Taphonomy of a late Eocene microvertebrate locality, Wind River Basin, Wyoming (U.S.A.)

Palaeogeography, Palaeoclimatology, Palaeoecology, 52 (1985): 123--142 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

123

TAPHONOMY OF A LATE EOCENE M I C R O V E R T E B R A T E LOCALITY, W I N D R I V E R B A S I N , W Y O M I N G (U.S.A.)

MARY C. MAAS

Department of Anthropology, State University of New York at Stony Brook, Stony Brook, N Y 11794 (U.S.A.) (Received March 12, 1984; revised and accepted January 8, 1985)

ABSTRACT Maas, M. C., 1984. Taphonomy of a late Eocene microvertebrate locality, Wind River Basin, Wyoming (U.S.A.). Palaeogeogr., Palaeoclimatol, Palaeoecol., 52: 123--142. The taphonomic history of a late Eocene micromammal assemblage (Badwater Locality 20, Wind River Basin, Wyoming) is examined using methods of data collection and analysis designed to elucidate three taphonomic processes: death and initial accumulation, transport and burial, and postburial alteration. The disarticulated fragmentary nature of the fossils and the low percentage of teeth and jaws in the taphonomic sample suggests that much of the assemblage was initially accumulated in predator scats or pellets. Nonpredator agents of mortality were responsible for an additional "attritional" component. There is evidence that the bones were exposed for a short period of time before burial and that the assemblage represents a time-lag soil accumulation. There is no evidence, either from orientation and surface characteristics of the bones or from the sediments, that the fossils were hydraulically transported and deposited. Postburial alteration included breakage of small bones due to mechanical stress. The greatest information loss occurred during initial accumulation. Little additional information was lost during burial or through postburial alteration. This information loss primarily concerned the relative abundance structure of the mammalian community. In contrast, information concerning species richness appears to be only slightly altered. INTRODUCTION The value o f t a p h o n o m y as a f o u n d a t i o n f o r the s t u d y o f the e c o l o g y and s t r u c t u r e o f past animal c o m m u n i t i e s has gained increasing r e c o g n i t i o n in the f o r t y years since E f r e m o v ( 1 9 4 0 ) c o i n e d the term, generating n u m e r o u s studies detailing the b i o s t r a t i n o m i c and diagenetic processes b y w h i c h animals die, are buried, and are fossilized. However, despite a b r o a d d a t a base, relatively few a t t e m p t s have been m a d e to synthesize this i n f o r m a t i o n into a useful f r a m e w o r k f o r t h e analysis o f t h e c o m p l e t e t a p h o n o m i c histories o f fossil assemblages or, m o s t i m p o r t a n t l y , t o assess the i m p l i c a t i o n s o f these t a p h o n o m i c histories f o r paleoecological i n t e r p r e t a t i o n ( b u t see, for example, B e h r e n s m e y e r , 1 9 7 5 ; S h i p m a n , 1 9 7 7 , Gnidovec, 1978). The lack o f comprehensive t a p h o n o m i c analyses is particularly m a r k e d in the case o f microv e r t e b r a t e fossil assemblages. 0031--0182/85/$03.30

© 1985 Elsevier Science Publishers B.V.

124

Microvertebrate assemblages are commonly considered to be of dubious value in paleoecological interpretation, a perception predicated largely on empirical data regarding modes of accumulation and transport of microvertebrate bones in modem environments (e.g., Dodson, 1973; Wolff, 1973; Mellett, 1974; Andrews and Nesbit Evans, 1983). On the one hand, the agents of accumulation of these assemblages (i.e., predators, hydraulic forces, etc.) are considered to be too selective: thus the information preserved in microvertebrate fossil assemblages represents only a biased sample of the paleocommunity. On the other hand, it has been suggested that the hypotheses explaining the accumulation of microvertebrate fossil assemblages may not reflect the taphonomic complexity of such occurrences, resulting in further misinterpretations of the biocoenoses (Dodson, 1980). In an effort to evaluate these concerns, and thus the value of paleoecological analyses based on data from microvertebrate assemblages, this study addresses the problems of methodology and interpretation in microvertebrate taphonomy, using the example of Badwater Locality 20, a late Eocene microvertebrate locality from the Wind River Basin. FORMATION OF M I C R O V E R T E B R A T E ASSEMBLAGES

Most recent studies of microvertebrate taphonomy have focused on one of two hypotheses of assemblage formation: the fluvial hypothesis and the scatological hypothesis (Korth, 1979). The first posits that microvertebrate assemblages accumulate as the result of sorting and concentration by hydraulic forces (Dodson, 1973; Wolff, 1973) while the second suggests that some such assemblages represent fecal deposits of predators (Mellett, 1974). These hypotheses, however, rather than representing alternative explanations for a single event (the accumulation and eventual fossilization of microvertebrate bones), may address different components of the taphonomic history of an assemblage. By recognizing that taphonomy comprises three processes (death and initial accumulation, transport and burial, postburial alteration), it is possible to develop a comprehensive methodology that allows an assessment of (1) the relative contributions of various agents to each of the three component processes and (2) the nature of information loss associated with each aspect of the taphonomic history of an assemblage. Death and initial accumulation

Identifying the agent or agents of death and initial accumulation is perhaps the most problematic part of the taphonomic assessment of any microvertebrate assemblage. Hypotheses related to death and initial accumulation of microvertebrates include accumulation as predator scats or pellets ("coprocoenoses", Mellett, 1974) and nonpredator mortality (e.g., accident, disease, starvation). Characteristics associated with these alternative hypotheses are summarized in Table I.

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126 It is clear that very few of these characteristics are unambiguously diagnostic of a single mode of accumulation. For instance, teeth and compact skeletal elements (e.g., those of the manus and pes) are preferentially preserved in several types of assemblages, including those of some owls, some mammals, and some nonpredator accumulations. In addition, some apparently diagnostic criteria, such as percentage representation of various skeletal elements by minimum numbers of individuals, and percentage of complete limb elements and crania may vary among members of the same general class of predators (e.g., mammalian carnivores). Although general statements have been made regarding diagnostic characteristics of different predator accumulations (for instance owls and mammalian carnivores, Korth, 1979; Mellett, 1974), comparative studies of different species within each broad class demonstrate a degree of variability within groups which cross-cuts the differences between groups (e.g., owls, Dodson and Wexlar, 1979; mammalian carnivores, Andrews and Nesbit Evans, 1983). Because of this ambiguity, conclusions regarding the specific agents of initial accumulation of an assemblage must be quite general (one exception may be the recognition of crocodilian accumulated assemblages, Fisher, 1981a). However, even the most general identification of the agent initially accumulating an assemblage makes it possible to assess something of the nature of the information lost during this phase of taphonomic history. Information lost during initial accumulation includes loss due to destruction (by digestion, weathering or breakage) and loss by selective agents of mortality. Bones preserved in scats may be so weakened by digestive processes that they have a low potential for fossilization. Because of this, Andrews and Nesbit Evans (1983) posit that bone accumulated by mammalian carnivores only rarely contributes to fossil assemblages. However, the variability of quality and quantity of bone preserved in scats of modern mammalian carnivores suggests that generalizations regarding the potential for preservation should be made with caution. The scatological hypothesis should n o t be rejected out of hand, but considered along with other hypotheses when assessing the taphonomic history of an assemblage. Assemblages where death and initial accumulation are not due to the actions of predators potentially preserve more information regarding community structure than do predator-accumulated assemblages (Bown and Kraus, 1981b; Winkler, 1983). For instance, Winkler (1983) states that the relative abundance structure of mammalian communities can be derived from some nonpredator accumulated assemblages, particularly those accumulating over a relatively short period of time. This is a level of information that is not preserved in concentrations accumulated by predators, where prey selection by species or size of prey is an important source of information loss. However, there is evidence that species diversity of small mammals (but not their relative abundances) is fairly represented in predator scats collected from a limited area over even a short period of time (Pearson, 1964; and see below). In either case, the final evaluation of information

127 preservation and loss must include consideration of the taphonomic processes that occur subsequent to initial accumulation. Transport and burial The next c o m p o n e n t of the taphonomic history of a microvertebrate assemblage involves modes of transport and burial. Two alternative hypotheses are generally invoked in discussions of this process. The first emphasizes the role of hydraulic forces in transporting and sorting the fossil material. Evidence indicates that hydraulic transport significantly alters the information preserved in an assemblage, both in terms of relative abundances (Wolff, 1973) and the mixing of fossils from different communities (Shotwell, 1955, 1958; Dodson, 1973). A number of studies have focused on the recognition of such assemblages in the fossil record {e.g., Wolff, 1973; Korth, 1979; Alexander, 1982). The alternative to the hydraulic hypothesis suggests that burial occurs soon after initial accumulation on a subaerially exposed surface, with little transport (hydraulic or otherwise) taking place after initial accumulation of the bones. This alternative involves very little alteration in the composition of the assemblage after initial accumulation, and thus introduces little bias into the transport and burial component of the taphonomic record (Bown and Kraus, 1981b; Winkler, 1983). The characteristics associated with hydraulically transported microvertebrate assemblages and with minimally or untransported assemblages are summarized in Table II. These include sorting by size or shape of skeletal element, preferential orientation of long bones, and wear, abrasion or weathering of bony surfaces, among others. Additional criteria by which a minimally transported assemblage can be recognized depend to a great extent on its mode of initial accumulation (see Table I) and the amount of time elapsed between accumulation and burial. Because many of the characteristics of the mode of transport and burial, particularly patterns of breakage, are also diagnostic of modes of initial accumulation, it is necessary to consider as many factors as possible when reconstructing these processes. Post burial processes Most recent taphonomic studies, particularly those addressing the unique characteristics of microvertebrate assemblages, have focused on the biostratinomic aspects of taphonomy. While this research sheds necessary light on the identification of the agents responsible for the accumulation and transport of an assemblage, the effects of postburial diagenetic and mechanical processes should also be considered. Recognition of whether bones have been broken before or after burial is of particular importance in the analysis of microvertebrate assemblages, since many of the criteria used to identify biostratinomic agents are based on patterns of breakage or percentage representation of skeletal elements.

128 TABLE II Characteristics associated with modes of transport and burial in microvertebrate assemblages Postaccumulation transport

Representation per M N I a (%)

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Orientation

Breakage Abrasion

Minimal

Variable

Minimal

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Low

By size and shape

Lack of preferred trend or plunge Preferred trend

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High, but variableby element

By size (Maximumsize of component species less than 150 g)

Lack of preferred trend or plunge

Minimal

Variable

Present, particularly if matrix includes pebble-sized sediments (i.e.,2--4 mm). Minimal

aMinimum number of individuals; bDodson, 1973; CWolff, 1973; dKorth, 1979, eAlexander, 1982; fShipman and Walker, 1980. Postfossilization breakage of bone due to diagenetic alteration or mechanical stress is a fairly c o m m o n occurrence and has been recognized at several fossil localities (e.g., Shipman, 1977, 1981). Some of the criteria used to distinguish between prefossilization and postfossilization breaks in bones of large animals (Shipman, 1977; Bonnichson, 1978; Gifford, 1980) are useful in assessing this parameter in small bone assemblages as well. However, other criteria, such as differences in colour between the broken bone surface and outer surface of the bone are sometimes equivocal in the case of very small bones or bone fragments and additional criteria are required. In situ provenience of bones and bone fragments provides a particularly useful line of evidence for discriminating between biostratinomic and postburial breakage in microvertebrate assemblages. If a bone is fractured after burial, the fragments of that bone should be found adjacent to one another (unless the deposit had been further altered by bioturbation, a factor t h a t can, in many cases, be elucidated by examination of sedimentary features). Identification of digestive corrosion or mechanical abrasion of broken surfaces is also useful for discriminating between pre- and postburial breakage. As a demonstration o f how the broad spectrum of taphonomic data reviewed here can be applied to the fossil record, the diagnostic criteria associated with hypotheses regarding each taphonomic process are compared with the characteristics of Badwater Locality 20, a late Eocene microvertebrate locality. The taphonomic reconstruction of this locality is of particular interest because of the paleoecological scenario suggested by the unique character of its fauna (Maas, 1983).

129 TAPHONOMIC ANALYSIS OF LOCALITY 20 Locality 20 is a microvertebrate assemblage of Duchesnean age, located in the Badwater Creek area of the northeast corner of the Wind River Basin, Wyoming (Fig.l). Late Eocene resedimented andesitic volcanic rocks are exposed in a narrow belt between the Owl Creek and Big Horn Mountains to the north, and the Cedar Ridge Fault to the south (Tourtelot, 1957; Riedel, 1969). The Cedar Ridge Fault, a large fault of normal displacement, separates these strata from the early Eocene Wind River Formation (Tourtelot, 1957; Love, 1978). Late Eocene rocks, the Hendry Ranch Member of the Wagon Bed Formation, crop out in poorly exposed weathered slopes in the vicinity of Locality 20. They consist of homogeneous volcanigenic mudstone and lack primary sedimentary structures such as bedding or any other evidence of deposition within a channel or high energy hydraulic regime. The grey to grey-green fossiliferous mudstone at Locality 20 is capped by a volcanic tuff, K/Ar dated at 40 + 1.2 m.y. (Black, 1969). The evidence of grain size (samples dominated by clay to silt-sized particles) and lack of bedding structures as well as the presence of pedotubules suggest that these deposits represent a paleosol, most probably an entisol (Maas, 1983), although further geochemical and petrographic analysis is needed to confirm this.

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130 TABLE III Faunal list, Badwater Locality 20, Wind River Basin, Wyoming, U.S.A. CLASS MAMMALIA Order Multituberculata Family Neoplagiaulacidae Ectypodus sp

Order Carnivora Family Amphicyonidae Daphoenus demilo Carnivora fam. indet.

Order Marsupicarnivora Order Perissodactyla Family Didelphidae Peratherium cf. P. knighti Peratherium sp. Nannodelphys cf. N. minutus Nannodelphys sp. Order Insectivora Family Adapisoricidae Ankylodon talonidus Amphilemur sp. Entomolestes sp. Family Nyctitheriidae Nyctitherium christopheri Nyctitherium robinsoni Nyctitherium sp. Family Geolabidae Centetodon magnus Centetodon sp. C. Family Apternodontidae Apternodus sp. cf. A. illifensis Family Lepticidae Palaeiclops sp. Family Soricidae Domnina sp. cf. D. gradata Domnina sp. Soricid Sl~ B. Order Chiroptera Chiroptera sp. Order Primates Family Paromomyidae Ignacius mcgrewi

Family Brontotheriidae ? Telmatherium cf. T. cultridens Order Artiodactyla Family Agriochoeridae Diplobunops matthewi Family Leptomerycidae Hendryomeryx wilsoni Family Protoceratidae Leptotragulus medius Poabromylus golzi Order Rodentia Family Ischyromyidae Leptotomus guildayi Spurimus scottii Spurimus selbyi Family Cylindrodontidae Pseudocylindrodon tobeyi Pseudocylindrodon near P. tobeyi ?Pseudocylindrodon sp. Pareumys lewisi Pareumys sp. Fmnily Sciuravidae Sciuravos sp. Sciuravidae sp. Family Eomyidae Eomyidae -~ 2 spp. Order Lagomorpha Family Leporidae Mytonolagus wyomingensis Leporid sp.

131

The late Eocene Badwater localities have been extensively studied over the last twenty years (Krishtalka and Setoguchi, 1977; Dawson, 1980, and references therein). Based on these studies, a faunal list comprising 41 mammalian taxa has been compiled for Locality 20 (Table III). It has been suggested that this fauna, comprised largely of micromammals, represents a unique assemblage for the late Eocene, containing on the one hand relict forms such as multituberculates and, on the other hand, earliest appearances, of forms including selenodont artiodactyls, soricid insectivorans, and certain rodent families (Krishtalka and Black, 1975; Black, 1978). However, before the paleoecological significance of this fauna can be assessed, the degree of bias resulting from its taphonomic history must be investigated. Methods The taphonomic analysis of Locality 20 included both field and laboratory data collection techniques designed to identify and distinguish between biostratinomic processes and diagenetic/mechanical processes. Evaluation of the in situ occurrence of the fossils comprised an important aspect of this analysis, one not emphasized in many studies of microvertebrate fossils (but see Gnidovec, 1978; Alexander, 1982). In order to determine whether or not a particular agent of death and initial accumulation is identifiable for the Locality 20 fauna, both the extent of the fossiliferous pocket and the physical characteristics of the in situ fossil material were examined. Excavation of a test trench from the top to the b o t t o m of the exposure allowed delimitation of the extent of the microvertebrate fossil pocket. Test holes were also dug to the east and west of the previously reported limits of the fossiliferous pocket. Sediment from the test holes was screen-washed and examined to ascertain the most densely fossiliferous portions of the exposure. To determine the condition of the fossil material in situ, four sample blocks, each approximately 0.5 m 2, were removed from Locality 20 and taken to the University of Colorado Museum laboratory for excavation, after recording provenience of each sample block in relation to a predetermined datum point, using standard archeological methodology. Screenwashing of 25 kg sacks of matrix, collected from points adjacent to each sample block, provided a basis for judging the completeness of recovery of material using excavating techniques. In addition, all fossil bones occurring on the surface of the locality were collected, and their locations and descriptions recorded. The fossil material recovered from screen-washing and from excavation of sample blocks was examined and sorted on the basis of the following criteria: skeletal element; degree of completeness; position of breaks (proximal or distal); and presence or absence of abraded, corroded or rounded surfaces, gnaw marks, and tooth punctures. The resulting data were included in analyses of both initial accumulation and transport and burial processes.

132 Based on the assumption that the fossil material had either been transported hydraulically and buried or accumulated in a subaerial environment and then buried, data were collected to allow analysis of orientation and breakage patterns of bone in situ. These included the recording of provenience, trend, and plunge for each long bone or long bone fragment recovered from sample blocks. These data were analyzed to determine whether or not the fossil material showed a preferred orientation. All excavated matrix was screen-washed and the residue dried and picked to recover bone overlooked during excavation. The fourth sample block and the sacked matrix were also screen-washed and picked. In addition to the specimens recovered during the taphonomic study, the Locality 20 fauna includes numerous specimens acquired during several seasons of quarrying, screen-washing and surface collecting by workers from the Carnegie Museum (e.g., Black, 1978, and references therein). These specimens were obviously not included in the taphonomic sample for analysis of orientation, provenience and in situ condition of the fossils, but were used in evaluating taxonomic and size diversity of the Locality 20 fauna. Results

Locality 20 consists of a limited exposure, no more than 50 m E--W and 10 m N--S. From south to north, the sediments slope approximately 15 m, from the top of the exposure, which consists of a volcanic tuff, to the bottom, where the exposure terminates in a small erosional gully. The vertical drop is approximately 3 m. The most densely fossiliferous portion of the Locality 20 exposures, based on relative amount of bone recovered from test holes and from examination of the test trench, extends 6 m E--W and 2 m N--S. The excavated blocks were removed from this portion of the locality. The Carnegie Museum specimens collected prior to the taphonomic study were largely derived from quarrying and screen-washing operations within the section of the fossiliferous area sampled in the taphonomic study (L. Krishtalka, personal communication, 1981). The fossils recovered from the Locality 20 taphonomic sample (both excavated material and specimens recovered through screen washing) comprise disarticulated and generally fragmentary postcranial bone, fragments of jaws, maxillae, and r~umerous isolated teeth. Percentages of the total specimens (contained in the excavated blocks) represented by each skeletal element are shown in Table IV. Only one complete limb bone (an ulna) was recovered during this study. The relatively small numbers of identifiable specimens recovered from the excavated blocks reflects the low density of fossils throughout the pocket. Material from the earlier Carnegie Museum collection was not included in the analysis of preservational characteristics of the assemblage since collecting procedures for the earlier sample may reflect biases in favor of

133 TABLE IV Percentage skeletal elements in Locality 20 taphonomic sample (N = 2261) Element

Percent of total

Isolated teeth Fragmentary teeth Maxillae with teeth Jaws with teeth Cranial fragments Vertebral fragments Limb girdle fragments

2.39 3.32 0.04 0.08 1.24 2.70 0.20

Element

Percent of total

Proximal limb fragments Distal limb fragments Diaphyses Complete limbs Pes and manus elements Unidentifiable fragments

0.66 0.66 3.36 0.04 9.29 76.00

complete elements, especially teeth and jaws. The postcranial specimens in the earlier collection do show, however, characteristics consistent with the taphonomic sample (e.g., most complete preservation of compact bones, such as carpals, tarsals, and phalanges and little evidence of abrasion or corrosion of outer surfaces of bones). The vast majority of bones and bone fragments in both the taphonomic sample and the earlier Carnegie Museum collection are less than 1 cm in longest dimensions, although some maxillary and mandibular fragments and the postcrania of a brontotheriid are much larger. With the exception of the brontotheriid, the lower molar length of the specimens (an indication of species body size) ranges from slightly less than 1 mm to 12.95 mm, with the bulk of the fauna falling into the 1--3 mm molar length group (Fig.2). This indicates an estimated body size range of less than 50--12 000 g (Maas, 1983 and references therein). None of the characteristics associated with nonmammalian predators, such as crocodilians (characteristic etching of teeth, transluscence of bone due to digestive processes), were observed on any of the material recovered from Locality 20. Split and cracked bone, indicative of the earliest stages of weathering due to subaerial exposure in modern bone assemblages (Behrensmeyer, 1978) were noted in a few of the large bone fragments. In addition, t o o t h puncture and gnaw marks were observed on several of the larger bones (greater than 2 cm in length) but on none of the small bone fragments (Fig.3).

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134

Fig.3 Fossil bones from Badwater Locality 20. Phalanx of brontotheriid showing tooth puncture mark (left) and unidentifiable bone fragment showing gnaw marks (right). Data recorded from the three excavated blocks indicate that the bones showed no preferred orientation (trend). These results are illustrated graphically in Fig.4. The trend data differ by 20% from a random model histogram (Shipman, 1977). Results of a Kolmogorov-Smirnov one-sample test (Siegel, 1956) confirm the lack of significant difference from a random distribution in these data (D = 0.1237, P greater than 0.20). Following Shipman (1977), plunge data were grouped in 5 ° increment categories. Correction for regional dip was not calculated, but was estimated in the field to be no more than 4 °. The majority of bones examined (59%) showed plunge of less than 15 °. Although 25% showed plunge of more than 30 °, only one specimen had a plunge greater than 65 °. Because of potential problems with the interpretation of hydraulic equivalency data (Maas, 1983) these data are not included in this study. However, it is interesting to note t h a t the computation of hydraulic equivalency (Korth, 1979) for the cheek tooth of a c o m m o n species from the Locality 20 fauna with a mean molar length of 2 mm (Eomyidae sp.), resulted in a hydraulic equivalency of 1.2 0, far larger than the modal grain size of less than 4 0 obtained from grain size analysis of the Locality 20 sediments (Maas, 1983). In contrast to the rounded edges of bone fragments and abrasion of ends and processes of whole elements reported by Korth (1979), both for experimentally abraded modern bone and fossil bone recovered from stream deposits,

135

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~Q Fig.4. Mirror i m a g e r o s e diagram of trend of fossil long bone fragments from Badwater Locality 20 taphonomic sample. N = 53. the majority of bone and bone fragments from Locality 20 show more sharply broken edges, similar to those observed in bone recovered from m o d e m carnivore scats (Mellett, 1974; Korth, 1979; Maas, 1983}. The results of excavation of bones from blocks of matrix indicate that this fragmentation was largely biostratinomic (preburial) in nature. Although bones exhibiting cracks or fractures in situ were fairly c o m m o n in the excavated material, most of this in situ breakage affected already fragmentary materials (as indicated by the isolated position of the fragments}. In a very few cases, as with the complete ulna and one fairly complete mandible, in situ or postburial fractures appeared to be the sole agent of breakage. DISCUSSION The patterns of breakage, percentage representation of skeletal elements, and surface characteristics of the bone, when compared with diagnostic characteristics of mammalian, avian, and crocodilian predator accumulated assemblages, indicate that much of the Locality 20 assemblage represents a coprocoenosis. In particular, the very fragmentary nature of most of the fossils (76% of the taphonomic sample composed of unidentifiable bone

136 fragments) and the patterns of breakage observed in the excavated material resemble characteristics of assemblages accumulated as modern mammalian predator scats. In contrast, assemblages accumulated by some types of avian predator, including certain species of owls, would be expected to have a much higher percentage of complete elements. It should be emphasized, however, that similarities in the diagnostic features of assemblages accumulated by some avian predators and some mammalian carnivores (see Table I) make a definitive identification of the accumulator of this assemblage extremely problematic. The absence of diagnostic characteristics associated with other nonmammalian predators (i.e., crocodilians}, however, indicates that these were not important agents in the accumulation of the Locality 20 fossil assemblage. In addition to the scatological contributions to the Locality 20 assemblage, there are also indications (weathered bone surfaces and gnaw marks on larger bone fragments) that the assemblage includes an attritional (nonpredator mortality) c o m p o n e n t and that the bones were subaerially exposed for a period of time before burial. Since characteristics of weathering or indications of scavenging are generally difficult to identify on very small bones, it is necessary to consider additional lines of evidence in order to assess the importance of this c o m p o n e n t in the Locality 20 assemblage. Studies of modern predators indicate that the b o d y size of the predator places a maximum, although not minimum, constraint on size of preferred prey (Rosenzweig, 1966). Thus, the absence of bones t o o large to serve as prey for the carnivores known from that locality could be interpreted as suggesting that the bones were not deposited as carnivore scats (Bown and Kraus, 1981b) (although the possibility that the predators responsible for an accumulation of scats may not be represented in the fossil assemblage must also be considered). A maximum b o d y size is also associated with assemblages of bones carried by harvester ants. Shipman and Walker (1980) proposed an expected maximum b o d y size of 150 g for species comprising these accumulations. The b o d y size distribution of species at Locality 20 (based on the total fauna, see Fig.2), as well as the sizes of specimens collected during the taphonomic study, are totally consistent with neither the carnivore nor harvester ant accumulation models. Although the distribution is depauperate in the larger end of the size distribution (estimated b o d y size greater than 5 kg) it is not devoid of these larger species. The distribution also contains an abundance of material too large to have been transported by harvester ants. However, b o d y sizes of most species do fall within the range of sizes expected in an accumulation of scats from a small-to-medium-sized carnivore (Rosenzweig, 1966). In summary, the size distribution of species suggests that the assemblage received a significant contribution from predator scats (although avian pellets cannot be excluded as source of bone). Some species t o o large to serve as prey for the known Locality 20 carnivores (estimated b o d y size 10--15 kg; Maas, 1983) are present. This, along with evidence of

137 subaerial exposure on some of the specimens, indicates the presence of a nonpredator mortality "attritional" c o m p o n e n t as well. The next aspect of the taphonomic history of the Locality 20 assemblage to be addressed is that of transport and burial. As noted above, two alternative hypotheses can be considered as possible explanations of this taphonomic component: the hydraulic transport hypothesis and the attritional hypothesis. Several lines of evidence, including data relating to the orientation of the bones, hydraulic equivalencies, and patterns of breakage and abrasion, are relevant to the evaluation of the effects of transport and burial processes on fossils from microvertebrate localities. While caution must be exercised in deriving generalizations from the small sample from which orientation data could be recorded (N = 43 for plunge and N = 53 for trend), several important observations can be made regarding the available data. First, the lack of preferred orientation is not unexpected, given the absence of sedimentary structures in the surrounding matrix or any other evidence that hydraulic current activity contributed significantly to the accumulation of the assemblage. Secondly, as Toots (1965) has demonstrated, bioturbation, which would be expected in a soil, is a c o m m o n cause of random orientation. These considerations, and the lack of surface characters preserved on the Locality 20 fossils indicating hydraulic transport, suggest that it is most likely that the bones were actually deposited w i t h o u t a strong preferred orientation. This lack of preferred orientation is expected of bones deposited in scats or pellets and dispersed slightly as these disintegrated and of bones attritionally accumulated and buried in a soil. The characteristic vertical or near-vertical orientation of bones of mediumsized animals that have been deposited on soft sediments and subject to trampling by larger animals as a part of the burial process (Andrews et al., 1981; K. Carpenter, personal communication, 1982) was not observed in the Locality 20 sample. The sharply fragmented, largely unabraded or rounded edges of bones and bone fragments from Locality 20 are also consistent with other lines of evidence which indicate that the assemblage underwent little transport, at least by hydraulic forces, after its initial accumulation. Examination of in situ occurrences of bone indicates that, while breakage of bone due to postburial mechanical processes (probably movement due to swelling and shrinking of the clayey matrix) contributed to the high percentage of unidentifiable material, most of the fragmentation occurred during initial accumulation. The high incidence of postburial fracture of bone suggests that some of the bone may have been weakened, perhaps by digestive processes, before burial. However, few of the bones exhibited characteristics which could be unambiguously attributed to corrosion during digestion, as predicted by Andrews and Nesbit Evans ( 1 9 8 3 ) f o r carnivore-accumulated assemblages. Again, the variability of bone preserved in carnivore scats suggests that caution should be exercised in either accepting or rejecting the scatological hypothesis on the basis of the presence or absence of a single characteristic, such as digestive corrosion.

138 The purpose of this taphonomic analysis is the evaluation of the type and degree of information lost from the Locality 20 fossil assemblage between death of the animals and their recovery as fossil material, in order to assess the usefulness of the assemblage, and other similar assemblages, in paleoecological interpretation. Possible sources of information loss can be considered for each of the taphonomic processes in turn. Postburial processes included a fairly large amount of breakage of small bones, probably due to postdepositional mechanical stress. This undoubtedly resulted in the destruction of some otherwise identifiable material. However, it probably did not lead to a significant loss of information from the assemblage since evidence suggests that most of the material was disarticulated and fragmented as a result of its initial accumulation, both by predators and due to subaerial exposure of the nonpredator accumulated component. Information loss due to destruction of skeletal elements by sorting or destruction of elements during hydraulic transport appears to have been minimal. However, information loss during initial accumulation may have been considerable. Perhaps the most important source of information loss from an assemblage containing a large contribution of bone from scats or pellets is that due to prey selection and transport by predators. The Locality 20 assemblage, however, represents at least 40 species of mammals (see Table III). Although this species list includes specimens from the entire Locality 20 collection and not only the taphonomic sample, it is assumed that the taphonomic reconstruction based on the sample is applicable for the entire locality. The mammalian species diversity for Locality 20 is similar to that reported from modern forest biomes that have been used as general analogues of early Tertiary ecological systems (e.g., Bown and Kraus, 1981a, b; Winkler, 1983). These included lowland forests in Africa (67 species, Andrews et al., 1979; Nesbit Evans et al., 1981) and Central America (40 species, Fleming, 1973), and Southeast Asian montane forests (40 species, Maas, unpublished data). Although these modern analogues cannot be considered perfect models of undisturbed Eocene biotas, the species numbers do suggest that the species diversity of Locality 20 is consistent with the number expected from a complete sampling of a forest fauna within a fairly limited geographical area. In addition, the size range of the species from Locality 20 (see Fig.2) is also similar to that reported from modern forest biomes, although slightly de° pauperate in the number of larger species (Maas, 1983). This paucity of large species is most probably an artifact of the predator-accumulated c o m p o n e n t of the Locality 20 assemblage, and reflects the maximum prey-size ceiling of the carnivores. The question of distance of transport by predators before the bone was deposited addresses the problem of degree of mixing of biome-types and is ,more problematic than evaluation of species diversity. There is little evidence from which the home ranges of late Eocene predators can be deduced; even reported home ranges of modern carnivores differ markedly. The prey of

139 the wider-ranging predatory species might include species from a variety of biomes or biome-types, particularly closely linked biome-types such as montane and lowland forest. The effects of such mixing on the ecological diversity patterns of an assemblage are difficult to assess. However, the pred o m i n a n t biome should be reflected in the community's ecological diversity pattern, albeit with some differences caused by the introduction of allochthonous species. The degree to which mixing of species from different biomes, as a result of transport by predators, has occured is best assessed in light of comparisons with the c o m m u n i t y structures of modern biomes. The adaptive diversity of species, the size range of species, and the number of species at Locality 20 suggests that species-selective predation is not a major source of sampling bias (Maas, 1983). It is probable that this assemblage received contributions from several predators over a period of time. In addition, it also contains a nonpredator accumulated component. It is important to note that no assertion is being made about the preservation of the relative abundance structure of this fauna, or any other similarly accumulated assemblage. Indeed, evidence from modern carnivore accumulated assemblages shows that such samples are not representative of relative abundances of species (Pearson, 1964; Andrews and Nesbit Evans, 1983). However, the wide taxonomic and size ranges of species recovered indicates that species diversity in the general area of Locality 20 during the late Eocene was fairly well sampled by the mammalian carnivores that comprised important agents of accumulation at the locality. CONCLUSIONS Both predator and nonpredator mortality contributed to the initial accumulation of fossils at Locality 20. The disarticulated, fragmentary nature of the fossils suggests that many of the small bones, in particular, were originally contained in scats or pellets. There is no evidence that the assemblage underwent significant transport after its initial accumulation, although at least some of the material, notably the large bone fragments, show evidence of subaerial exposure. By isolating the different components of the taphonomic history of this assemblage, it has been possible to elucidate the processes that most significantly altered its information content, and so to assess the impact of that information loss. It is clear from this study that identification of the various components of taphonomic history is not an easy task: many of the same characteristics of a fossil assemblage (breakage, percentage representation of elements, etc.) can be the result of very different agencies. Because of this ambiguity, it is important to systematically examine as many criteria as possible, including both features of the fossils and their sedimentological environment, as well as the interaction of the two, and compare these with predictions generated by different taphonomic hypotheses. This approach, implicit in many taphonomic studies, provides a sound basis for taphonomic

140

reconstruction, and thus the evaluation of the paleoecological information contained in a fossil assemblage. ACKNOWLEDGEMENTS

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