Palaeogeography, Palaeoclimatology, Palaeoecology, 63 (1988): 183 199 183 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

VERTEBRATE PRESERVATION IN FLUVIAL CHANNELS

A N N A K. B E H R E N S M E Y E R

Department of Paleobiology, NHB-E207 MRC 121, Smithsonian Institution, Washington, DC 20560 (U.S.A.)

(Received July 21, 1987)

Ab str ac t

Behrensmeyer, A. K., 1988. Vertebrate preservation in fluvial channels. Palaeogeogr., Palaeoclimatol., Palaeoecol., 63:183 199.

Two taphonomic modes for attritional vertebrate assemblages in channels are proposed, based on the sedimentary context of the vertebrate remains and taphonomic features of the bones themselves. The channel-lag mode includes bones that are buried with coarse lithologies near the bases of active channels. The channel-fill mode occurs in fine- grained to mixed fills of abandoned channels. The extreme for a channel-lag assemblage would be a cluster of allochthonous, abraded, unidentiflable fragments, and the extreme for a channel-fill assemblage would be a cluster of autochthonous, unbraded, complete skeletons. Between these extremes there is a broad spectrum of possible taphonomic histories for bones in channels, but distinct channel-lag vs. channel-fill modes can be recognized in fluvial deposits in different tectonic and climatic settings throughout the Phanerozoic. Physical and biological processes that affect the different modes produce different samples of vertebrate paleocommunities, with bones in the channel-lag mode representing transported remains from a variety of habitats, whereas channel-fill assemblages are more autochthonous and habitat-specific.

Channel facies, channel pattern, and alluvial architecture are used to develop hypotheses concerning how the taphonomic modes relate to different scales of fluvial processes. Fluvial systems with numerous abandoned channels provide more sites for preservation of relatively complete fossil vertebrates in channel-fills, while systems that continually rework sediments by lateral migration preserve more vertebrate remains as channel-lags. Large-scale physical controls on channel pattern and fluvial architecture probably have had significant effects on the quality and quantity of the verrtebrate record throughout the history of land vertebrates.

Taphonomic modes provide a basis for comparing faunas with similar preservational histories throughout the geologic record, and they can help to minimize biases in important paleobiological parameters such as diversity estimates and the timing of appearance and extinction events.

I n t r o d u c t i o n c h a n n e l s r e c u r i n d i f f e r en t rock s e q u e n c e s

t h r o u g h o u t the h i s t o r y of l a n d v e r t e b r a t e s . A s i g n i f i c a n t p a r t of t he v e r t e b r a t e fossi l A t a p h o n o m i c mode is def ined he re as a

r e c o r d o c c u r s w i t h i n f luv ia l c h a n n e l depos i t s r e c u r r i n g p a t t e r n of p r e s e r v a t i o n of o r g a n i c

a n d has b e e n a f fec ted by s e d i m e n t a r y p rocesses r e m a i n s in a p a r t i c u l a r s e d i m e n t a r y con t ex t ,

a s s o c i a t e d w i t h c h a n n e l f o r m a t i o n a n d deposi- a c c o m p a n i e d by c h a r a c t e r i s t i c t a p h o n o m i c

t ion . C h a n n e l s c a n be cu t g r a d u a l l y or i n s t a n - f ea tu res . The goa l of th i s p a p e r is to deve lop

t a n e o u s l y , t h e y c a n m o v e l a t e r a l l y or ver t i - t he h y p o t h e s i s t h a t two t a p h o n o m i c modes ,

c a l l y t h r o u g h t ime, a n d t h e y e v e n t u a l l y a re " c h a n n e l - l a g " a n d " c h a n n e l - f i l l ' '1, o c c u r w i t h fi l led w i t h s e d i m e n t s r a n g i n g f rom coa r se

c o n g l o m e r a t e to m u d a n d p l a n t debr i s . T h e s e p rocesses r e s u l t i n a wide r a n g e of t a p h o n o m i c 1Channel-fill and channel-lag will be hyphenated when

referring to the taphonomic modes but will not be histories for bones preserved in channel de- hyphenated when used in a more general sedimentological posi ts . Some of the p a t t e r n s of p r e s e r v a t i o n i n sense, e.g., "channel filling", "channel lag deposits".

0031-0182/88/$03.50 © 1988 Elsevier Science Publishers B.V.

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different frequencies in different types of glomeratic lenses (e.g., Olson, 1962; Carroll fluvial regimens. These two modes represent et al., 1972; Hlavin, 1972; Behrensmeyer, 1975; end points on a range of possible taphonomic Cross et al., 1979; Salisbury, 1982; Eberth and histories from transported attri t ional bone Berman, 1983; Badgley, 1986), dispersed, disar- assemblages to untransported attr i t ional as- ticulated remains (e.g., Wolff, 1973; Badam and semblages. They are not all-inclusive, and Ganjoo, 1986); mass accumulations of bones other modes could be defined based on descrip- (e.g., Lawton, 1977), and isolated skeletons or tive criteria (e.g., density of fossil material), partial skeletons (e.g., Olson, 1962; Gradzinski, inferred causes of death (e.g., catastrophic vs. 1970; Dodson, 1971; Gradzinski and Jerzyk- attritional), or other characters. The channel- iewicz, 1974). Fossils are also found in channels lag and channel-fill modes (as well as other filled with poorly sorted mixtures of mudclast modes not discussed in this paper) preserve conglomerates, sand and mudstone (e.g., Voor- different types of biological and ecological hies, 1969; Hunt, 1978; Stewart, 1981; Berman information, resulting in biases that affect et al., 1985; Behrensmeyer, 1987) and finer- interpretations of vertebrate evolution, extinc- textured sediment including organic debris tion, and paleocommunity structure. (e.g., Hook and Ferm, 1985). The vertebrate

remains found in examples given above range V e r t e b r a t e b o n e s in c h a n n e l s from abraded fragments to undamaged whole

skeletons. Paleontologists have long recognized the Obviously processes associated with chan-

association between vertebrate fossils and nels can promote fossil preservation, but the channel deposits. Rapid burial by energetic wide variation in sediment type and bone flow in channels is an obvious way to preserve condition indicates that such processes do not bones, although it also may damage them and always involve energetic currents and rapid reduce their value as paleontological speci- sedimentation rates. Within the overall chan- mens. Preservation in channel deposits is often nel context, there may be a range of tapho- taken as an indication that carcasses and/or nomic histories linked to different patterns of individual bones experienced substantial channel formation and filling. Prior generali- t ransport prior to burial, and that this period zations concerning the taphonomy of bones in of transport left its signature on the composi- channels need reexamination based on theoret- tion of the bone assemblage. The assumption ical considerations and a range of examples that bones in channels usually are transported from the fossil record. also implies that they represent mixtures of Part icular fluvial formations often appear to animals from different habitats, and this have characterist ic patterns of vertebrate pres- affects how the preserved vertebrates are used ervation that persist through significant in paleoecological reconstructions (Shotwell, periods of time. In some stratigraphic se- 1958; Behrensmeyer, 1982). quences fossils occur in both channel and

A review of l i terature on fossil vertebrates overbank lithofacies (e.g., Olson, 1962; found in channels shows that there is wide Behrensmeyer, 1975; Dodson et al., 1980; Bad- variabili ty in lithologies associated with bone gley, 1986); in others remains are found almost assemblages as well as in their taphonomic exclusively in overbank deposits (e.g., Smith, features. Often bones are found as part of~'lag '' R . M . H . , 1980; Bown and Kraus, 1981; Kraus deposits, defined here as winnowed and sorted et al., 1985) or exclusively in channels (e.g., residues composed of relatively large or heavy Gradzinski, 1970; Dodson, 1971; Salisbury, particles that are above the threshold compe- 1982). Such patterns suggest large-scale sedi- tence (transporting ability) of local currents, mentological and/or taphonomic controls on Bones from sand and gravel channel deposits how vertebrates are preserved in different include lag assemblages associated with con- fluvial systems.

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In the following paper, relationships be- erosion. The latter are characteristic of chutes, tween fluvial environments and vertebrate crevasse splay channels, and some anastamos- assemblages in the Siwalik sequence of Paki- ing channels (Friend et al., 1979; Smith, D. G. stan and the Permian deposits of Texas will be and Smith, N.D., 1980; Smith, D.G. and used to construct a general hypothesis con- Putnam, 1980; Bridge, 1985). cerning sedimentological controls on the oc- The depositional phase of a channel results currence of channel-lag and channel-fill modes in the sediments and structures used to charac- in the vertebrate record. Prior to description of terize its flow, although these (including or- the specific examples from Pakis tan and Texas, ganic remains) may represent only the later the following introduction to channel pro- phases of activity when currents are depositing cesses is offered as a basis for understanding more than they remove from a particular how such processes can affect the occurrence reach. Bar structures formed of relatively and frequency of the two different taphonomic coarse sediment are characteristic of laterally modes, migrating channels with sustained or frequent

high-energy flow, while both coarse and fine- Channe l depos i t s grained sediments occur as channel fill de-

posits in abandoned channels (Fisk, 1944, 1947; Channel deposits bear several classes of Bridge, 1985). In stratified fluvial deposits, a

information that pertain to different scales of sediment package that fills a U-shaped trough processes operating in fluvial environments: and/or forms a distinct lens indicates an (1) sedimentary textures and structures that abandoned channel. Sandy fills are evidence record the rate and mode of local deposition, for gradual abandonment, with the current (2) evidence for original channel patterns in maintaining its capacity to transport sand sedimentary structures and overall geometry, until filling is complete. In contrast, fine- which reflect the balance of sediment input, grained channel fills indicate sudden abandon- slope, and other factors at a part icular point in ment and either a drastic reduction in the time, (3) evidence for longer-term patterns of energy needed to transport coarser sediment or basin subsidence in the preserved shapes of a barrier (e.g., vegetation) to the supply of channel deposits and their occurrence in sediment (Bridge, 1985). Mixed fills generally stratigraphic sequences. Channel facies, chan- fine upward and reflect gradual abandonment nel patterns, and the geometry or "architec- with periods of energetic flow (as during floods) ture" of the channel deposits all can be used to alternating with quiet water deposition or relate fluvial processes to different modes of paleosol formation within the channel. There vertebrate preservation, is a complete spectrum of channel fills between

The cutting of a channel by energetic flow the fine and coarse end-members (Fig.l), and a and its subsequent filling with sediment repre- single abandoned channel segment may have sent two distinct, potentially independent sand fills at either end and clay fill in its middle phases of sedimentary history. The shape of section (Smith, D. G., 1983). channel deposit in a stratigraphic sequence Channel patterns characterize the fluvial reflects the mode of erosion. Sheet sands result regimen, which ultimately is controlled by from sustained bank cutting and deposition by climatic and tectonic processes. Slope, dis- meandering, braided, or anastomosing streams charge, sediment load and vegetation all are in tectonic settings where there is a low rate of known to affect channel patterns (Leopold subsidence (Bridge, 1985). Ribbon sands occur et al., 1964; Schumm, 1977; Baker, 1978; Rust, when vertical down-cutting is combined with 1981). In general, meandering rivers are associ- lateral erosion within a restricted channel ated with low slope and low sediment loads belt, and shoe-string sands typically result while anastomosed and braided rivers reflect from down-cutting events with minimal lateral increased slope and higher sediment loads. All

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LITHOLOGY SEDIMENTATION PATTERN

A.

S a p r ~ l ~ TIH£ )

B.

Co

D.

Fig.1. A generalized model for channel fills, showing a progression from finer to coarser-grained deposits from A through D. Hypothetical graphs to the right show the pattern of sedimentation in relation to time, with finer-grained deposits representing slow, relatively steady deposition. Coarser deposits reflect sporadic erosion and deposition by active currents and more rapid channel filling overall. Channel fills comparable to B-D (but not A) occur in the Siwalik deposits of Pakistan and fills comparable to A-D occur in the Lower Permian deposits of Texas. Stippling = sand, gray = sandy to clayey silt, black = clay or coal.

types of r ivers can deposi t sheet sands when 1985; Smith, D.G. , 1986). Such systems are condi t ions favor sus ta ined la te ra l agg rada t ion cha rac te r i zed by f requent avuls ion and chan- (e.g., in s table to slowly subsiding basins) nel abandonment , resu l t ing in t rough-shaped (Bridge, 1985; Kraus and Middleton, 1987). In shoes t r ing deposits wi th coarse to f ine-grained subsiding basins, the same r ivers can form a fills. P rese rved segments of meande r ing chan- di f ferent type of f luvial a r c h i t e c t u r e (Allen, nels are k n o w n in both fluvial and del ta ic 1978), wi th d iscre te channe l bel t (i.e., r ibbon) se t t ings (Elliot, 1965; Gardner , 1983; Smith, sand bodies dispersed t h r o u g h ove rbank de- R . M . H . , 1987), and in s i tua t ions where deposi- posits (Bridge, 1985; Kraus and Middle ton , t ion and subsidence were affected by local 1987). It appears tha t wha t eve r the or ig inal s t ruc tu ra l cont ro l (Hook and Ferm, this issue). channe l pa t te rn , more sheet- l ike channe l de- Loca l cycles of avuls ion are in t e rp re t ed as the posits will be p reserved in a reas of slow cause of abandoned s inuous channe ls wi th subsidence, whi le more d iscre te r ibbon or mixed to f ine-grained fill in fluvial sequences shoes t r ing channe l bodies will be p reserved in from areas of modera te subsidence (Hopkins, areas of increased subsidence (e.g., K raus and 1985; Gordon and Bridge, 1987; Behrensmeyer , Middleton, 1987). 1987).

Anas tomos ing channe l pa t t e rns of ten are Sedimentologis ts have t r ad i t iona l ly used associa ted wi th rapidly subsiding basins or ver t i ca l profiles and sed imenta ry s t ruc tu re s to o the r s i tua t ions where ver t i ca l agg rada t ion is classify r iver pa t te rns as meande r ing or dominan t (Fr iend et al., 1979; Smith, D. G. and bra ided (Vischer, 1965; Miall , 1978). However , Smith, N.D. , 1980; Smith, N .D. and Cross, i t is now appa ren t t ha t s imilar ve r t i ca l se-

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quences and sedimentary structures can result laterally discontinuous, fine-grained litho- from different kinds of channel patterns (Miall, facies indicating a mosaic of localized deposi- 1980, 1984). Consequently, there is new empha- tional settings such as ponds. The sheet sands sis on using lateral control in channel deposits are interpreted as channel deposits of a major to establish the characterist ics of ancient river (on the scale of the modern Indus or rivers (Bridge and Gordon, 1985). Analysis of Ganges), and the facies along their upper fluvial systems also has expanded from the surfaces as infillings of depressions left after study of single channels to the interrelation- channel avulsion (Behrensmeyer and Tauxe, ships of multiple channel and overbank se- 1982; Behrensmeyer, 1987). quences, or alluvial architecture (Allen, 1978; Ribbon sands occur within the fine-grained Bridge and Leeder, 1979; Allen and Williams, deposits and vary in frequency throughout the 1982; Miall, 1987). different formations in the Siwalik Group

(Behrensmeyer, 1987). These sands are usually Siwal ik c h a n n e l depos i t s single-storied and show little evidence for

point-bar accretion or lateral erosion of flood- Types of channels plain sediments. Their widths range from tens

to hundreds of meters and thicknesses from < 1 Throughout the sequence of Miocene forma- to 10 m. They typically have trough-shaped

tions in northern Pakistan, which spans lower contacts and poorly preserved bedding approximately 12 m.y., there are two distinctly structures. Upper contacts are gradational different types of sand bodies (Fig.2). Sheet into silts and clays, often with root traces and sands with thicknesses between 6 and 20m other paleosol features. At the edges of the alternate with thicker sequences of fine- trough, the upper part of the channel sand may grained deposits. Internal stratification indi- pass laterally into sandy levee facies, indicat- cates complex, large-scale bar structures of a ing in situ vertical aggradation. These chan- braided or anastomosing, sand-dominated nels include lenses of carbonate and mudclast river. The upper parts of these sheets include conglomerate, but mud-drapes and other fine-

L o w e r C h i n j i F o r m a t i o n - 1 3 . 1 m.y .

50 ~,, : : : : , . :=, : : : . : . : . : . : . : . : : :~: ." . . : : : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . :

0 m . . . . . . . . . . . . ~ . . . . . . . ; - , . , :.:- . . . . . .

0 1.0 km. [ I I ~ V e r t e b r a t e L o c a l i t i e s

C h a n n e l Lag F a c i e s

Channe l Fi l l F a c i e s

Fig.2. Example of fluvial lithofacies in the lower part of the Siwalik deposits of the Potwar Plateau, Pakistan. Vertebrate localities are concentrated in the middle part of the fill of a large.scale abandoned channel, but also occur in smaller channel fills, in channel lag facies, and in other contexts in the overbank deposits. Size of bone icon is roughly proportional to number of fossils at each site. Coarse stippling indicates major sheet sandstones, white represents overbank lithologies.

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grained sediments are rare. They are inter- homogeneous and lack distinct bedding. Adja- preted as crevasse-splay channels that were cent floodplain deposits also may have a well- cut and filled by short-term pulses of overbank developed paleosol at the same horizon as the flow which did not cause lateral channel less mature paleosol capping the channel fill movement through time. Fossil vertebrates (Fig.2). occur in abundance in lenses of conglomerate, in the upper, fine-grained parts of the channel, Sedimentary context and taphonomy of and in adjacent levee deposits, but almost vertebrate remains never within the channel sands themselves.

Fine-grained and mixed channel fills occur Throughout the Siwalik sequence, verte- at a variety of scales, from small trough-shaped brate fossils occur in low frequencies in sheet lenses a few meters in width to complex sands deposited by the major channel belts. deposits overlying large-scale, irregular ero- Isolated bones, teeth, and bone pebbles are the sional depressions 1 km or more in width, rule, al though there are rare occurrences of normal to current direction (Fig.2). The larger- partial skeletons or skulls of larger animals. scale erosional features are interpreted as Typically, bones in this context are fragmen- "failed avulsions", in which flow from the tary and abraded. In contrast, some of the major river eroded very large crevasse chan- richest fossil localities occur in fine-grained nels during floods but then failed to shift facies that fill depressions on the tops of the course into these channels (Behrensmeyer, sands. These assemblages include unabraded 1987). Analogous large, abandoned channels skeletal material from a wide range of taxa and occur in braided river systems presently drain- body sizes, with taphonomic features similar to ing the Himalaya Mountains (Gole and Chi- those in the channel fills described below. tale, 1966; Holmes, 1968; Coleman, 1969). The Fossils can be extraordinarily abundant in large Siwalik channel fills have discontinuous fine-grained channel fills, and there is a basal conglomeritic lags and some coarse sand marked pattern of association with the middle lenses, but the predominant fill is fine sand and to upper parts of these fills rather than with clayey silt, often with complex cut-and-fill basal units (about 75°//0 of the channel-fill fossil bedding. Thin lenses of mud- and carbonate- occurrences follow this pattern; Behrens- clast conglomerate within the finer facies meyer, 1987). Concentrations of bones and alternate with weakly developed paleosols, bone fragments occur in thin nodule-clast and indicating sporadic flow in the abandoned mudclast conglomerates and in bioturbated channel. The degree of bioturbation increases units of mixed clay, silt, and fine sand. The upward, and the channel fills typically are former are often size-sorted and include capped by silty clays that lack bedding and abraded material, while the latter are charac- have more mature paleosol features. Smaller terized by articulated, undamaged skeletal scale channel fills are similar, but less complex parts. Micro- and macro-vertebrates occur internally and finer-grained overall, together in both of these facies.

Fine-grained channel fills are very similar to Bones larger than 1 cm (maximum diameter) floodplain facies, especially in the large-scale occur at densities up to 14/m 2 in excavated abandoned channels. Establishing the bottom samples from the channel fill deposits. These and at least one edge of the erosional trough is show extensive pre-burial breakage and sur- the surest way to confirm the existence of an face scratching indicating the combined effects abandoned channel. In the absence of well- of carnivore activity and trampling (Behrens- established geometry, local textural complex- meyer et al., 1986; Behrensmeyer et al., in ity and well-preserved bedding usually differ- press). The subvertical orientation of a num- entiate the channel fill facies from laterally ber of the excavated specimens is added contiguous floodplain deposits, which are more evidence that trampling was an important

189

process in bone modification and burial supported by comparing the taphonomy of (Behrensmeyer et al., 1986; Behrensmeyer et organic concentrations from different tempo- al., in press). Variable orientations of bones in ral or physical settings. the conglomeratic facies do not suggest strong, Deposits of the Wichita and Clear Fork unidirectional flow, and the presence of skele- Groups in central Texas represent fluvio- tal parts with a wide range of hydraulic deltaic environments where vertebrates lived equivalences also implies that the assemblages and died over a considerable span of time in the were not subjected to extensive current action. Early Permian (Case, 1915; Romer, 1935, 1958; Rather, it appears that bones accumulated Olson and Beerbower, 1953; Olson, 1958; Dal- through attri t ional processes within the aban- quest and Kocurko, 1986). Associations of doned channels; some were transported short faunas and subenvironments in the Clear Fork distances, winnowed, and concentrated by Group provided the basis for E.C. Olson's sporadic flow, and others were buried without pioneering work in vertebrate community evo- transport by the vertical build-up of fine- lution (Olson, 1952, 1976, 1977; Olson and grained sediments and by trampling into soft Mead, 1982). More recently, paleoecological as substrates, well as taxonomic history has been extended

The greatest number of the paleontologically downward into the Wichita Group by N. Hot- important localities in the Potwar region occur ton (pers. comm., 1984). The Permian terres- in middle, upper, and occasionally lower parts trial facies are predominantly red siltstones of fine-grained channel fills. Vertebrate fossils and mudstones, with some formations charac- can occur in fine-grained floodplain deposits terized by increased amounts of sandstone. throughout the Siwalik Group, but they are There are occasional thin, intercalated units of relatively uncommon in this context. According marine limestones, indicating that the overall to data assembled by Badgley for the Dhok setting was much closer to deltaic and marine Pathan Formation (Badgley, 1982: table6-3), environments than the Siwalik sequence. only 7% of the recovered vertebrate fossils Moreover, the tectonic setting was quite differ- occur in the floodplain context, while 520//o are ent; Permian fluvial deposits accumulated on a from channels or channel margins. The remain- stable craton whereas the Siwalik sequence der are associated with mud- or carbonate-clast resulted from the collision between India and conglomerate units which probably represent Asia. In spite of these different settings, local crevassesplay sheets or shallow crevassesplay sedimentary processes deposited similar fluvial channels. This pattern appears to hold for the facies representing coarse and fine-grained Siwalik sequence in general in the Potwar channel fills, floodplains, and levees. Plateau region (Raza, 1983). Channel deposits in the Belle Plains and

Arroyo Formations (Wichita Group) include The P e r m i a n depos i t s o f Texas sheet sands and small-scale (<102m wide)

abandoned channels with fine-grained to mixed The following observations are based on a fills. Sheet sandstones are better exposed and

brief survey of notable exposures and verte- apparently more abundant in the Arroyo brate localities in Seymour County, Texas. Formation and are characterized by well- While the data are limited compared to what is preserved lateral accretion surfaces indicative available for the Siwalik vertebrate record, of large-scale (> 102 m wide) meandering chan- there are similarities in the patterns of bone nels (Edwards et al., 1983). Vertebrate fossils occurrence across approximately 260 million are uncommon in the sheet sands, as also noted years, in markedly different biotic and tectonic for overlying formations (Olson, 1962). settings. The Permian examples also demon- Fine-grained channel fill deposits in the strate the value of comparative taphonomy, in lower Wichita Group were first reported by which general patterns of preservation are Case (1915). They are often difficult to distin-

190

A.

~1 m.[

~ 1 0 rn.

B.

~1 m.[

~ I 0 m,

Fig.3. Two examples of channel-fill contexts for fossil vertebrates from the Lower Permian deposits of Texas. A. The Craddock Bone Quarry in the Arroyo Formation, Seymour County, showing fill of channel (probably a chute) cut into point- bar deposits of a major sheet sandstone. Unfossiliferous carbonate nodule conglomerate occurs at the base, and the rest of the fill consists of red to purple mudstone with pedogenic carbonate nodules and rare lenses of sand. Well-preserved vertebrate material occurs in the lower 1.5 m of the channel-fill. B. Channel-fill in the Belle Plains Formation, Seymour County, showing context of size-sorted vertebrate material associated with thin nodule conglomerates within fine sands and silts. Basal unit is a purple silty clay, with minor sand at the base. Channel is cut into red overbank silts.

guish f rom s u r r o u n d i n g f loodpla in deposi ts occu r in the d r a b b e r m u d s t o n e s and m a y be and m a y be more a b u n d a n t in the redbed in te rbedded wi th the v e r t e b r a t e - b e a r i n g con- sequences t h a n they appear . Typ ica l ly the g l o m e r a t e s (a r a r e i n s t ance of co-occur r ing lower con tac t s can be de t e rmined by a lag p l an t and v e r t e b r a t e r ema ins in these s t ra ta) . c o n g l o m e r a t e of mudc la s t s and c a r b o n a t e nod- In c o n t r a s t to the c o n g l o m e r a t e s wi th in the ules, which defines the e ros iona l t rough . Fi l ls c h a n n e l fills, lags a t the bases of channe l s v a r y f rom d rab gray, purp le or red m u d s t o n e s gene ra l l y l ack v e r t e b r a t e remains . to in t e rbedded fine sands and sil ts wi th well- V e r t e b r a t e fossils occur in o the r con tex t s p rese rved bedding (Fig.3). Thin , d i s con t i nuous wi th in the o v e r b a n k depos i t s of the Belle beds of l imoni t i c c o n g l o m e r a t e wi th mud and P la ins and Ar royo Forma t ions , inc lud ing c a r b o n a t e c las t s o c c u r a t va r i ous levels wi th in " c l u s t e r s " of a r t i c u l a t e d ind iv idua ls of the these c h a n n e l fills. M u d s t o n e s a re typ ica l ly same t a x o n in f loodpla in facies and the occa- b i o t u r b a t e d and occas iona l ly p r e se rve root- s ional pa r t i a l ske le ton or i so la ted bone wi th in t r aces and b u r r o w s ind ica t ive of the in i t ia l the shee t sands tones . The p a t t e r n s obse rved in phases of soil fo rmat ion , these fo rma t ions a re b road ly s imi la r to Olson ' s

Fossi l v e r t e b r a t e s occur in the th in conglom- desc r ip t ions of fossil o c c u r r e n c e s in the over- e ra t e s w i th in channe l fills, and the r e m a i n s ly ing Lower P e r m i a n depos i t s of Texas and typ ica l ly a re f r a g m e n t a r y , size-sorted, and in O k l a h o m a (Olson and Beerbower , 1953; Olson, v a r y i n g s t a t e s f rom f resh to abraded . Well- 1958, 1962). The a s soc i a t i on of bone assem- p rese rved bones occu r in some of the mud- b lages wi th smal l -scale a b a n d o n e d channe l s tones and are cha r ac t e r i z ed by a r t i cu l a t ed or deposi ts a lso has been d o c u m e n t e d for compar- a s soc ia ted pa r t i a l ske le tons , a wide r a n g e of able s t r a t a on the wes t e rn side of the Lower body and bone sizes, and gene ra l ly fresh, P e r m i a n s e a w a y in New Mexico (Eber th and u n a b r a d e d bone surfaces . P l a n t r e m a i n s a lso Berman , 1983; B e r m a n et al., 1985).

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Similarities to the Miocene examples of cal processes such as transport and trampling vertebrate fossil occurrences include: (1) the affect vertebrate remains, but overlap in their tendency for the best-preserved assemblages to taphonomic attributes is to be expected. be within channel fills, (2) the pattern of The channel-lag mode refers to bones in the association with fine-grained deposits above lower part of an erosional channel feature the basal-lag conglomerate, (3)the occurrence which are in direct association with coarse of size-sorted fragmentary material in thin clastic material (TableI). The base of the nodule and mudclast conglomerates within the channel may be eroded into fine-grained de- channel fills, and (4) preservation of associated posits or into previous channel deposits, as in skeletal material in fine-grained units. Tapho- local scours or multistoried sand bodies. Chan- nomic features of bones associated with the nels may be of any size but are filled with sand finer versus the coarser facies of the channel or coarser material and usually represent the fills also appear to be similar in the Permian more continuously active rivers in a drainage and Miocene examples. Although the physical basin. Bones have taphonomic features (e.g., taphonomic processes associated with these abraded edges and processes, size-sorting) indi- fluvial facies probably have remained fairly cating that they have experienced sustained constant through time, biological processes interaction with moving water and sediment. A (e.g., scavenging)undoubtedly have changed, model for the formation of this mode in Detailed comparisons of patterns of breakage meandering channels has been proposed pre- and other features in the Permian bone assem- viously (Behrensmeyer, 1982). blages and the excavated Miocene samples The channel-fill mode refers to bones pre- must await further taphonomic work in the served in mixed to fine-grained deposits that Permian deposits, fill a channel after it has been abandoned by

sustained, active flow. Usually such deposits occur in the middle to upper parts of a channel

P a t t e r n s of v e r t e b r a t e p r e s e r v a t i o n in c h a n n e l s fill, al though they also may be found immedi-

ately above the basal lag. Vertebrate material can be associated with thin beds of coarse

The channel-fill and channel-lag modes of clastics, especially mudclast or nodule con- preservation in channels are based on docu- glomerates, or with mudstones and clays. A mentation of vertebrate occurrences in the characteristic of channel-fill assemblages is Pakistan sequence, with supporting evidence that taphonomic features are highly variable, from the Texas Permian (Table I) and pub- but there is a larger component of fresh, well- lished studies of vertebrate occurrences in preserved, material (e.g., unabraded and un- channels. These modes are defined primarily sorted bones, articulated skeletons) than in on overall sedimentary context and secondar- channel-lag assemblages. ily on taphonomic features of the bone assem- Potential overlap between these two modes blages. Sedimentary context appears to differ- exists when a sand- or gravel-filled channel entiate them rather clearly, while there is occurs within the overall context of an aban- considerable overlap in taphonomic features, doned channel that has predominantly fine- This overlap may be due in part to present lack grained fill, or vice-versa. In such cases, the of detailed information on the taphonomy of relative scale of the two modes must be bone assemblages from the different sedimen- assessed, along with which depositional context tary contexts. However, it also reflects the fact is most relevant to the genesis of the fossil that similar processes interact with bones in occurrences. both channel-lag or channel-fill contexts. Par- The common association of fossils with the ticular channel environments may shift the middle parts of the Siwalik Miocene and Texas degree to which specific physical and biologi- Permian channel fills indicates that conditions

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TABLE I

Characteristics of two taphonomic modes in channel vertebrate assemblages composed of attritional skeletal remains (i.e., accumulated gradually over periods of 102-104 yr, not due to single-event mass deaths)

Channel-lag Channel-fill

Sedimentary context Large-scale Lower parts of channels or erosional Above basal lags, usually in middle to

troughs upper parts of channels

Small-scale Basal lag deposits, scour pockets, Discontinuous, thin, coarse beds, channels within channels thicker fine-grained units

Lithology Sands, gravels, mudclast and nodule Mudstones, silts, clays, fine sands, conglomerates nodule conglomerates

Taphonomic attributes Sorting Larger, heavier, robust elements more Size-sorting in coarser sediments;

common (e.g., jaws, teeth); usually variable to poor sorting otherwise well-sorted

Abrasion Edges often rounded, bone pebbles Edges fresh to rounded, usually common fresh in mudstones

Fragmentation Variable; usually broken parts Variable; more complete in finer sediments

Associated Skeletal Rare Variable; more frequent in mudstones Parts

Orientation Commonly aligned with paleocurrent; Variable alignment with paleocurrent; usually horizontal random in mudstones; often at angles

to a horizontal plane

Body Sizes Variable; large usually more common A wide range usually present, including microfauna

Interpretative notes Bones usually allochthonous; may Bones at death site or transported represent large areas of the drainage short distances; most are autochthonous basin with respect to the local channel

Channels represent active drainages Channels are abandoned and have with recurring energetic flow and sporadic, waning flow with minor reworking of banks and bedload reworking of bank and bedload sediments

often favored bone concen t r a t i on dur ing wan- the al luvial plain ( Jarman, 1972; G. Haynes , ing r a the r t han act ive phases of channe l pers. comm., 1986), (3) localized t ranspor t , act ivi ty. Absence of bones in basal conglom- sort ing, and hydrau l i c concen t r a t i on of bones, era tes and sands implies t ha t they were no t such as migh t occur dur ing sporadic floods, (4) being concen t r a t ed dur ing the cu t t ing or sed imenta t ion pa t te rns charac te r ized by init ial filling s tages of the channel . Circum- periods of non-deposi t ion when a t t r i t iona l s tances tha t could promote bone concen t r a t i on remains could accumula te , a l t e rna t ing wi th and bur ia l in abandoned channe l fills include: periods of re la t ive ly rapid deposi t ion tha t (1) topograph ic lows l ikely to receive and would bury bone-r ich horizons, (5) low rates of pro tec t o rganic remains, (2) concen t r a t ions of des t ruc t ion by soil o rganisms and chemical animals nea r abandoned channe ls due to dissolut ion because of the combined effects of localized avai labi l i ty of food and water , partic- rapid bur ia l and (perhaps) anaerobic condi- u lar ly dur ing times of res t r ic ted resources on tions. Bone preserva t ion migh t also be en-

193

hanced by local concentrations of nutrients land surfaces. If sedimentation is too rapid, such as Ca and P from increased biotic activity there will be insufficient time for bones to in this environment, accumulate, and they are likely to be dispersed

In modern environments, inactive channels or damaged by repeated interaction with other often harbor bodies of standing water and sedimentary particles and high-energy cur- dense patches of vegetation (Fisk, 1947; Jar- rents. The timing of sedimentation events also man, 1972; Gagliano and Howard, 1983), mak- is important, and there may be an ~'on-off" ing them attractive places for herbivores and periodicity at particular stages of channel predators. Attri t ional bone assemblages would filling which is optimal for bone preservation. be expected in such situations, and periods of All of the factors enumerated above can low sedimentation could result in high bone occur in non-channel environments, but appar- densities on and in soils developed within the ently at least some of them were more fre- channel fill. Trampling by animals within the quently associated with abandoned channels abandoned channels would contribute to disar- in the Miocene and Permian fluvial deposits. t iculation and breakage of bones but also For these examples, it is not yet possible to say would enhance burial in soft substrates, which factors were most important in creating

Abandoned channels have a pattern of the observed taphonomic patterns. However, clastic sedimentation that might be more the scarcity of bones in paleosols that cap conducive to vertebrate preservation than channel deposits and in floodplain paleosols in other fluvial sub-environments, at least in Miocene and Permian examples suggests that some systems (for a contrasting situation with they were less likely to be preserved where the low clastic input, see Hook and Ferm, this more mature soils developed in these fluvial issue). Given constant input of bones, there systems. This implies that rate and mode of must be a balance between sedimentation and sedimentation is one of the most critical rates of bone dispersal and decomposition to factors promoting preservation in channel fills. create a bone concentration (Fig.4). If sedimen- The channel-fill mode of fossil occurrence tation is too slow, bones will decompose faster persists throughout the Miocene sequence in than they can be buried, or they will be northern Pakistan and transcends major destroyed after burial as soils mature on stable changes in the fluvial systems during this

period of time (Behrensmeyer and Tauxe, 1982; Behrensmeyer, 1987). Therefore it appears that ASSUME CONSTANT BONE INPUT : conditions favoring bone preservation are

k Bo,~ TooO~.,.0, .... Too O ........ linked more strongly to local processes associ-

T ~ - ated with abandoned channels than to the ,at. o, ~ . - ~ overall fluvial regimen. This also is supported

S e d i m e n t O ...... B ...... by the occurrence of the channel-fill mode in Accumulation ~ ~ / of Bone Input

a,,S ..... the Permian deposits of Texas, which were formed by a different fluvial system in a different tectonic (and probably a different ~ Bones Decornpo~eBones Decornpo~e

~asterThanTl~ey~eBuried climatic) setting. However, the overall verte- brate record in the Siwalik sequence is also Quality and Quantity

of P . . . . . . . d B . . . . ~ affected by the number of abandoned channels

Fig.4. Hypothet ical relat ionship between rate of sediment that occur at different stratigraphic levels accumulat ion and the quali ty of the ver tebra te record for (Behrensmeyer, 1987), and this appears to be at t r i t ional assemblages in channel deposits. Rate on the y- related more directly to the fluvial system and axis refers to individual sedimentary uni ts tha t preserve its tectonic setting. bones; absolute values for optimal rates of sediment accumulat ion for bone preservat ion would depend on bone Paleoecological implications of the channel- input and on bone sizes. (See text for fur ther explanation.) fill and channel-lag modes can be inferred from

194

their differing context and taphonomic fea- in fluvial deposits other than the Siwalik tures. In channel-lag assemblages, attr i t ional Miocene and Texas Permian sequences. How- vertebrate remains may be derived from vari- ever, these two modes appear to be generally ous sources (e.g., channel banks, upstream represented in theve r t eb ra t e reco rd . Processes drainages) (Behrensmeyer, 1982) but in general associated with channels affect fossil preserva- are al lochthonous with respect to the deposi- tion at three discernable levels: (1) local tional site. Multiple cycles of channel erosion circumstances that control whether bones are and deposition can be represented in the lag preserved in channel-lags or channel-fills (e.g., assemblage, depending on the pattern of chan- rates and modes of depositional events), (2) nel migration within the fluvial system. In characteristics of the fluvial system that affect some cases, this mode also can record a single the frequency of different types of channels, event of erosion, transport, and concentrat ion (3) local to basin-scale rates of subsidence that of at tr i t ional bones from a restricted source affect fluvial regimen and the ultimate preser- area. The species in the assemblage may vation of channel deposits. represent members of the community or com- In both channel-fill and channel-lag con- munities inhabiting different environments in texts, local channel processes have a major the drainage basin, time-averaged over effect on the balance of bone input versus 102-104 yr (Behrensmeyer, 1982). In channel- destruction or dispersal in creating a fossil fill assemblages, the vertebrate remains are assemblage (Fig.4). Channel pattern (e.g., derived from a smaller area within the channel braided, meandering, anastomosing) and or from adjacent overbank environments, and change through time (lateral migration, avul- they are autochthonous with respect to the sion) affect the degree to which bones are abandoned channel environment. Time-aver- transported, winnowed, abraded, or left undis- aging depends on the rate of filling of the turbed. Highly sinuous channels are subject to channel, which can occur in < 102 yr based on neck cut-off, and resulting oxbow lakes typi- modern examples (Gagliano and Howard, cally have low clastic input compared to less 1983). sinuous chutes and braid channels (Fisk, 1947;

Allen, 1965; Hook and Ferm, this issue). G e n e r a l imp l i ca t ions fo r t he v e r t e b r a t e Depositional processes affecting bone concen- r e c o r d trations in channel fills thus are controlled in

part by channel sinuousity. Braided and anas- The channel-lag mode has been recognized tomosing systems are characterized by mul-

for some time and characterizes what many tiple channels that repeatedly form and reform paleontologists and sedimentologists visualize around bars or islands (Leopold and Wolman, when they think of bones in channels. The 1957; Schumm, 1963), while meandering sys- channel-fill mode also has been noted pre- terns progressively rework their own deposits viously (e.g., Boy, 1977; Eberth and Berman, by lateral erosion and deposition, usually over 1983), and evidence from the Miocene and longer time periods. Channel-lag assemblages Permian examples discussed above supports would be subjected to different degrees of the recurring association of vertebrate re- short- versus long-term reworking and concen- mains with channel fills in widely different trat ion in fluvial systems with different chan- time periods and alluvial settings. The broader nel patterns. significance of the channel fill context in the Given local conditions favorable to bone vertebrate record probably has been underesti- concentrat ion in the channel-lag and/or the mated because it is more difficult to discern in channel-fill mode, fluvial systems with active, outcrop than the channel-lag context, migrating channels in well-established chan-

Pat terns of occurrence of the channel-lag nel belts would be likely to generate more and channel-fill modes should be tested further channel-lag assemblages, while systems with

195

and channel-lag modes, with more secondary A. B. channels probably favoring the former. Both

~ ! suspended load (meandering) and bedload (braided and anastomosing) river systems have crevassesplays and/or secondary floodplain channels, with number and scale depending on overall rates of aggradation and local climatic conditions that affect flood cycles and flood-

S plain drainage. " There is presently little direct information

/ f l on how tectonics and climate control the ~ / ~ frequency of abandoned versus active channels

~ at a given point in time in different fluvial settings. It does appear that avulsion-domi-

[ ~ , nated systems frequently characterize areas o f

A___2 A ~ ' ~ ~i.~ high sediment input and rapid subsidence ,~ ~ ~ ~ ' ~ ~ f ~ (Smith, 1986), resulting in alluvial architecture

~ K ~ with ribbon or shoe-string sands isolated . ~ within the floodplain deposits (Fig.hb).

Laterally migrating systems that deposit mul- tistoried ribbon and sheet sands are more

h A' B B' ~~.~-------~,--,~ ~ typical of lower rates of subsidence and

~'~:~:=~ . . . . . sediment input (e.g., Kraus and Middleton, ~-"~-"~~':'~ ' 1987). Thus, areas of rapid subsidence would

Fig.5. Comparison of a meandering, lateral ly migrat ing generate and preserve more instances of the fluvial system within a restricted channel belt (A) and an channel-fill mode, while channel-lag a s s e m - anastomosing, avulsion-dominated system that is not blages would occur more frequently in areas of confined to a restricted channel belt (B), showing the lower subsidence. The vertebrate record in the resul t ing alluvial archi tec ture and the different types of channeldeposi ts . The lateral ly migrat ing system generates first case would be biased toward well-pre- sheet sandstones and would be expected to have more served samples from a particular habitat while channel-lag vertebrate assemblages than the avulsion- in the second it would represent a broader dominated system, which would have more channel-fill habitat spectrum with less complete fossil assemblages, material.

Ultimately, the preservation of whole se- frequent avulsion and channel-belt abandon- quences of vertebrate-bearing strata depends ment would have more instances of preserva- on the large-scale tectonic setting. As sug- tion in the channel-fill mode (Fig.5). Based on gested by R. Hook (pers. comm., 1986), it is observations in modern environments, aban- possible that fluvial deposits at colliding conti- doned channel environments would be ex- nental margins (e.g., the Siwaliks) contribute pectedin most river systems. Because of lateral significantly only to the latest part of the erosion of a river within its channel belt, vertebrate record, since older rocks from this however, deposits in these environments are setting have been tectonically deformed and repeatedly destroyed until the river avulses to subducted. The more long-lived deposits are a new position on the alluvial plain, likely to occur on trailing margins, rift basins,

The frequencies and scales of different kinds and stable cratons. Thus, the fossil record of of secondary channels (e.g., crevasse splay, land vertebrate evolution and paleoecology chute, and tributary) also would affect the may be derived from significantly different number of opportunities for the channel-fill continental settings through the Phanerozoic.

196

C o n c l u s i o n basin-scale faunal change through long periods of time. This also would be a good

The channel-lag vs. channel-fill modes are a taphonomic mode for determining the strati- way of classifying vertebrate fossil assem- graphic position of immigration and extinction blages according to sedimentary context and events since the appearance or disappearance taphonomy and are analogous to "taphofacies" of a taxon could be affected by which habitats as defined for marine invertebrate assemblages are being sampled in a fossil assemblage. The (see Speyer and Brett, this issue). The extreme channel-lag mode would represent different case of a channel-lag assemblage would be a habitats on the alluvial plain, while other cluster of rounded, unidentifiable bone pebbles types of samples from channel fills or flood- in a conglomerate within a channel deposit, plain paleosols might be more habitat-specific while the opposite extreme would be complete, (see also Badgley and Gingerich, this issue). art iculated skeletons preserved in a sapropel Since channel-fill assemblages provide samples in a channel-fill. Most channel vertebrate of smaller areas and shorter time periods, they assemblages lie between these extremes; in are better suited for analyses of habitat- some fluvial systems they will tend toward the specific paleocommunities as well as anatomi- channel-lag mode while in others they will cal studies of populations from similar environ- more commonly occur in channel fills. Both ments. modes should be present in most fluvial de- posits that preserve vertebrates in channels, A c k n o w l e d g m e n t s but in different frequencies depending on the fluvial regimen. The relative proportion of The ideas in this paper have benefitted from fossil occurrences in these modes can be tested productive discussions with A. Aslan, C. Bad- further to determine if there is a general gley, J. Barry, J. Bridge, G. Haynes, R. Hook, correlation with the type of fluvial system and N. Hotton, L. McRae, H. Sues and S. Wing. a link to broader climatic and tectonic con- Helpful comments on the manuscript were trols, provided by C. Badgley, M. Kraus, R. Hook,

Taphonomic modes provide a basis for com- and H. Sues. I have greatly appreciated assist- paring faunas with similar preservational his- ance in the field from I. Khan, K. Sheikh and tories throughout the geological record, continuing support from Dr. Ibrahim Shah of thereby helping to moderate the effects of the Geological Survey of Pakistan. The field taphonomic biases in diversity estimates, tim- work has been supported by Smithsonian ing of appearance and extinction events, and Foreign Currency Program grants to D. Pil- other important paleobiological parameters, beam and J. Barry (Harvard University) with Traditionally, paleontologists have combined additional funding from the Smithsonian Re- samples with different preservational histories search Opportunities Program. to make the most of anatomical, biostrati- graphic, and paleogeographic information in R e f e r e n c e s developing an overview of vertebrate evolu- tion and paleoecology. The recognition of Allen, J. R. L., 1965. A review of the origin and taphonomic modes can help to refine this characteristics of recent alluvial sediments. Sedimento- approach because different kinds of samples logy, 5: 89-191. Allen, J. R. L., 1970. Physical Processes of Sedimentation. from in the fossil record can be matched with Allen and Unwin, London, 248 pp. more specific evolutionary and paleoecological Allen, J. R. L., 1978. Studies in fluviatile sedimentation: an questions. For example, vertebrate fossils from exploratory quantitative model for the architecture of at tr i t ional channel-lag assemblages should avulsion-controlled alluvial suites. Sediment. Geol., 21:

129-147. provide the most homogeneous sample of the Allen, J. R. L. and Williams, B. P. J., 1982. The architecture overall paleocommunity for the analysis of of an alluvial suite: rocks between the Townsend Tuff

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