meters. Oph-thalmosaurus fell into the medium-sizegroup and was by no means the most ag-gressive of the lot. Its company wouldhave been considerably more pleasantthan that of a ferocious Temnodonto-saurus, or “cutting-tooth lizard,” whichsometimes dined on large vertebrates.

When paleontologists uncovered thefirst ichthyosaur fossils in the early1800s, visions of these long-vanishedbeasts left them awestruck. Dinosaurshad not yet been discovered, so everyunusual feature of ichthyosaurs seemedintriguing and mysterious. Examina-tions of the fossils revealed that ichthyo-saurs evolved not from fish but fromland-dwelling animals, which them-selves had descended from an ancientfish. How, then, did ichthyosaurs makethe transition back to life in the water?To which other animals were they most

related? And why did they evolve bizarrecharacteristics, such as backbones thatlooked like a stack of hockey pucks andeyes as big around as bowling balls?

Despite these compelling questions,the opportunity to unravel the enigmat-ic transformation from landlubbing rep-tiles to denizens of the open sea wouldhave to wait almost two centuries. Whendinosaurs such as Iguanodan grabbedthe attention of paleontologists in the1830s, the novelty of the fish-lizards fad-ed away. Intense interest in the rulers ofthe Jurassic seas resurfaced only a fewyears ago, thanks to newly available fos-sils from Japan and China. Since then,fresh insights have come quickly.

Murky Origins ALTHOUGH MOST PEOPLE forgotabout ichthyosaurs in the early 1800s, afew paleontologists did continue to thinkabout them throughout the 19th centuryand beyond. What has been evident since

their discovery is that the ichthyosaurs’adaptations for life in water made themquite successful. The widespread ages ofthe fossils revealed that these beasts ruledthe ocean from about 245 million untilabout 90 million years ago—roughly theentire era that dinosaurs dominated thecontinents. Ichthyosaur fossils werefound all over the world, a sign that theymigrated extensively, just as whales dotoday. And despite their fishy appear-ance, ichthyosaurs were obviously air-breathing reptiles. They did not havegills, and the configurations of their skulland jawbones were undeniably reptilian.What is more, they had two pairs oflimbs (fish have none), which impliedthat their ancestors once lived on land.

Paleontologists drew these conclu-sions based solely on the exquisite skele-tons of relatively late, fish-shaped ich-thyosaurs. Bone fragments of the firstichthyosaurs were not found until 1927.Somewhere along the line, those early an-

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FACT: The smallest ichthyosaur was no longer than a human arm;

ORIGINS OF ICHTHYOSAURS baffled paleontologists for nearly two centuries. At timesthought to be closely related to everything from fish to salamanders to mammals,ichthyosaurs are now known to belong to the group called diapsids. New analysesindicate that they branched off from other diapsids at about the time lepidosaurs andarchosaurs diverged from each other—but no one yet knows whether ichthyosaursappeared shortly before that divergence or shortly after.

Sharks and rays

Ray-finnedfishes Amphibians Mammals Snakes Lizards Tuatara

ANCESTRALVERTEBRATE

Crocodiles Birds

DIAPSIDS

LEPIDOSAURS ARCHOSAURS

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imals went on to acquire a decidedly fishybody: stocky legs morphed into flippers,and a boneless tail fluke and dorsal finappeared. Not only were the advanced,fish-shaped ichthyosaurs made for aquat-ic life, they were made for life in the openocean, far from shore. These extremeadaptations to living in water meant thatmost of them had lost key features—suchas particular wrist bones and ankle-bones—that would have made it possibleto recognize their distant cousins onland. Without complete skeletons of thevery first ichthyosaurs, paleontologistscould merely speculate that they musthave looked like lizards with flippers.

The early lack of evidence so con-fused scientists that they proposed al-most every major vertebrate group—notonly reptiles such as lizards and croco-diles but also amphibians and mam-mals—as close relatives of ichthyosaurs.As the 20th century progressed, scientistslearned better how to decipher the rela-tionships among various animal species.On applying the new skills, paleontolo-gists started to agree that ichthyosaurswere indeed reptiles of the group Diap-sida, which includes snakes, lizards, croc-odiles and dinosaurs. But exactly when

ichthyosaurs branched off the family treeremained uncertain—until paleontolo-gists in Asia unearthed new fossils of theworld’s oldest ichthyosaurs.

The first big discovery occurred onthe northeastern coast of Honshu, themain island of Japan. The beach is dom-inated by outcrops of slate, the layeredblack rock that is often used for the ex-pensive ink plates of Japanese calligra-phy and that also harbors bones of theoldest ichthyosaur, Utatsusaurus. MostUtatsusaurus specimens turn up frag-mented and incomplete, but a group ofgeologists from Hokkaido Universityexcavated two nearly complete skele-tons in 1982. These specimens eventual-ly became available for scientific study,thanks to the devotion of Nachio Mi-noura and his colleagues, who spentmuch of the next 15 years painstakinglycleaning the slate-encrusted bones. Be-cause the bones are so fragile, they hadto chip away the rock carefully with finecarbide needles as they peered through amicroscope.

As the preparation neared its end in1995, Minoura, who knew of my inter-est in ancient reptiles, invited me to jointhe research team. When I saw the skele-

ton for the first time, I knew that Utatsu-saurus was exactly what paleontologistshad been expecting to find for years: anichthyosaur that looked like a lizard withflippers. Later that same year my col-league You Hailu, then at the Institutefor Vertebrate Paleontology and Paleo-anthropology in Beijing, showed me asecond, newly discovered fossil—theworld’s most complete skeleton of Chao-husaurus, another early ichthyosaur.Chaohusaurus occurs in rocks the sameage as those harboring remains of Utat-susaurus, and it, too, had been found be-fore only in bits and pieces. The newspecimen clearly revealed the outline of aslender, lizardlike body.

Utatsusaurus and Chaohusaurus il-luminated at long last where ichthyo-saurs belonged on the vertebrate familytree, because they still retained some keyfeatures of their land-dwelling ancestors.Given the configurations of the skull andlimbs, my colleagues and I think thatichthyosaurs branched off from the restof the diapsids near the separation oftwo major groups of living reptiles,lepidosaurs (such as snakes and lizards)and archosaurs (such as crocodiles andbirds). Advancing the family-tree debate


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NEW FOSSILS of the first ichthyosaurs,including Chaohusaurus (right), have begun to illuminate how theselizard-shaped creatures evolved into masters of the open ocean, such asStenopterygius, shown below with a baby exiting the birth canal.


was quite an achievement, but the mys-tery of the ichthyosaurs’ evolution re-mained unsolved.

From Feet to FlippersPERHAPS THE MOST exciting out-come of the discovery of these two Asianichthyosaurs is that scientists can nowpaint a vivid picture of the elaborateadaptations that allowed their descen-dants to thrive in the open ocean. Themost obvious transformation for aquat-ic life is the one from feet to flippers. Incontrast to the slender bones in the frontfeet of most reptiles, all bones in the front“feet” of the fish-shaped ichthyosaurs arewider than they are long. What is more,they are all a similar shape. In most oth-er four-limbed creatures it is easy to dis-tinguish bones in the wrist (irregularlyrounded) from those in the palm (longand cylindrical). Most important, the

bones of fish-shaped ichthyosaurs areclosely packed—without skin in be-tween—to form a solid panel. Having allthe toes enclosed in a single envelope ofsoft tissues would have enhanced therigidity of the flippers, as it does in livingwhales, dolphins, seals and sea turtles.Such soft tissues also improve the hydro-dynamic efficiency of the flippers becausethey are streamlined in cross section—ashape impossible to maintain if the digitsare separated.

But examination of fossils rangingfrom lizard- to fish-shaped—especially

those of intermediate forms—revealedthat the evolution from fins to feet wasnot a simple modification of the foot’sfive digits. Indeed, analyses of ichthyo-saur limbs reveal a complex evolution-ary process in which digits were lost,added and divided. Plotting the shape offin skeletons along the family tree of ich-thyosaurs, for example, indicates thatfish-shaped ichthyosaurs lost the thumbbones present in the earliest ichthyo-saurs. Additional evidence comes fromstudying the order in which digits be-came bony, or ossified, during the growthof the fish-shaped ichthyosaur Stenop-terygius, for which we have specimensrepresenting various growth stages. Lat-er, additional fingers appeared on bothsides of the preexisting ones, and someof them occupied the position of the lostthumb. Needless to say, evolution doesnot always follow a continuous, direc-tional path from one trait to another.

Built for SwimmingTHE NEW LIZARD-SHAPED fossilshave also helped resolve the origin of theskeletal structure of their fish-shaped de-scendants. The descendants have back-bones built from concave vertebrae theshape of hockey pucks. This shape,though rare among diapsids, was alwaysassumed to be typical of all ichthyo-saurs. But the new creatures from Asiasurprised paleontologists by having amuch narrower backbone, composed ofvertebrae more closely resembling can-isters of 35-millimeter film than hockeypucks. It appeared that the vertebraegrew dramatically in diameter and short-ened slightly as ichthyosaurs evolvedfrom lizard- to fish-shaped. But why?

My colleagues and I found the an-swer in the swimming styles of livingsharks. Sharks, like ichthyosaurs, comein various shapes and sizes. Cat sharksare slender and lack a tall tail fluke, also


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ANCIENT SKELETONS have helped scientists trace how the slender, lizardlike bodies of the firstichthyosaurs (top) thickened into a fish shape with a dorsal fin and a tail fluke.

CHAOHUSAURUS GEISHANESIS0.5 to 0.7 meter • Lived 245 million years ago (Early Triassic)

DORSAL FIN

TAIL FLUKE

MIXOSAURUS CORNALIANUS0.5 to 1 meter • Lived 235 million years ago (Middle Triassic)

OPHTHALMOSAURUS ICENICUS3 to 4 meters • Lived from 165 million to 150 million years ago (Middle to Late Jurassic)

RYOSUKE MOTANI is assistant professor of paleontology at the University of Oregon anda former researcher at the Royal Ontario Museum in Toronto. As a child he thought ichthyo-saurs “looked too ordinary in my picture books,” but his view changed during his under-graduate years at the University of Tokyo, after a professor allowed him to study the onlydomestic reptilian fossil they had: an ichthyosaur. Motani explored ichthyosaur evolutionfor his doctoral degree from the University of Toronto in 1997 and did postdoctoral re-search at the University of California, Berkeley.

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known as a caudal fin, on their lowerbacks, as did early ichthyosaurs. In con-trast, mackerel sharks such as the greatwhite have thick bodies and a crescent-shaped caudal fin similar to the later fish-shaped ichthyosaurs. Mackerel sharksswim by swinging only their tails, where-as cat sharks undulate their entire bodies.Undulatory swimming requires a flexiblebody, which cat sharks achieve by hav-ing a large number of backbone seg-ments. They have about 40 vertebrae inthe front part of their bodies—the samenumber scientists find in the first ichthyo-saurs, represented by Utatsusaurus andChaohusaurus. (Modern reptiles andmammals have only about 20.)

Undulatory swimmers, such as catsharks, can maneuver and accelerate suf-ficiently to catch prey in the relativelyshallow water above the continentalshelf. Living lizards also undulate toswim, though not as efficiently. It is log-ical to conclude, then, that the first ich-thyosaurs—which looked like cat sharksand descended from a lizardlike ances-tor—swam in the same fashion and livedabove the continental shelf.

Undulatory swimming enables preda-tors to thrive near shore, where food isabundant, but it is not the best choice for

an animal that has to travel long dis-tances to find a meal. Offshore preda-tors, which hunt in the open oceanwhere food is less concentrated, need amore energy-efficient swimming style.Mackerel sharks solve this problem byhaving stiff bodies that do not undulateas their tails swing back and forth. A

crescent-shaped caudal fin, which acts asan oscillating hydrofoil, also improvestheir cruising efficiency. Fish-shaped ich-thyosaurs had such a caudal fin, andtheir thick body profile implies that theyprobably swam like mackerel sharks.

Inspecting a variety of shark speciesreveals that the thicker the body from top


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SWIMMING STYLES—and thus the habitats(above)—of ichthyosaurs changed as the shapeof their vertebrae evolved. The narrow backboneof the first ichthyosaurs suggests that theyundulated their bodies like eels (right). Thismotion allowed the quickness and maneuver-ability needed for shallow-water hunting. As thebackbone thickened in later ichthyosaurs, thebody stiffened so it could remain still as the tailswung back and forth (bottom). This stillnessfacilitated the energy-efficient cruising neededto hunt in the open ocean.

Chaohusaurus

CHAOHUSAURUS CONTINENTAL SHELF

Ophthalmosaurus

OPHTHALMOSAURUS

Backbone segment

FACT: No other reptile group ever evolved a fish-shaped body


to bottom, the larger the diameter of thevertebrae. It seems that sharks and ich-thyosaurs solved the flexibility problemresulting from having high numbers ofbody segments in similar ways. As thebodies of ichthyosaurs thickened overtime, the number of vertebrae stayedabout the same. To add support to themore voluminous body, the backbonebecame at least one and a half timesthicker than those of the first ichthyo-saurs. As a consequence, the body be-came less flexible, and the individual

vertebrae acquired their hockey-puckappearance.

Drawn to the DeepTHE ICHTHYOSAURS’ inva-sion of open water also meant adeeper exploration of the ma-rine environment. We knowfrom the fossilized stomachcontents of fish-shaped ich-thyosaurs that they mostly atesquidlike creatures. Squid-eat-ing whales hunt anywherefrom about 100 to 1,000 me-ters deep and sometimes downto 3,000 meters. The greatrange in depth is hardly surpris-

ing considering that food re-sources are widely scattered be-

low about 200 meters. But to huntdown deep, whales and other air-

breathing divers have to go there andget back to the surface in one breath—

no easy task. Reducing energy use dur-ing swimming is one of the best ways toconserve precious oxygen stored in theirbodies. Consequently, deep divers todayhave streamlined body shapes that re-duce drag—and so did the fish-shapedichthyosaurs.

Characteristics apart from diet andbody shape also indicate that at leastsome fish-shaped ichthyosaurs were deepdivers. The ability of an air-breathingdiver to stay submerged depends rough-ly on its body size: the heavier the diver,the more oxygen it can store in its mus-cles, blood and certain organs—and theslower the consumption of oxygen per


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ICHTHYOSAUR EYES were surprisingly large. Analyses ofdoughnut-shaped eye bones called sclerotic rings revealthat Ophthalmosaurus had the largest eyes relative tobody size of any adult vertebrate, living or extinct, andthat Temnodontosaurus had the biggest eyes, period.The beige shape in the background is the size of anOphthalmosaurus sclerotic ring. The photograph depictsa well-preserved ring from Stenopterygius.

FACT: Their eyes were the largest of any animal, living or dead

APPROXIMATE MAXIMUMDIAMETER OF EYE

African elephant5 centimeters

Blue whale15 centimeters

Ophthalmosaurus23 centimeters

Giant squid25 centimeters

Temnodontosaurus26 centimeters


unit of body mass. The evolution of athick, stiff body increased the volumeand mass of fish-shaped ichthyosaurs rel-ative to their predecessors. Indeed, a fish-shaped ichthyosaur would have been upto six times heavier than a lizard-shapedichthyosaur of the same length. Calcula-tions based on the aerobic capacities of to-day’s air-breathing divers (mostly mam-mals and birds) indicate that an animalthe weight of fish-shaped Ophthalmo-saurus, which was about 950 kilograms,could hold its breath for at least 20 min-utes. Ophthalmosaurus could easily havedived to 600 meters—possibly even 1,500meters—and returned to the surface inthat time span.

Bone studies also indicate that fish-shaped ichthyosaurs were deep divers.Limb bones and ribs of four-limbed ter-restrial animals include a dense outershell that enhances the strength neededto support a body on land. But that denselayer is heavy. Because aquatic verte-brates are fairly buoyant, they do notneed the extra strength it provides.Heavy bones can impede the ability ofdeep divers to return to the surface. Agroup of French biologists has estab-lished that modern deep-diving mam-mals solve that problem by making theouter shell of their bones spongy and lessdense. The same type of spongy layeralso encases the bones of fish-shaped ich-thyosaurs, creating lighter skeletons.

Perhaps the best evidence for thedeep-diving habits of later ichthyosaursis their remarkably large eyes, up to 23centimeters across for Ophthalmosaur-us. Relative to a logarithmically correct-ed comparison of body size, that fish-shaped ichthyosaur had the biggest eyesof any animal ever known.

The size of their eyes also suggeststhat visual capacity improved as ichthyo-saurs moved up the family tree. These es-timates are based on measurements ofthe sclerotic ring, a doughnut-shapedbone that was embedded in their eyes.(Humans do not have such a ring, butmost other vertebrates have bones intheir eyes.) In the case of ichthyosaurs,

the ring presumably helped to maintainthe shape of the eye against the forces ofwater passing by as the animals swam.

The diameter of the sclerotic ringmakes it possible to calculate the eye’sminimum f-number—an index, used torate camera lenses, for the relative-brightness-sensing ability of an opticalsystem. Low-quality lenses have a valueof f/3.5 and higher; high-quality lenseshave values as low as f/1.0. The f-num-ber for the human eye is about 2.1,whereas the number for the eye of a noc-turnal cat is about 0.9. Calculations sug-gest that a cat would be capable of see-ing at depths of 500 meters or greater inmost oceans. Ophthalmosaurus alsohad a minimum f-number of about 0.9but with its much larger eyes couldprobably outperform a cat.

Gone for GoodMANY CHARACTERIST ICS of ich-thyosaurs—including the shape of theirbodies and backbones, the size of theireyes, their aerobic capacity, and theirhabitat and diet—seem to have changedin a connected way during their evolu-tion. Such adaptations enabled ichthyo-

saurs to reign for 155 million years. Newfossils of the earliest of these sea dwellersare now making it clear just how theyevolved so successfully for aquatic life,yet still no one knows why ichthyosaurswent extinct.

Loss of habitat may have clinched thefinal demise of lizard-shaped ichthyo-saurs, whose inefficient, undulatoryswimming style limited them to near-shore environments. A large-scale dropin sea level could have snuffed out thesecreatures, along with many others, byeliminating their shallow-water niche.Fish-shaped ichthyosaurs, on the otherhand, could make a living in the openocean. Because their habitat never dis-appeared, something else must haveeliminated them. The period of their dis-appearance roughly corresponds to theappearance of advanced sharks, but noone has found direct evidence of compe-tition between the two groups.

Paleontologists may never fully ex-plain the extinction of ichthyosaurs. Butas we explore their evolutionary history,we are sure to learn a great deal moreabout how these fascinating creatureslived.


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SMALL ISLAND in northeast Japan harbored two almost complete skeletons of

Utatsusaurus, the oldest ichthyosaur.

Dinosaurs, Spitfires, and Sea Dragons. Christopher McGowan. Harvard University Press, 1991.

Eel-like Swimming in the Earliest Ichthyosaurs. Ryosuke Motani, You Hailu and ChristopherMcGowan in Nature, Vol. 382, pages 347–348; July 25, 1996.

Ichthyosaurian Relationships Illuminated by New Primitive Skeletons from Japan. RyosukeMotani, Nachio Minoura and Tatsuro Ando in Nature, Vol. 393, pages 255–257; May 21, 1998.

Large Eyeballs in Diving Ichthyosaurs. Ryosuke Motani, Bruce M. Rothschild and William Wahl, Jr.,in Nature, Vol. 402, page 747; December 16, 1999.

Ryosuke Motani’s Web site: www.ucmp.berkeley.edu/people/motani/ichthyo/

M O R E T O E X P L O R E


Mammals Conquered

The

New fossils and DNA analyses elucidate theremarkable evolutionary history of whales

That


the Seas By Kate Wong

“They say the sea is cold, but the sea contains the hottest blood of all, and the wildest, the most urgent.”

—D. H. Lawrence, “Whales Weep Not!”

Dawn breaks overthe Tethys Sea, 48 million

years ago, and the blue-

green water sparkles with

the day’s first light. But for

one small mammal, this

new day will end almost as

soon as it has started.

ANCIENT WHALE Rodhocetus (right and left front)feasts on the bounty of the sea, while Ambulocetus(rear) attacks a small land mammal some 48 millionyears ago in what is now Pakistan.


Tapir-like Eotitanops has wandered perilously close to the wa-ter’s edge, ignoring its mother’s warning call. For the brute lurk-ing motionless among the mangroves, the opportunity is sim-ply too good to pass up. It lunges landward, propelled by pow-erful hind limbs, and sinks its formidable teeth into the calf,dragging it back into the surf. The victim’s frantic strugglingsubsides as it drowns, trapped in the unyielding jaws of its cap-tor. Victorious, the beast shambles out of the water to devourits kill on terra firma. At first glance, this fearsome predator re-sembles a crocodile, with its squat legs, stout tail, long snoutand eyes that sit high on its skull. But on closer inspection, it hasnot armor but fur, not claws but hooves. And the cusps on itsteeth clearly identify it not as a reptile but as a mammal. In fact,this improbable creature is Ambulocetus, an early whale, andone of a series of intermediates linking the land-dwelling an-cestors of cetaceans to the 80 or so species of whales, dolphinsand porpoises that rule the oceans today.

Until recently, the emergence of whales was one of the mostintractable mysteries facing evolutionary biologists. Lacking furand hind limbs and unable to go ashore for so much as a sip offreshwater, living cetaceans represent a dramatic departurefrom the mammalian norm. Indeed, their piscine form led Her-man Melville in 1851 to describe Moby Dick and his fellowwhales as fishes. But to 19th-century naturalists such as CharlesDarwin, these air-breathing, warm-blooded animals that nurse

their young with milk distinctly grouped with mammals. Andbecause ancestral mammals lived on land, it stood to reason thatwhales ultimately descended from a terrestrial ancestor. Exact-ly how that might have happened, however, eluded scholars.For his part, Darwin noted in On the Origin of Species that abear swimming with its mouth agape to catch insects was aplausible evolutionary starting point for whales. But the propo-sition attracted so much ridicule that in later editions of thebook he said just that such a bear was “almost like a whale.”

The fossil record of cetaceans did little to advance the studyof whale origins. Of the few remains known, none were suffi-ciently complete or primitive to throw much light on the mat-ter. And further analyses of the bizarre anatomy of living whalesled only to more scientific head scratching. Thus, even a centu-ry after Darwin, these aquatic mammals remained an evolu-tionary enigma. In fact, in his 1945 classification of mammals,famed paleontologist George Gaylord Simpson noted thatwhales had evolved in the oceans for so long that nothing in-formative about their ancestry was left. Calling them “on thewhole, the most peculiar and aberrant of mammals,” he in-serted cetaceans arbitrarily among the other orders. Wherewhales belonged in the mammalian family tree and how theytook to the seas defied explanation, it seemed.

Over the past two decades, however, many of the pieces ofthis once imponderable puzzle have fallen into place. Paleon-tologists have uncovered a wealth of whale fossils spanning theEocene epoch, the time between 55 million and 34 million yearsago when archaic whales, or archaeocetes, made their transi-tion from land to sea. They have also unearthed some cluesfrom the ensuing Oligocene, when the modern suborders ofcetaceans—the mysticetes (baleen whales) and the odontocetes(toothed whales)—arose. That fossil material, along with analy-ses of DNA from living animals, has enabled scientists to painta detailed picture of when, where and how whales evolved fromtheir terrestrial forebears. Today their transformation—fromlandlubbers to leviathans—stands as one of the most profoundevolutionary metamorphoses on record.

Evolving IdeasAT AROUND THE SAME TIME that Simpson declared the re-lationship of whales to other mammals undecipherable on thebasis of anatomy, a new comparative approach emerged, onethat looked at antibody-antigen reactions in living animals. Inresponse to Simpson’s assertion, Alan Boyden of Rutgers Uni-versity and a colleague applied the technique to the whale ques-tion. Their results showed convincingly that among living ani-mals, whales are most closely related to the even-toed hoofedmammals, or artiodactyls, a group whose members includecamels, hippopotamuses, pigs and ruminants such as cows. Still,

14 S C I E N T I F I C A M E R I C A N U p d a t e d f r o m t h e M a y 2 0 0 2 i s s u e

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CETACEA is the order of mammals that comprises livingwhales, dolphins and porpoises and their extinct ancestors,the archaeocetes. Living members fall into two suborders: theodontocetes, or toothed whales, including sperm whales, pilotwhales, belugas, and all dolphins and porpoises; and themysticetes, or baleen whales, including blue whales and finwhales. The term “whale” is often used to refer to all cetaceans.

ARTIODACTYLA is the order of even-toed, hoofed mammalsthat includes camels; ruminants such as cows; hippos;and, most researchers now agree, whales.

MESONYCHIDS are a group of primitive hoofed, wolflikemammals once widely thought to have given rise to whales.

EOCENE is the epoch between 55 million and 34 millionyears ago, during which early whales made their transitionfrom land to sea.

OLIGOCENE is the epoch between 34 million and 24 millionyears ago, during which odontocetes and mysticetesevolved from their archaeocete ancestors.

Guide to Terminology


the exact nature of that relationship remained unclear. Werewhales themselves artiodactyls? Or did they occupy their ownbranch of the mammalian family tree, linked to the artiodactylbranch via an ancient common ancestor?

Support for the latter interpretation came in the 1960s, fromstudies of primitive hoofed mammals known as condylarthsthat had not yet evolved the specialized characteristics of ar-tiodactyls or the other mammalian orders. Paleontologist Leigh

Van Valen, then at the American Museum of Natural Historyin New York City, discovered striking resemblances betweenthe three-cusped teeth of the few known fossil whales and thoseof a group of meat-eating condylarths called mesonychids. Like-wise, he found shared dental characteristics between artio-dactyls and another group of condylarths, the arctocyonids,close relatives of the mesonychids. Van Valen concluded thatwhales descended from the carnivorous, wolflike mesonychids


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brought about radical changes in the quantity and distributionof nutrients in the sea, generating a whole new set of ecologicalopportunities for the cetaceans.

As posited by paleontologist Ewan Fordyce of theUniversity of Otago in New Zealand, that set the stage for thereplacement of the archaeocetes by the odontocetes andmysticetes (toothed and baleen whales, respectively). Theearliest known link between archaeocetes and the moderncetacean orders, Fordyce says, is Llanocetus, a 34-million-year-old protobaleen whale from Antarctica that may well havetrawled for krill in the chilly Antarctic waters, just as livingbaleen whales do. Odontocetes arose at around the same time,he adds, specializing to become echolocators that could hunt in the deep.

Unfortunately, fossils documenting the origins ofmysticetes and odontocetes are vanishingly rare. Low sealevels during the Middle Oligocene exposed most potentialwhale-bearing sediments from the Early Oligocene to erosivewinds and rains, making that period largely “a fossilwasteland,” says paleontologist Mark Uhen of the CranbrookInstitute of Science in Bloomfield Hills, Mich. The later fossilrecord clearly shows, however, that shortly after, by about 30million years ago, the baleen and toothed whales haddiversified into many of the cetacean families that reign overthe oceans today. —K.W.

It might seem odd that 300 million years after vertebratesfirst established a toehold on land, some returned to the sea.But the setting in which early whales evolved offers hints asto what lured them back to the water. For much of the Eoceneepoch (roughly between 55 million and 34 million years ago), a sea called Tethys, after a goddess of Greek mythology,stretched from Spain to Indonesia. Although the continents andocean plates we know now had taken shape, India was stilladrift, Australia hadn’t yet fully separated from Antarctica, andgreat swaths of Africa and Eurasia lay submerged underTethys. Those shallow, warm waters incubated abundantnutrients and teemed with fish. Furthermore, the spacevacated by the plesiosaurs, mosasaurs and other large marinereptiles that perished along with the dinosaurs created roomfor new top predators (although sharks and crocodiles stillprovided a healthy dose of competition). It is difficult toimagine a more enticing invitation to aquatic life for a mammal.

During the Oligocene epoch that followed, sea levels sankand India docked with the rest of Asia, forming the crumpledinterface we know as the Himalayas. More important,University of Michigan paleontologist Philip Gingerich notes,Australia and Antarctica divorced, opening up the SouthernOcean and creating a south circumpolar current that eventuallytransformed the balmy Eocene Earth into the ice-capped planetwe inhabit today. The modern current and climate systems

50 Million Years Ago Present

PROTO-INDIA

PROTO-AUSTRALIA

BASILOSAURIDSFOSSIL LOCATIONS

PROTOCETIDS

THE WHALE’S CHANGING WORLD

LLANOCETUSPAKICETIDS AMBULOCETIDS REMINGTONOCETIDS

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HIPPOS = HIPPOPOTAMIDSARTIOS = ARTIODACTYLS OTHER THAN HIPPOS MESOS = MESONYCHIDS

OLD MESONYCHID HYPOTHESIS

MESOS ARTIOS HIPPOS WHALES

ARTIOS HIPPOS MESOS WHALES

HIPPOPOTAMID HYPOTHESIS

ARTIOS HIPPOS MESOS WHALES

NEW MESONYCHID HYPOTHESIS

MESOS ARTIOS HIPPOS WHALES

ARTIODACTYL HYPOTHESIS

FAMILY TREE OF CETACEANS shows the descent of the two modernsuborders of whales, the odontocetes and mysticetes, from theextinct archaeocetes. Representative members of each archaeocetefamily or subfamily are depicted (left). Branching diagrams illustratevarious hypotheses of the relationship of whales to other mammals(right). The old mesonychid hypothesis, which posits that extinctwolflike beasts known as mesonychids are the closest relatives ofwhales, now seems unlikely in light of recent fossil whale discoveries.The anklebones of those ancient whales bear the distinctivecharacteristics of artiodactyl ankles, suggesting that whales are

themselves artiodactyls, as envisioned by the artiodactylhypothesis. Molecular studies indicate that whales are more closelyrelated to hippopotamuses than to any other artiodactyl group.Whether the fossil record can support the hippopotamid hypothesis,however, remains to be seen. A fourth scenario, denoted here asthe new mesonychid hypothesis, proposes that mesonychids couldstill be the whale’s closest kin if they, too, were included in theartiodactyl order, instead of the extinct order Condylarthra, in whichthey currently reside. If so, they would have to have lost the ankletraits that characterize all known artiodactyls. —K.W.

CETACEAN RELATIONS

BASILOSAURUS18.2 meters

DORUDON4.5 meters

RODHOCETUS3 meters

KUTCHICETUS1.75 meters

AMBULOCETUS4.15 meters

PAKICETUS1.75 meters

Millions of Years Ago55 50 45 40 35

PAKICETIDAE

AMBULOCETIDAE

PROTOCETIDAE

BASILOSAURIDAEODONTOCETES

MYSTICETES

CETACEA

DORUDONTINAE

BASILOSAURINAE

REMINGTONOCETIDAE


and thus appeared to be linked to artiodactyls through thecondylarths.

Walking WhalesA DECADE OR SO PASSED before paleontologists finally be-gan unearthing fossils close enough to the evolutionary branch-ing point of whales to address Van Valen’s mesonychid hy-pothesis. Even then the significance of these finds took a whileto sink in. It started when University of Michigan paleontolo-gist Philip D. Gingerich went to Pakistan in 1977 in search ofEocene land mammals. The expedition proved disappointingbecause just marine fossils turned up. Finding traces of ancientocean life in Pakistan, far from the country’s modern coast, isnot surprising: during the Eocene, the vast Tethys Sea periodi-cally covered great swaths of what is now the Indian subconti-nent [see box on page 15]. Intriguingly, though, the team dis-covered among those ancient fish and snail remnants two pelvisfragments that appeared to have come from relatively large,walking beasts. “We joked about walking whales,” Gingerichrecalls with a chuckle. “It was unthinkable.” Curious as thepelvis pieces were, the only fossil collected during that field sea-son that seemed important at the time was a primitive artio-dactyl jaw that had turned up in another part of the country.

Two years later, in the Himalayan foothills of northern Pa-kistan, Gingerich’s team located another weird whale clue: a par-tial braincase from a wolf-size creature—found in the companyof 50-million-year-old land mammal remains—that bore dis-tinctive cetacean characteristics. All modern whales have fea-tures in their ears that do not appear in any other vertebrates.Although the fossil skull lacked the anatomy necessary for hear-ing directionally in water (a critical skill for living whales), itclearly had the diagnostic cetacean ear traits. The team had dis-covered the oldest and most primitive whale then known—onethat must have spent some, if not most, of its time on land. Gin-gerich christened the creature Pakicetus for its place of originand, thus hooked, began hunting for ancient whales in earnest.

Meanwhile another group recovered additional remains ofPakicetus—a lower jaw fragment and isolated teeth—that bol-stered the link to mesonychids through strong dental similari-ties. With Pakicetus showing up around 50 million years agoand mesonychids known from around the same time in thesame part of the world, it seemed increasingly likely thatcetaceans had indeed descended from the mesonychids or some-thing closely related to them. Still, what the earliest whales looked

like from the neck down was a mystery.

Further insights from Pakistan would have to wait, howev-er. By 1983 Gingerich was no longer able to work there becauseof the Soviet Union’s invasion of Afghanistan. He decided tocast his net in Egypt instead, journeying some 95 miles south-west of Cairo to the Western Desert’s Zeuglodon Valley, sonamed for early 20th-century reports of fossils of archaicwhales—or zeuglodons, as they were then known—in the area.Like Pakistan, much of Egypt once lay submerged under Tethys.Today the skeletons of creatures that swam in that ancient sealie entombed in sandstone. After several field seasons, Gingerichand his crew hit pay dirt: tiny hind limbs belonging to a 60-foot-long sea snake of a whale known as Basilosaurus and the firstevidence of cetacean feet.

Earlier finds of Basilosaurus, a fully aquatic monster thatslithered through the seas between about 40 million and 37 mil-lion years ago, preserved only a partial femur, which its dis-coverers interpreted as vestigial. But the well-formed legs andfeet revealed by this discovery hinted at functionality. At lessthan half a meter in length, the diminutive limbs probablywould not have assisted Basilosaurus in swimming and cer-tainly would not have enabled it to walk on land, but they maywell have helped guide the beast’s serpentine body during thedifficult activity of aquatic mating. Whatever their purpose, ifany, the little legs had big implications. “I immediately thought,we’re 10 million years after Pakicetus,” Gingerich recounts ex-citedly. “If these things still have feet and toes, we’ve got 10 mil-lion years of history to look at.” Suddenly, the walking whalesthey had scoffed at in Pakistan seemed entirely plausible.

Just such a remarkable creature came to light in 1992. Ateam led by J.G.M. (Hans) Thewissen of the Northeastern OhioUniversities College of Medicine recovered from 48-million-year-old marine rocks in northern Pakistan a nearly completeskeleton of a perfect intermediate between modern whales andtheir terrestrial ancestors. Its large feet and powerful tail be-spoke strong swimming skills, while its sturdy leg bones andmobile elbow and wrist joints suggested an ability to locomoteon land. He dubbed the animal Ambulocetus natans, the walk-ing and swimming whale.

Shape ShiftersSINCE THEN, Thewissen, Gingerich and others have uneartheda plethora of fossils documenting subsequent stages of thewhale’s transition from land to sea. The picture emerging fromthose specimens is one in which Ambulocetus and its kin—themselves descended from the more terrestrial pakicetids—spawned needle-nosed beasts known as remingtonocetids as

well as the intrepid protocetids, the first whales seaworthyenough to fan out from Indo-Pakistan across the

globe. From the protocetids arose the dolphinlikedorudontines, the probable progenitors of thesnakelike basilosaurines and modern whales

[see box on opposite page]. In addition to furnishing supporting branches

for the whale family tree, these discoveries have enabledresearchers to chart many of the spectacular anatomical and

w w w . s c i a m . c o m S C I E N T I F I C A M E R I C A N 17COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.

physiological changes that allowed cetaceans to establish per-manent residency in the ocean realm. Some of the earliest ofthese adaptations to emerge, as Pakicetus shows, are those re-lated to hearing. Sound travels differently in water than it doesin air. Whereas the ears of humans and other land-dwelling an-imals have delicate, flat eardrums, or tympanic membranes, forreceiving airborne sound, modern whales have thick, elongatetympanic ligaments that cannot receive sound. Instead a bonecalled the bulla, which in whales has become quite dense and istherefore capable of transmitting sound coming from a densermedium to deeper parts of the ear, takes on that function. ThePakicetus bulla shows some modification in that direction, butthe animal retained a land mammal–like eardrum that couldnot work in water.

What, then, might Pakicetus have used its thickened bullaefor? Thewissen suspects that, much as turtles hear by picking upvibrations from the ground through their shields, Pakicetus mayhave employed its bullae to pick up ground-borne sounds. Tak-ing new postcranial evidence into consideration along with theear morphology, he envisions Pakicetus as an ambush predatorthat may have lurked around shallow rivers, head to the ground,preying on animals that came to drink. Ambulocetus is evenmore likely to have used such inertial hearing, Thewissen says,because it had the beginnings of a channel linking jaw and ear.By resting its jaw on the ground—a strategy seen in modern croc-odiles—Ambulocetus could have listened for approaching prey.The same features that allowed early whales to receive soundsfrom soil, he surmises, preadapted them to hearing in the water.

Zhe-Xi Luo of the Carnegie Museum of Natural Historyin Pittsburgh has shown that by the time of the basilosaurinesand dorudontines, the first fully aquatic whales, the ropelike


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DORUDON, a 4.5-meter-long, dolphinlike archaeocete that roamed the seas between roughly 40 million and 37 million years ago, may be the ancestor of modern whales.

BECOMING LEVIATHAN

REPRESENTATIVE ARCHAEOCETES in the lineage leading to modern odontocetesand mysticetes trace some of the anatomical changes that enabled theseanimals to take to the seas (reconstructed bone appears in lavender). In just 15million years, whales shed their terrestrial trappings and became fully adaptedto aquatic life. Notably, the hind limbs diminished, the forelimbs transformedinto flippers, and the vertebral column evolved to permit tail-powered swimming.Meanwhile the skull changed to enable underwater hearing, the nasal openingmoved backward to the top of the skull, and the teeth simplified into pegs forgrasping instead of grinding. Later in whale evolution, the mysticetes’ teethwere replaced with baleen.

PAKICETUS AMBULOCETUS

MODERN MYSTICETE


tympanic ligament had probably already evolved. Additional-ly, air sinuses, presumably filled with spongy tissues, hadformed around the middle ear, offering better sound resolutionand directional cues for underwater hearing. Meanwhile, withthe external ear canal closed off (a prerequisite for deep-sea div-ing), Luo adds, the lower jaw was taking on an increasingly im-portant auditory role, developing a fat-filled canal capable ofconducting sound back to the middle ear.

Later in the evolution of whale hearing, the toothed andbaleen whales parted ways. Whereas the toothed whales evolvedthe features necessary to produce and receive high-frequencysounds, enabling echolocation for hunting, the baleen whalesdeveloped the ability to produce and receive very low frequen-cy sounds, allowing them to communicate with one anotherover vast distances. Fossil whale ear bones, Luo says, show thatby around 28 million years ago early odontocetes already hadsome of the bony structures necessary for hearing high-pitchedsound and were thus capable of at least modest echolocation.The origin of the mysticete’s low-frequency hearing is far murki-er, even though the fossil evidence of that group now dates backto as early as 34 million years ago.

Other notable skull changes include movement of the eyesockets from a crocodilelike placement atop the head in Pa-kicetus and Ambulocetus to a lateral position in the more aquat-ic protocetids and later whales. And the nasal opening migrat-ed back from the tip of the snout in Pakicetus to the top of thehead in modern cetaceans, forming the blowhole. Whale den-tition morphed, too, turning the complexly cusped, grindingmolars of primitive mammalian ancestors into the simple,prong-shaped teeth of modern odontocetes, which grasp andswallow their food without chewing. Mysticetes lost their teethaltogether and developed plates of baleen that hang from theirupper jaws and strain plankton from the seawater.

The most obvious adaptations making up the whale’s pro-tean shift are those that produced its streamlined shape and un-matched swimming abilities. Not surprisingly, some bizarreamphibious forms resulted along the way. Ambulocetus, forone, retained the flexible shoulder, elbow, wrist and fingerjoints of its terrestrial ancestors and had a pelvis capable of sup-porting its weight on land. Yet the creature’s disproportion-ately large hind limbs and paddlelike feet would have madewalking rather awkward. These same features were perfect forpaddling around in the fish-filled shallows of Tethys, however.

Moving farther out to sea required additional modifications,many of which appear in the protocetid whales. Studies of onemember of this group, Rodhocetus, indicate that the lower armbones were compressed and already on their way to becominghydrodynamically efficient, says University of Michigan pale-ontologist William J. Sanders. The animal’s long, delicate feetwere probably webbed, similar to the fins used by scuba divers.Rodhocetus also exhibits aquatic adaptations in its pelvis,where the fusion between the vertebrae that form the sacrumis reduced, loosening up the lower spine to power tail move-ment. These features, says Gingerich, whose team discoveredthe creature, suggest that Rodhocetus performed a leisurely dogpaddle at the sea surface and a swift combination of otterlikehind-limb paddling and tail propulsion underwater. When itwent ashore to breed or perhaps to bask in the sun, he propos-es, Rodhocetus probably hitched itself around in the manner ofa modern eared seal or sea lion.

By the time of the basilosaurines and dorudontines, whaleswere fully aquatic. As in modern cetaceans, the shoulder re-mained mobile while the elbow and wrist stiffened, forming flip-pers for steering and balance. Farther back on the skeleton, onlytiny legs remained, and the pelvis had dwindled accordingly.Analyses of the vertebrae of Dorudon, conducted by Mark D.


MODERN ODONTOCETE

RODHOCETUS DORUDON


Uhen of the Cranbrook Institute of Science in Bloomfield Hills,Mich., have revealed one tail vertebra with a rounded profile.Modern whales have a similarly shaped bone, the ball vertebra,at the base of their fluke—the flat, horizontal structure cappingthe tail. Uhen thus suspects that basilosaurines and dorudon-tines had tail flukes and swam much as modern whales do, us-ing so-called caudal oscillation. In this energetically efficientmode of locomotion, motion generated at a single point in the

vertebral column powers the tail’s vertical movement throughthe water, and the fluke generates lift.

Exactly when whales lost their legs altogether remains un-known. In fact, a recent discovery made by Lawrence G. Barnesof the Natural History Museum of Los Angeles County hints atsurprisingly well developed hind limbs in a 27-million-year-oldbaleen whale from Washington State, suggesting that whale legspersisted far longer than originally thought. Today, however,some 50 million years after their quadrupedal ancestors firstwaded into the warm waters of Tethys, whales are singularlysleek. Their hind limbs have shrunk to externally invisible ves-tiges, and the pelvis has diminished to the point of serving mere-ly as an anchor for a few tiny muscles unrelated to locomotion.

Making WavesTHE FOSSILS UNCOVERED during the 1980s and 1990s ad-vanced researchers’ understanding of whale evolution by leapsand bounds, but all morphological signs still pointed to amesonychid origin. An alternative view of cetacean roots wasgaining currency in genetics laboratories in the U.S., Belgiumand Japan, however. Molecular biologists, having developedsophisticated techniques for analyzing the DNA of living crea-tures, took Boyden’s 1960s immunology-based conclusions astep further. Not only were whales more closely related to ar-tiodactyls than to any other living mammals, they asserted, butwhales were themselves artiodactyls, one of many twigs on thatbranch of the mammalian family tree. Moreover, a number ofthese studies pointed to an especially close relationship betweenwhales and hippopotamuses. Particularly strong evidence forthis idea came in 1999 from analyses of snippets of noncodingDNA called SINES (short interspersed elements), conducted byNorihiro Okada and his colleagues at the Tokyo Institute ofTechnology.

The whale-hippo connection did not sit well with paleon-tologists. “I thought they were nuts,” Gingerich recollects.“Everything we’d found was consistent with a mesonychid ori-gin. I was happy with that and happy with a connection throughmesonychids to artiodactyls.” Whereas mesonychids appearedat the right time, in the right place and in the right form to beconsidered whale progenitors, the fossil record did not seem tocontain a temporally, geographically and morphologically plau-sible artiodactyl ancestor for whales, never mind one linkingwhales and hippos specifically. Thewissen, too, had largely dis-missed the DNA findings. But “I stopped rejecting it when Oka-da’s SINE work came out,” he says.

It seemed the only way to resolve the controversy was tofind, of all things, an ancient whale anklebone. Morphologistshave traditionally defined artiodactyls on the basis of certainfeatures in one of their anklebones, the astragalus, that enhancemobility. Specifically, the unique artiodactyl astragalus has twogrooved, pulleylike joint surfaces. One connects to the tibia, orshinbone; the other articulates with more distal anklebones.If whales descended from artiodactyls, researchers reasoned,those that had not yet fully adapted to life in the sea should ex-hibit this double-pulleyed astragalus.


WATER, WATER EVERYWHEREMOST MAMMALS—big ones in particular—cannot live withoutfreshwater. For marine mammals, however, freshwater isdifficult to come by. Seals and sea lions obtain most of theirwater from the fish they eat (some will eat snow to getfreshwater), and manatees routinely seek out freshwaterfrom rivers. For their part, cetaceans obtain water both fromtheir food and from sips of the briny deep.

When did whales, which evolved from a fairly large (andtherefore freshwater-dependent) terrestrial mammal, developa system capable of handling the excess salt load associatedwith ingesting seawater? Evidence from so-called stableoxygen isotopes has provided clues. In nature, oxygen mainlyoccurs in two forms, or isotopes: 16O and 18O. The ratios ofthese isotopes in freshwater and seawater differ, withseawater containing more 18O. Because mammals incorporateoxygen from drinking water into their developing teeth andbones, the remains of those that imbibe seawater can bedistinguished from those that take in freshwater.

J.G.M. (Hans) Thewissen of the Northeastern OhioUniversities College of Medicine and his colleagues thusanalyzed the oxygen isotope ratios in ancient whale teeth togain insight into when these animals might have moved froma freshwater-based osmoregulatory system to a seawater-based one. Oxygen isotope values for pakicetids, the mostprimitive whales, indicate that they drank freshwater, aswould be predicted from other indications that these animalsspent much of their time on land. Isotope measurements fromamphibious Ambulocetus, on the other hand, vary widely, andsome specimens show no evidence of seawater intake. Inexplanation, the researchers note that although Ambulocetusis known to have spent time in the sea (based on the marinenature of the rocks in which its fossils occur), it may still havehad to go ashore to drink. Alternatively, it may have spent theearly part of its life (when its teeth mineralized) in freshwaterand only later entered the sea.

The protocetids, however, which show more skeletaladaptations to aquatic life, exhibit exclusively marine isotopevalues, indicating that they drank only seawater. Thus, just afew million years after the first whales evolved, theirdescendants had adapted to increased salt loads. Thisphysiological innovation no doubt played an important role infacilitating the protocetids’ dispersal across the globe. —K.W.


That piece of the puzzle appeared in 2001, when Gingerichand Thewissen both announced discoveries of new primitivewhale fossils in Pakistan. In the eastern part of BaluchistanProvince, Gingerich’s team had found partially articulatedskeletons of Rodhocetus balochistanensis and a new protocetidgenus, Artiocetus. Thewissen and his colleagues recoveredfrom a bone bed in Punjab much of the long-sought postcra-nial skeleton of Pakicetus, as well as that of a smaller memberof the pakicetid family, Ichthyolestes. Each came with an as-tragalus bearing the distinctive artiodactyl characteristics.

The anklebones convinced both longtime proponents of themesonychid hypothesis that whales instead evolved from ar-tiodactyls. Gingerich has even embraced the hippo idea. Al-though hippos themselves arose long after whales, their pur-ported ancestors—dog- to horse-size, swamp-dwelling beastscalled anthracotheres—date back to at least the Middle Eoceneand may thus have a forebear in common with the cetaceans.In fact, Gingerich notes that Rodhocetus and anthracotheresshare features in their hands and wrists not seen in any otherlater artiodactyls. Thewissen agrees that the hippo hypothesisholds much more appeal than it once did. But he cautions thatthe morphological data still do not point to a particular artio-dactyl, such as the hippo, being the whale’s closest relative, orsister group. “We don’t have the resolution yet to get themthere,” he remarks, “but I think that will come.”

What of the evidence that seemed to tie early whales tomesonychids? In light of the recent ankle data, most workersnow suspect that those similarities probably reflect convergentevolution rather than shared ancestry and that mesonychidsrepresent an evolutionary dead end. But not everyone is con-vinced. Maureen O’Leary of Stony Brook University arguesthat until all the available evidence—both morphological andmolecular—is incorporated into a single phylogenetic analysis,the possibility remains that mesonychids belong at the base ofthe whale pedigree. It is conceivable, she says, that mesony-

chids are actually ancient artiodactyls but ones that reversedthe ankle trend. If so, mesonychids could still be whales’ clos-est relative and hippos could be their closest living relative [seebox on page 16]. Critics of that idea, however, point out thatalthough folding the mesonychids into the artiodactyl order of-fers an escape hatch of sorts to supporters of the mesonychidhypothesis, it would upset the long-standing notion that theankle makes the artiodactyl.

Investigators agree that determining the exact relationshipbetween whales and artiodactyls will most likely require find-ing additional fossils—particularly those that can illuminate thebeginnings of artiodactyls in general and hippos in particular.Yet even with those details still unresolved, “we’re really get-ting a handle on whales from their origin to the end of ar-chaeocetes,” Uhen reflects. The next step, he says, will be to fig-ure out how the mysticetes and odontocetes arose from the ar-chaeocetes and when their modern features emerged. Researchersmay never solve all the mysteries of whale origins. But if the ex-traordinary advances made over the past two decades are anyindication, with continued probing, answers to many of theselingering questions will surface from the sands of time.

Kate Wong is editorial director of ScientificAmerican.com


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1 2 3

The Emergence of Whales: Evolutionary Patterns in the Origin ofCetacea. Edited by J.G.M. Thewissen. Plenum Publishing, 1998.

Skeletons of Terrestrial Cetaceans and the Relationship of Whales toArtiodactyls. J.G.M. Thewissen, E. M. Williams, L. J. Roe and S. T. Hussainin Nature, Vol. 413, pages 277–281; September 20, 2001.

Origin of Whales from Early Artiodactyls: Hands and Feet of EoceneProtocetidae from Pakistan. Philip D. Gingerich, Munir ul Haq, Iyad S. Zalmout, Intizar Hussain Khan and M. Sadiq Malkani in Science,Vol. 293, pages 2239–2242; September 21, 2001.

The Encyclopedia of Marine Mammals. Edited by W. F. Perrin, Bernd G. Würsig and J.G.M. Thewissen. Academic Press, 2002.

M O R E T O E X P L O R E

HIND LIMB of an ancientwhale, Rodhocetus, preserves

a long-sought ankleboneknown as the astragalus

(at right). Shown in the insetbeside a mesonychid

astragalus (1) and one from a modern artiodactyl (2), the

Rodhocetus astragalus (3)exhibits the distinctive

double-pulley shape thatcharacterizes all artiodactyl

astragali, suggesting thatwhales descended not frommesonychids, as previously

thought, but from an ancient artiodactyl.


Breathing Life into

TYRANNOSAURUS REXdefends its meal, a Triceratops, from other hungry T. rex.Troodontids, the smallcreatures at the bottomleft and right, wait forscraps left by thetyrannosaurs, whilepterosaurs circleoverhead on this typicalday some 65 millionyears ago. Trees andflowering plants completethe landscape; grasseshave yet to evolve.

TyrannosaurusrexTyrannosaurusrexBreathing Life into


By analyzing previously overlooked fossilsand by taking a second look at some

old finds, paleontologists are providing the first glimpses of the actual behavior

of the tyrannosaurs

By Gregory M. Erickson

By analyzing previously overlooked fossilsand by taking a second look at some

old finds, paleontologists are providing the first glimpses of the actual behavior

of the tyrannosaurs

By Gregory M. Erickson


24 S C I E N T I F I C A M E R I C A N

inosaurs ceased towalk the earth 65 mil-lion years ago, yet theystill live among us. Velociraptors star in

movies, and Triceratops toys clutter tod-dlers’ bedrooms. Of these charismaticanimals, however, one species has alwaysruled our fantasies. Children, filmmakerSteven Spielberg and professional pale-ontologists agree that the superstar wasand is Tyrannosaurus rex.

The late Harvard University paleon-tologist Stephen Jay Gould said that everyspecies designation represents a theoryabout that animal. The very name Tyran-

nosaurus rex—“tyrant lizard king”—evokes a powerful image of this species.John R. Horner of Montana State Uni-versity and science writer Don Lessemwrote in their book The Complete T.Rex, “We’re lucky to have the opportu-nity to know T. rex, study it, imagine it,and let it scare us. Most of all, we’relucky T. rex is dead.” And paleontologistRobert T. Bakker of the Glenrock Pale-ontological Museum in Wyoming de-scribed T. rex as a “10,000-pound road-runner from hell,” a tribute to its obvi-ous size and power.

In Spielberg’s Jurassic Park, whichboasted the most accurate popular de-

piction of dinosaurs ever, T. rex was, asusual, presented as a killing machinewhose sole purpose was aggressive,bloodthirsty attacks on helpless prey. T. rex’s popular persona, however, is asmuch a function of artistic license as ofconcrete scientific evidence. A century ofstudy and the existence of 30 fairly com-plete T. rex specimens have generatedsubstantial information about its anato-my. But inferring behavior from anat-omy alone is perilous, and the true natureof T. rex continues to be largely shroud-ed in mystery. Whether it was even pri-marily a predator or a scavenger is stillthe subject of debate.

MASSIVE FORCE generated by T. rex in the “puncture and pull” bitingtechnique was sufficient to have created the huge furrows on the surface of the section of a fossil Triceratops pelvis shown in the inset at the right. The enormous body of the T. rex (skeleton at right) and its powerful neckmusculature enabled the “pull” in “puncture and pull.”

NIPPING STRATEGY enabled T. rex to remove strips of flesh in tight spots, such as between vertebrae, using only the front teeth.

D


Over the past decade or so, a newbreed of scientists has begun to unravelsome of T. rex’s better-kept secrets.These paleobiologists try to put a crea-ture’s remains in a living context—theyattempt to animate the silent and stillskeleton of the museum display. T. rexis thus changing before our eyes as pa-leobiologists use fossil clues, some newand some previously overlooked, to de-velop fresh ideas about the nature ofthese magnificent animals.

Rather than draw conclusions aboutbehavior based solely on anatomy, pa-leobiologists demand proof of actual ac-tivities. Skeletal assemblages of multiple

individuals shine a light on the interac-tions among T. rex and between themand other species. In addition, so-calledtrace fossils reveal activities through phys-ical evidence, such as bite marks on bonesand wear patterns on teeth. Also of greatvalue as trace fossils are coprolites, fos-silized feces. (Remains of a herbivore,such as Triceratops or Edmontosaurus, inT. rex coprolites certainly provide smok-ing-gun proof of species interactions!)

One assumption that paleobiologistsare willing to make is that closely relatedspecies may have behaved in similarways. T. rex data are therefore being cor-roborated by comparisons with those of

earlier members of the family Tyranno-sauridae, including their cousins Alberto-saurus, Gorgosaurus and Daspletosaur-us, collectively known as albertosaurs.

Solo or Social?TYRANNOSAURS are usually depictedas solitary, as was the case in JurassicPark. (An alternative excuse for thatfilm’s loner is that the movie’s genet-ic wizards wisely created only one.)Mounting evidence, however, points togregarious T. rex behavior, at least forpart of the animals’ lives. Two T. rex ex-cavations in the Hell Creek Formation ofeastern Montana are most compelling.

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In 1966 Los Angeles County Muse-um researchers attempting to exhume aHell Creek adult were elated to find an-other, smaller individual resting atop theT. rex they had originally sought. Thissecond fossil was identified at first as amore petite species of tyrannosaur. Myexamination of the histological evi-dence—the microstructure of the bones—now suggests that the second animal wasactually a subadult T. rex [see top illus-tration on page 28]. A similar discoverywas made during the excavation of“Sue,” the largest and most completefossil T. rex ever found. Sue is perhaps asfamous for its $8.36-million auctionprice following ownership haggling asfor its paleontological status [see “No

Bones about It,” by Karin Vergoth; Newsand Analysis, Scientific American,December 1997]. Remains of a subadultand a juvenile T. rex were later found inSue’s quarry by researchers from theBlack Hills Institute of Geological Re-search in Hill City, S.D. Experts whohave worked the Hell Creek Formation,myself included, generally agree that longodds argue against multiple, loner T. rexfinding their way to the same burial. Themore parsimonious explanation is thatthe animals were part of a group.

An even more spectacular find from1910 further suggests gregarious behav-ior among the Tyrannosauridae. Re-searchers from the American Museum ofNatural History in New York Cityworking in Alberta, Canada, found abone bed—a deposit with fossils of manyindividuals—holding at least nine of T. rex’s close relatives, albertosaurs.

Philip J. Currie and his team from theRoyal Tyrrell Museum of Paleontologyin Alberta have relocated the 1910 findand are conducting the first detailedstudy of the assemblage. Such aggrega-tions of carnivorous animals can occurwhen one after another gets caught in atrap, such as a mud hole or soft sedimentat a river’s edge, in which a prey animalthat has attracted them is already en-

snared. Under those circumstances, how-ever, the collection of fossils should alsocontain those of the hunted herbivore.The lack of such herbivore remainsamong the albertosaurs (and among thethree–T. rex assemblage that includedSue) indicates that the herd most likelyassociated with one another naturallyand perished together from drought, dis-ease or drowning.

From examination of the remainscollected so far, Currie estimates that theanimals ranged from four to almost ninemeters (13 to 29 feet) in length. This vari-ation in size hints at a group composedof juveniles and adults. One individual isconsiderably larger and more robustthan the others. Although it might have

been a different species of albertosaur, amixed bunch seems unlikely. I believethat if T. rex relatives did indeed have asocial structure, this largest individualmay have been the patriarch or matri-arch of the herd.

Tyrannosaurs in herds, with complexinterrelationships, are in many ways anentirely new species to contemplate. Butscience has not morphed them into a be-nign and tender collection of CretaceousCare Bears: some of the very testimonyfor T. rex group interaction is partiallyhealed bite marks that reveal nasty inter-personal skills.

A paper published by Currie andDarren Tanke, also at the Royal TyrrellMuseum, highlights this evidence. Tankeis a leading authority on paleopathol-ogy—the study of ancient injuries anddisease. He has detected a unique pat-tern of bite marks among theropods, thegroup of carnivorous dinosaurs that en-compasses T. rex and other tyranno-saurs. These bite marks consist of gougesand punctures on the sides of the snout,on the sides and bottom of the jaws, andoccasionally on the top and back of the skull.

Interpreting these wounds, Tankeand Currie reconstructed how thesedinosaurs fought. They believe that the

animals faced off but primarily gnawedat one another with one side of theircomplement of massive teeth rather thansnapping from the front. The workersalso surmise that the jaw-gripping be-havior accounts for peculiar bite marksfound on the sides of tyrannosaur teeth.The bite patterns imply that the combat-ants maintained their heads at the samelevel throughout a confrontation. Basedon the magnitude of some of the fossilwounds, T. rex clearly showed little re-serve in battle and sometimes inflicted se-vere damage to its conspecific foe. Onetyrannosaur studied by Tanke and Cur-rie sports a souvenir tooth embedded inits own jaw, perhaps left by a fellowcombatant.

The usual subjects—food, mates andterritory—may have prompted the vig-orous clashes among tyrannosaurs. What-ever the motivation behind the fighting,the fossil record demonstrates that thebehavior was repeated throughout a ty-rannosaur’s life. Injuries among youngerindividuals seem to have been more com-mon, possibly because a juvenile wassubject to attack by members of its ownage group as well as by large adults.(Nevertheless, the fossil record may alsobe slightly misleading and simply containmore evidence of injuries in young T. rex.Nonlethal injuries to adults would haveeventually healed, destroying the evi-dence. Juveniles were more likely to diefrom adult-inflicted injuries, and theycarried those wounds to the grave.)

Bites and BitsIMAGINE THE LARGE canine teeth ofa baboon or lion. Now imagine a mouth-ful of much larger canine-type teeth, thesize of railroad spikes and with serratededges. Kevin Padian of the University ofCalifornia at Berkeley has summed upthe appearance of the huge daggers thatwere T. rex teeth: “lethal bananas.”

Despite the obvious potential of suchweapons, the general opinion among pa-leontologists had been that dinosaur bite

26 S C I E N T I F I C A M E R I C A N U p d a t e d f r o m t h e S e p t e m b e r 1 9 9 9 i s s u e

Mounting evidence indicates that tyrannosaurs WERE NOT LONERS BUT MOVED IN GROUPS.


marks were rare. The few published re-ports before 1990 consisted of briefcomments buried in articles describingmore sweeping new finds, and the cluesin the marred remains concerning be-havior escaped contemplation.

Some researchers have nonethelessspeculated about the teeth. As early as1973, Ralph E. Molnar, now at the Mu-seum of Northern Arizona in Flagstaff,began musing about the strength of theteeth, based on their shape. Later, JamesO. Farlow of Indiana University–PurdueUniversity Fort Wayne and Daniel L.Brinkman of Yale University performedelaborate morphological studies oftyrannosaur dentition, which made themconfident that the “lethal bananas” wererobust, thanks to their rounded cross-sectional configuration, and would en-dure bone-shattering impacts duringfeeding.

In 1992 I was able to provide mater-ial support for such speculation. KennethH. Olson, a Lutheran pastor and superbamateur fossil collector for the Museumof the Rockies in Bozeman, Mont., cameto me with several specimens. One was aone-meter-wide, 1.5-meter-long partialpelvis from an adult Triceratops. Theother was a toe bone from an adultEdmontosaurus (duck-billed dinosaur).I examined Olson’s specimens and foundthat both bones were riddled with gougesand punctures up to 12 centimeters longand several centimeters deep. The Tricer-atops pelvis had nearly 80 such indenta-tions. I documented the size and shape ofthe marks and used orthodontic dentalputty to make casts of some of the deep-er holes. The teeth that had made theholes were spaced some 10 centimetersapart. They left punctures with eye-shaped cross sections. They clearly in-cluded carinae, elevated cutting edges, ontheir anterior and posterior faces. Andthose edges were serrated. The totality ofthe evidence pointed to these indenta-tions being the first definitive bite marksfrom a T. rex.

This finding had considerable behav-ioral implications. It confirmed for thefirst time the assumption that T. rex fedon its two most common contempo-raries, Triceratops and Edmontosaurus.

Furthermore, the bite patterns opened awindow into T. rex’s actual feeding tech-niques, which apparently involved twodistinct biting behaviors. T. rex usuallyused the “puncture and pull” strategy, inwhich biting deeply with enormous forcewas followed by drawing the teeththrough the penetrated flesh and bone,which typically produced long gashes. Inthis way, a T. rex appears to have de-tached the pelvis found by Olson fromthe rest of the Triceratops torso. T. rexalso employed a nipping approach inwhich the front (incisiform) teethgrasped and stripped the flesh in tightspots between vertebrae, where only themuzzle of the beast could fit. This meth-od left vertically aligned, parallel furrowsin the bone.

Many of the bites on the Triceratopspelvis were spaced only a few centimetersapart, as if the T. rex had methodicallyworked its way across the hunk of meatas we would nibble an ear of corn. Witheach bite, T. rex appears also to have re-moved a small section of bone. We pre-sumed that the missing bone had beenconsumed, confirmation for which short-ly came, and from an unusual source.

In 1997 Karen Chin, now at the Uni-versity of Colorado, received a peculiar,tapered mass that had been unearthed bya crew from the Royal SaskatchewanMuseum. The object, which weighed 7.1kilograms and measured 44 by 16 by 13centimeters, proved to be a T. rex copro-lite [see bottom illustration on nextpage]. The specimen, the first ever con-firmed from a theropod and more thantwice as large as any previously report-ed meat eater’s coprolite, was chock-fullof pulverized bone. Once again makinguse of histological methods, Chin and Idetermined that the shattered bone camefrom a young herbivorous dinosaur. T.rex did ingest parts of the bones of itsfood sources and, furthermore, partiallydigested these items with strong enzymesor stomach acids.

Following the lead of Farlow andMolnar, Olson and I have argued vehe-mently that T. rex probably left multi-tudinous bite marks, despite the paucityof known specimens. Absence of evi-dence is not evidence of absence, and webelieve two factors account for thistoothy gap in the fossil record. First, re-searchers have never systematically


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GREGORY M. ERICKSON is assistant professor of biological science at Florida State Univer-sity and has studied dinosaurs since his first expedition to the Hell Creek Formation bad-lands of eastern Montana in 1986. He received his master’s degree under John B. Hornerin 1992 at Montana State University and a doctorate with Marvalee Wake in 1997 from theUniversity of California, Berkeley. Erickson conducted postdoctoral research at Stanfordand Brown universities aimed at understanding the form, function, development and evo-lution of the vertebrate skeleton, with Tyrannosaurus rex as one of his favorite study ani-mals. He has won the Romer Prize from the Society of Vertebrate Paleontology, the StoyeAward from the American Society of Ichthyologists and Herpetologists, and the Davis Awardfrom the Society for Integrative and Comparative Biology.

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BITE-FORCE graph shows that T. rex is the undisputed champion. The author, working with bioengineerDennis R. Carter of Stanford University, simulated the production of feeding bite marks, which aretypically less than full strength, using a cast of a T. rex tooth on cow pelvises. They made a co

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