Bone Surface Modifications Research Articles

New taphonomic advances in 3D digital microscopy: A morphological characterisation of trampling marks

The concept of equifinality has become one of the greatest difficulties in the field of taphonomy However, new advances in technology have diminished the margins of error in the interpretation of archaeological sites. The use of multivariate statistics and the most recent advances in microscopic analysis of Bone Surface Modifications (BSMs) have enable a less subjective interpretation of site formation processes. Nevertheless, this broader range of methodological approaches also presents some problems. The capacity of laser scanners in processing inconspicuous and superficial cortical alterations, such as trampling marks, has proven to be problematic. This study presents a new advance towards resolving this problem through the use of the HIROX KH-8700 Digital Microscope, whereby detailed digital three-dimensional (3D) reconstructions are able to pick up such minute BSMs. Through the statistical comparison of the David Laser scanner and the HIROX KH-8700 Digital Microscope, this paper contributes to our understanding of said equipment, followed by a significant advance in the characterisation of superficial BSMs. The combination of advanced microscopy and the application of geometric morphometrics highlights a morphological differentiation between two different types of trampling marks, hereby named scratch and graze trampling marks.

Morphological characteristics of preparator air-scribe marks: Implications for taphonomic research.

Taphonomic analyses of bone-surface modifications can provide key insights into past biotic involvement with animal remains, as well as elucidate the context(s) of other biostratinomic (pre-burial) processes, diagenesis, excavation, preparation and storage. Such analyses, however, first require researchers to rigorously disambiguate between continuums of damage morphologies prior to attributing individual marks to specific actors and effectors (e.g., carnivore tooth, stone tool cutting edge, etc.). To date, a number of bone-modifying agents have been identified, and criteria for identifying their traces have been published. Relatively little research, however, has focused on bone-surface modifications imparted during specimen preparation. Herein we report that air scribes, small pneumatic tools commonly used for preparation in museum contexts, can generate unintentional marks that may mimic surficial modification caused by carnivores. To aid investigators in assessing the hypothesis that a mark in question is derived from air-scribe preparation activities, we provide high-resolution, detailed morphological information imaged with scanning electron microscopy (SEM). The main diagnostic characteristic of air-scribe damage is the occurrence of sequential, variously spaced, sub-millimeter scallop-like stepped bone removals. This morphology can resemble damage imparted by carnivore teeth. In contrast to marks produced by trampling, stone tools and carnivores, however, no continuous internal features, such as linear microstriations, were observed within grooves produced by the air scribe. Thus, the presence of such features can be used to disprove an air-scribe origin. A culmination of the morphological criteria presented herein, cross-cutting relationships with other surficial features (e.g., diagenetic discoloration, weathering textures), the position of occurrence, and an overall contextual framework for the assemblage is suggested for accurate identification of such traces. The ability to recognize or disprove air-scribe damage will allow researchers to confidently proceed with interpreting past biological and sedimentological interactions with animal remains.

Subsistence strategies throughout the African Middle Pleistocene: Faunal evidence for behavioral change and continuity across the Earlier to Middle Stone Age transition

The African Middle Pleistocene (781–126 ka) is a key period for human evolution, witnessing both the origin of the modern human lineage and the lithic turnover from Earlier Stone Age (ESA) Acheulean bifacial tools to Middle Stone Age (MSA) prepared core and point technologies. This ESA/MSA transition is interpreted as representing changing landscape use with greater foraging distances and more active hunting strategies. So far, these behavioral inferences are mainly based on the extensive stone tool record, with only a minor role for site-based and regional faunal studies. To provide additional insights into these behavioral changes, this paper details a pan-African metastudy of 63 Middle Pleistocene faunal assemblages from 40 sites. A hierarchical classification system identified 26 well-contextualized assemblages with quantitative paleontological and/or zooarcheological data available for detailed comparative analyses and generalized linear mixed modeling. Modeling of ungulate body size classes structured around three dimensions (context, antiquity and technology) illustrates no one-to-one correlation between changes in lithic technology (Acheulean vs. MSA) and changes in prey representation. All assessed faunal assemblages are dominated by medium-sized bovids, and variations between smaller and larger body size classes are linked to site context (cave vs. open-air), with an increase in cave sites during the Middle Pleistocene. Current data do not signal a broadening of the hominin dietary niche during the Middle Pleistocene; no meaningful variation was visible in the exploitation of smaller-sized bovids or dangerous game, with coastal resources exploited when available. Proportions of anthropogenic bone surface modifications, and hence carcass processing intensity, do increase over time although more zooarcheological data is crucial before making behavioral inferences. Overall, this paper illustrates the potential of broad scale comparative faunal analyses to provide additional insights into processes of human behavioral evolution and the mechanisms underlying patterns of technological, chronological and contextual change.

Successful classification of experimental bone surface modifications (BSM) through machine learning algorithms: a solution to the controversial use of BSM in paleoanthropology?

Here, I show that powerful machine learning (ML) algorithms can efficiently classify most BSM of any given controlled experimental assemblage. In some cases, classification may reach an accuracy of 100%. No other statistical tool commonly used in taphonomy had been this successful at classification before. However, the heuristics of ML algorithms depend tightly on the objectivity in raw data collection. The use of multivariate approaches in which variables are independently scored by the analyst introduces a subjective bias. In this work, I show that different analysts producing raw data on the same testing data set can lead to widely divergent BSM classifications and interpretations using the same powerful ML algorithms. It is emphasized that until an objective non-biasing method of raw data collection is implemented, BSM classification carried out via statistical tests will remain heuristically limited. As a consequence, the application of these referents to archeological BSM does not guarantee a correct interpretation beyond the analyst expertise.

The role of small prey in hunter–gatherer subsistence strategies from the Late Pleistocene–Early Holocene transition site in NE Iberia: the leporid accumulation from the Epipalaeolithic level of Balma del Gai site

In Mediterranean Europe during the Pleistocene–Holocene, transition changes in the intensification of small prey exploitation by humans are detected. In the NE of the Iberian Peninsula, these changes are mainly evidenced by an increase in the number of rabbit remains, normally exceeding the 90% of the recovered animal specimens. The archaeological site of Balma del Gai (Moia, Barcelona) provides one of the most significant archaeological records of this kind. Through zooarchaeological and taphonomic analysis, our work’s aims are: to elucidate the human activity on leporids (rabbits and hares) in comparison to other predators, to understand the different ways of handling and exploiting rabbit carcasses and to assess the importance of this small prey for the Epipalaeolithic hunter–gatherers. Results on anatomical representation, breakage and bone surface modifications show that rabbit remains recovered respond to an anthropogenic contribution. High proportions of thermo-altered bones, cut marks caused by lithic tools and teeth marks are observed. All parameters indicate intense exploitation of rabbit fur and meat. This study shows clear evidence of the importance of small prey for human subsistence during this period.

Assessing the effects of fluvial abrasion on bone surface modifications using high-resolution 3-D scanning

Stone tool cut marks and carnivore tooth marks on fossil bones are invaluable traces of the feeding behavior and ecology of our ancestors. Recent research has focused on quantifying the micromorphology of experimentally created cut and tooth marks using high-resolution 3-D scanning technology with the goal of improving the reliability and accuracy of interpretations that are based on these feeding traces. However, to apply these experimental data in the identification and interpretation of bone surface modifications on fossil bones, the effect of post-depositional processes on the micromorphology of feeding traces must be known. Previous research has shown that fluvial abrasion has a substantial impact on the qualitative criteria that are used to identify cut marks, but the specific effect of hydraulic transport and abrasion on quantifiable attributes of bone surface modifications is completely unknown.The objective of this research is to understand the effects of fluvial abrasion on the micromorphology of cut marks and mammalian carnivore tooth marks using high-resolution 3-D data. An experimental study was undertaken by tumbling cattle and deer bones in a rock tumbler filled with sand and water to simulate the effects of fluvial abrasion on the bone surface modifications. Variables were measured using qualitative (visual observation of rounding and polishing of the bone surfaces, and the effect of these processes on the potential identifiability of cut and tooth marks) and quantitative (surface area, volume, maximum depth, mean depth, maximum length, maximum width, roughness, angle, and radius of the marks) criteria. The 3-D data from cut and tooth marks was collected using a white-light confocal profilometer and analyzed with specialized surface metrology software. Analyses of both qualitative and quantitative data indicate that fluvial abrasion has a greater effect on cut marks than tooth marks, suggesting that the frequency of cut marks could be underestimated relative to carnivore tooth marks in archaeological assemblages preserved in fluvial environments. This could affect interpretations of hominin feeding behavior based on the abundance of bone surface modifications when relying on qualitative methods alone. However, 98.6% of marks were identified correctly after tumbling when using multivariate quantitative methods of analysis bolstering the need for applying these methods in the interpretation of bone surface modifications on fossil specimens.

Testing accuracy in 2D and 3D geometric morphometric methods for cut mark identification and classification.

The analysis of bone surface modifications (BSMs) is a prominent part of paleoanthropological studies, namely taphonomic research. Behavioral interpretations of the fossil record hinge strongly upon correct assessment of BSMs. With the significant impact of microscopic analysis to the study of BSMs, multiple authors have discussed the reliability of these technological improvements for gaining resolution in BSM discrimination. While a certain optimism is present, some important questions are ignored and others overemphasized without appropriate empirical support. This specifically affects the study of cut marks. A diversity of geometric morphometric approaches applied to the study of cut marks have resulted in the coexistence (and competition) of different 2D and 3D methods. The present work builds upon the foundation of experiments presented by Maté-González et al. (2015), Courtenay et al. (2017) and Otárola-Castillo et al. (2018) to contrast for the first time 2D and 3D methods in their resolution of cut mark interpretation and classification. The results presented here show that both approaches are equally valid and that the use of sophisticated 3D methods do not contribute to an improvement in accuracy.

Revisiting the hunting-versus-scavenging debate at FLK Zinj: A GIS spatial analysis of bone surface modifications produced by hominins and carnivores in the FLK 22 assemblage, Olduvai Gorge, Tanzania

This study presents a taphonomic examination of the ca. 1.84 million year old (Ma) bovid fauna from FLK Level 22 (FLK Zinj), Bed I, Olduvai Gorge using a new GIS spatial analysis approach. The FLK Zinj site contains some of the best preserved evidence for meat eating by hominins and has been used extensively in reconstructions of early hominin behavior. However, despite numerous interpretations of site function, there is currently no consensus regarding how hominins acquired carcasses at FLK Zinj. Some interpretations argue that Early Stone Age hominins had early access to fleshed carcasses through hunting or active scavenging, while others argue hominins had late access to largely defleshed carcasses through passive scavenging. In this study, GIS was used to record bone preservation in the FLK Zinj assemblage as well as in several modern, experimentally-modified bone assemblages, which are used for comparison. The placement of hominin- and carnivore-produced modifications was documented on bones from these assemblages, and the GIS Spatial Analyst was used to identify patterning in tooth mark clusters as well as patterning in cut mark locations. This analysis suggests hominins had early access to fleshed carcasses at FLK Zinj, particularly of smaller prey, which they may have acquired through hunting. Damage patterns on larger carcasses are more difficult to interpret, but are consistent with early access through hunting or aggressive scavenging. A reanalysis of carnivore tooth mark frequencies on the FLK Zinj bovid fauna also supports an early access scenario.

An experimental study of the patterned nature of anthropogenic bone breakage and its impact on bone surface modification frequencies

The analysis of bone breakage is one of the most relevant issues of current taphonomic studies. Available experimental analogies aim at differentiating agencies in the production of fractured bones. Possible equifinality presented by different agents can hinder the characterisation of bone breakage at archaeological sites. Equally important is the potential distortion that bone-breaking processes introduce in bone surface modification (BSM) frequencies. This study presents an experimental approach to the problem of identifying signatures for anthropogenic bone breaking as a product of direct hammerstone percussion. This study also contributes to improving the existing analogical framework on processes related to bone breakage of medium-sized animals (80–200 kg), since most previous experimentation has focused on smaller (10–80 kg) and larger (200–800 kg) carcasses. It has been possible to verify the existence of non-random and non-intentional breakage patterns on long bones due to their shape and structural properties. Thus, this introduces the possibility of correctly identifying anthropogenic fracture patterns in the archaeological record. Additionally, it also opens up the possibility of finding different cultural patterns. It has frequently been argued that the frequency of bone surface modifications correlates with fragmentation intensity. However, this assertion remained untested until now. Here, we test the frequency and occurrence of percussion and cut marks in faunal assemblages according to the intensity of green bone fragmentation. The results also improve the current referential framework in reference to interpretation of notches produced by dynamic loading.

Distinguishing butchery cut marks from crocodile bite marks through machine learning methods

All models of evolution of human behaviour depend on the correct identification and interpretation of bone surface modifications (BSM) on archaeofaunal assemblages. Crucial evolutionary features, such as the origin of stone tool use, meat-eating, food-sharing, cooperation and sociality can only be addressed through confident identification and interpretation of BSM, and more specifically, cut marks. Recently, it has been argued that linear marks with the same properties as cut marks can be created by crocodiles, thereby questioning whether secure cut mark identifications can be made in the Early Pleistocene fossil record. Powerful classification methods based on multivariate statistics and machine learning (ML) algorithms have previously successfully discriminated cut marks from most other potentially confounding BSM. However, crocodile-made marks were marginal to or played no role in these comparative analyses. Here, for the first time, we apply state-of-the-art ML methods on crocodile linear BSM and experimental butchery cut marks, showing that the combination of multivariate taphonomy and ML methods provides accurate identification of BSM, including cut and crocodile bite marks. This enables empirically-supported hominin behavioural modelling, provided that these methods are applied to fossil assemblages.

Taphonomy of bird (Aves) remains at Laili Cave, Timor-Leste, and implications for human-bird interactions during the Pleistocene

The significance of the role of birds in hominin evolution in Island Southeast Asia is not clear. Few avian vertebrate deposits have been recovered from archaeological or fossil sites in the region, and their association with either hominin or natural deposition in caves and rock shelters complicates their usefulness in hominin behavioural and palaeoecological reconstructions. In this paper, we assess the taphonomic history of the Pleistocene avian vertebrate remains recovered from Laili Cave, Timor-Leste, dated to between ca. 44.6 to 11.2 ka and in association with abundant lithic material. We use avian taxonomic composition, skeletal element abundance, and bone surface modification data to determine the agent of avian skeletal deposition. Our analyses indicate that the small grassland and woodland birds (quail, buttonquail, song birds), which dominate the assemblage, were deposited by avian predators (probably barn owls) throughout the sequence. Humans possibly hunted the small quantity of larger birds (imperial pigeon, duck). The bird remains suggest that grasslands, woodland savannahs, wetlands, and forest environments were present locally during the Pleistocene.

Differentiating between cutting actions on bone using 3D geometric morphometrics and Bayesian analyses with implications to human evolution

Studies of bone surface modifications (BSMs) such as cut marks are crucial to our understanding of human and earlier hominin subsistence behavior. Over the last several decades, however, BSM identification has remained contentious, particularly in terms of identifying the earliest instances of hominin butchery; there has been a lack of consensus over how to identify or differentiate marks made by human and non-human actors and varying effectors. Most investigations have relied on morphology to identify butchery marks and their patterning. This includes cut marks, one of the most significant human marks. Attempts to discriminate cut marks from other types of marks have employed a variety of techniques, ranging from subjectively characterizing cut mark morphology using the naked eye, to using high-powered microscopy such as scanning electron microscopy (SEM) or micro-photogrammetry. More recent approaches use 3D datasets to obtain even more detailed information about mark attributes, and apply those to the fossil record. Although 3D datasets open promising new avenues for investigation, analyses of these datasets have not yet taken advantage of the full 3D surface morphology of BSM. Rather, selected cross-sectional slices of 3D scans have been used as proxies for overall shape. Here we demonstrate that 3D geometric morphometrics (GM), under the “Procrustes paradigm” and coupled with a Bayesian approach, probabilistically discriminates between marks caused by different butchery behaviors. At the same time, this approach provides a complete set of 3D morphological measurements and descriptions. Our results strengthen statistical confidence in cut mark identification and offer a novel approach that can be used to discriminate subtle differences between cut mark types in the fossil record. Furthermore, this study provides an incipient digital library with which to make future quantitative comparisons to archaeological examples, including contentious specimens that are key to understanding the earliest hominin butchery.

Use and abuse of cut mark analyses: The Rorschach effect

A series of experimental cut marks have been analyzed by eleven taphonomists with the goal of assessing if they could identify similarly 14 selected microscopic variables which would identify those marks as cut marks. The main objective was to test if variable identification could be made scientifically; that is, different researchers using the same method and criteria making the same assessment of each variable. This experiment shows that even in researchers trained in the same laboratories and following the same protocols divergences in the perception of each variable are significant. This indicates that mark perception and interpretation is a highly subjective process. If this basic analytical stage is subjective, subjectivity permeates to a greater degree the higher inferential stages leading from mark identification to reconstruction of butchering behaviors based on mark frequencies, mark anatomical distribution, actor-effector-trace processes, and statistical interpretations of the stochastic mark-imparting butchering processes. Here, we emphasize that the use of bone surface modifications for behavioral interpretations remains a non-scientific endeavor because of lack of independent replicability of criteria and processes, divergences in how variables are selected and used and epistemologically flawed analogs. This constitutes a major call to taphonomy to engage in more scientific (i.e., objective) approaches to the study of bone surface modifications for taphonomic inference elaboration.

Identifying the accumulator: Making the most of bone surface modification data

Taphonomic analysis is an essential component of zooarchaeology, but is employed in different ways within different research traditions. Within the Africanist Palaeolithic literature, there is a strong emphasis on quantitative comparison of proportions of different bone surface modifications to one another and to proportions observed on modern experimental collections. This work has been driven by debates about the taphonomic histories of Oldowan sites that document the subsistence strategies of early Homo, but this specific approach can be usefully applied to a range of contexts across many different time periods and geographic locations. One obstacle to the cross-fertilization of this taphonomic tradition with other zooarchaeological work is the restrictive manner in which data are selected from an assemblage for analysis. To ensure comparability between fossil and modern assemblages, analysts typically exclude specimens with evidence for post-depositional modification not modeled in the experimental data. Although this adds interpretive robustness, it can diminish sample size significantly, sometimes to the point of affecting statistical analyses, and results in much time invested in collecting data that ultimately are not used. Here, we describe a new method for maximizing the number of specimens that can be incorporated into analysis, thus resolving the persistent problem of poor sample sizes to make more statistically robust comparisons to actualistic datasets.

The trajectory of bone surface modification studies in paleoanthropology and a new Bayesian solution to the identification controversy

A critical issue in human evolution is how to determine when hominins began incorporating significant amounts of meat into their diets. This fueled evolution of a larger brain and other adaptations widely considered unique to modern humans. Determination of the spatiotemporal context of this shift rests on accurate identification of fossil bone surface modifications (BSM), such as stone tool butchery marks. Multidecade-long debates over the agents responsible for individual BSM are indicative of systemic flaws in current approaches to identification. Here we review the current state of BSM studies and introduce a novel probabilistic approach to identifying agents of BSM. We use control assemblages of bones modified by modern agents to train a multivariate Bayesian probability model. The model then identifies BSM agents with associated uncertainties, serving as the basis for a predictive probabilistic algorithm. The multivariate Bayesian approach offers a novel, probabilistic, and analytical method for BSM research that overcomes much of the bias that has typified previous, more qualitative approaches.

FOLSOM MAMMOTH HUNTERS? THE TERMINAL PLEISTOCENE ASSEMBLAGE FROM OWL CAVE (10BV30), WASDEN SITE, IDAHO

The 1960s and 1970s excavations at Owl Cave (10BV30) recovered mammoth bone and Folsom-like points from the same strata, suggesting evidence for a post-Clovis mammoth kill. However, a synthesis of the excavation data was never published, and the locality has since been purged from the roster of sites with human/extinct megafauna associations. Here, we present dates on bone from the oldest stratum, review provenience data, conduct a bone-surface modification study, and present the results of a protein-residue analysis. Our study fails to make the case for mammoth hunting by Folsom peoples. Although two of the point fragments tested positive for horse or elephant protein, recent AMS dates indicate that all of the mammoth remains predate Folsom, and horse remains are absent from the Owl Cave collection. Further, no unambiguously cultural surface modifications were identified on any of the mammoth remains. Given the available data, the Owl Cave deposits are most parsimoniously read as containing a Folsom-age occupation in a buried context, the first of its kind in the desert West, but one nonetheless part of a palimpsest of terminal Pleistocene materials.

Taphonomic signature of golden eagles (Aquila chrysaetos) on bone prey remains

The golden eagle (Aquila chrysaetos) is one of the most important birds of prey in the Northern Hemisphere. This raptor is used to building large nests in high cliffs to which they return for several breeding years accumulating important amounts of their prey skeletal remains. This makes the golden eagle one of the major predators able to accumulate faunal remains in archaeological sites. Despite this fact, the taphonomic signature of golden eagles has not been properly characterized. Here we present the analysis of ingested and non-ingested faunal remains predated and accumulated by this raptor in two different nesting areas from the Iberian Peninsula. Results show how the faunal taxonomic record may vary depending on the ecological zone. Leporids and terrestrial carnivores are the best represented. The observed anatomical representation, breakage and bone surface modification patterns are discussed for different taxa. The taphonomic pattern varies depending on the type of prey and the origin of skeletal materials (non-ingested vs. pellets). Finally, after comparing our results with marks left by other predators, several characteristic features are noted to recognise golden eagles as agents of animal bones accumulations in the fossil record.

A new high-resolution 3-D quantitative method for identifying bone surface modifications with implications for the Early Stone Age archaeological record

Bone surface modifications have become important indicators of hominin behavior and ecology at prehistoric archaeological sites. However, the method by which we identify and interpret these marks remains largely unchanged despite decades of research, relying on qualitative criteria and lacking standardization between analysts. Recently, zooarchaeologists have begun using new technologies capable of capturing 3-D data from bone surface modifications to advance our knowledge of these informative traces. However, an important step in this research has been overlooked and after years of work, we lack both a universal and replicable protocol and an understanding of the precision of these techniques. Here we propose a new standard for identifying bone surface modifications using high-resolution 3-D data and offer a systematic and replicable approach for researchers to follow. Data were collected with a white-light non-contact confocal profilometer and analyzed with Digital Surf's Mountains® software. Our data show that when methods are standardized, results between researchers are statistically indistinguishable. Multivariate analyses using the measured parameters allow discrimination between stone tool cut marks and mammalian carnivore tooth marks with 97.5% accuracy. Application of this method to fossil specimens resulted in 100% correspondence with identifications made by an experienced analyst using macroscopic observations of qualitative features of bone surface modifications. High-resolution 3-D analyses of bone surface modifications have great potential to improve the reliability and accuracy of taphonomic research, but only if our methods are replicable and precise.

Direct evidence for human exploitation of birds in the Middle Stone Age of South Africa: The example of Sibudu Cave, KwaZulu-Natal

Here, we present direct taphonomic evidence for the exploitation of birds by hunter-gatherers in the Middle Stone Age of South Africa as far as ∼77 ka. The bird assemblage from Sibudu Cave, KwaZulu-Natal, was analysed for bone surface modifications. Cut-marks associated with skinning, defleshing, and disarticulation, perforations on distal humeri produced during disarticulation of the forewing, peeling, and human tooth marks were observed on bird bones (i.e., mostly pigeons, doves, Galliformes, waders, and raptors) recovered from pre-Still Bay, Still Bay, Howiesons Poort, and post-Howiesons Poort techno-complexes. We conducted experiments to butcher, disarticulate, cook, and consume pigeon and dove carcasses, in order to create a comparative collection of bone surface modifications associated with human consumption of these birds. Human/bird interactions can now be demonstrated outside of Europe and prior to 50 ka. The evidence sheds new light on Middle Stone Age subsistence strategies in South Africa and introduces a fresh argument to the debate regarding the early emergence of behaviours usually associated with Later Stone Age hunter-gatherers.

When wildcats feed on rabbits: an experimental study to understand the taphonomic signature of European wildcats (Felis silvestris silvestris)

Studies of the feeding ecology of the European wildcat (Felis silvestris silvestris) demonstrate that leporids, mostly European rabbit (Oryctolagus cuniculus), dominate their diet in regions where they are present. The remains of wildcats have been found at Pleistocene and Holocene archaeological sites, raising the possibility that they actively accumulated leporid bones in caves and shelters shared with other terrestrial carnivores, raptors and humans. We present the first taphonomic study of rabbit remains consumed by this terrestrial carnivore, with the ultimate aim of understanding their role in bone accumulations at archaeological sites. An experimental study was carried out with a wildcat female, who was fed with nine complete rabbit carcasses. Non-ingested remains and scats were recovered for the analysis of anatomical representation, breakage and bone surface modification. This revealed that non-ingested remains and scats of the European wildcat can be discriminated from most other agents of accumulation. The referential framework provided will permit the discrimination of hominids and wildcats as agents of fossil accumulations of rabbits.