Anna K. Behrensmeyer

Taphonomic and ecologic information from bone weathering

Bones of recent mammals in the Amboseli Basin, southern Kenya, exhibit distinctive weathering characteristics that can be related to the time since death and to the local conditions of temperature, humidity and soil chemistry. A categorization of weathering characteristics into six stages, recognizable on descriptive criteria, provides a basis for investigation of weathering rates and processes. The time necessary to achieve each successive weathering stage has been calibrated using known-age carcasses. Most bones decompose beyond recognition in 10 to 15 yr. Bones of animals under 100 kg and juveniles appear to weather more rapidly than bones of large animals or adults. Small-scale rather than widespread environmental factors seem to have greatest influence on weathering characteristics and rates. Bone weathering is potentially valuable as evidence for the period of time represented in recent or fossil bone assemblages, in- cluding those on archeological sites, and may also be an important tool in censusing populations of animals in modern ecosystems.

Systematic Butchery by Plio/Pleistocene Hominids at Olduvai Gorge, Tanzania

Human origins research by archaeologists has expanded the evidence of the diet and subsistence activities of ancient hominids. We examine an important component of that evidence, the 1.75-million-year-old faunal assemblage from the FLK Zinjanthropus site at Olduvai Gorge, Tanzania. Skeletal-part frequencies are used to evaluate hominid access to and differential transport of carcass portions of differing nutritional value. Cut-mark frequencies and locations are used to evaluate butchery patterns including skinning, disarticulation, and defleshing of carcasses. In contrast to other recently published assessments of the FLK Zinjanthropus data, we conclude that (1) ancient hominids had full access to meaty carcasses of many small and large animals prior to any substantial loss of meat or marrow bones through other predator or scavenger feeding; (2) ancient hominids were butchering animal carcasses by an efficient and systematic technique that involved skinning, disarticulation, and defleshing; and (3) the FLK Zinjanthropus site represents a place where the secondary butchering of selected carcass portions and the consumption of substantial quantities of meat and marrow occurred.

Taphonomy and paleobiology

Abstract Taphonomy plays diverse roles in paleobiology. These include assessing sample quality relevant to ecologic, biogeographic, and evolutionary questions, diagnosing the roles of various taphonomic agents, processes and circumstances in generating the sedimentary and fossil records, and reconstructing the dynamics of organic recycling over time as a part of Earth history. Major advances over the past 15 years have occurred in understanding (1) the controls on preservation, especially the ecology and biogeochemistry of soft-tissue preservation, and the dominance of biological versus physical agents in the destruction of remains from all major taxonomic groups (plants, invertebrates, vertebrates); (2) scales of spatial and temporal resolution, particularly the relatively minor role of out-of-habitat transport contrasted with the major effects of time-averaging; (3) quantitative compositional fidelity; that is, the degree to which different types of assemblages reflect the species composition and abundance of source faunas and floras; and (4) large-scale variations through time in preservational regimes (megabiases), caused by the evolution of new bodyplans and behavioral capabilities, and by broad-scale changes in climate, tectonics, and geochemistry of Earth surface systems. Paleobiological questions regarding major trends in biodiversity, major extinctions and recoveries, timing of cladogenesis and rates of evolution, and the role of environmental forcing in evolution all entail issues appropriate for taphonomic analysis, and a wide range of strategies are being developed to minimize the impact of sample incompleteness and bias. These include taphonomically robust metrics of paleontologic patterns, gap analysis, equalizing samples via rarefaction, inferences about preservation probability, isotaphonomic comparisons, taphonomic control taxa, and modeling of artificial fossil assemblages based on modern analogues. All of this work is yielding a more quantitative assessment of both the positive and negative aspects of paleobiological samples. Comparisons and syntheses of patterns across major groups and over a wider range of temporal and spatial scales present a challenging and exciting agenda for taphonomy in the coming decades.

Faunal and environmental change in the late Miocene Siwaliks of northern Pakistan

Abstract The Siwalik formations of northern Pakistan consist of deposits of ancient rivers that existed throughout the early Miocene through the late Pliocene. The formations are highly fossiliferous with a diverse array of terrestrial and freshwater vertebrates, which in combination with exceptional lateral exposure and good chronostratigraphic control allows a more detailed and temporally resolved study of the sediments and faunas than is typical in terrestrial deposits. Consequently the Siwaliks provide an opportunity to document temporal differences in species richness, turnover, and ecological structure in a terrestrial setting, and to investigate how such differences are related to changes in the fluvial system, vegetation, and climate. Here we focus on the interval between 10.7 and 5.7 Ma, a time of significant local tectonic and global climatic change. It is also the interval with the best temporal calibration of Siwalik faunas and most comprehensive data on species occurrences. A methodological focus of this paper is on controlling sampling biases that confound biological and ecological signals. Such biases include uneven sampling through time, differential preservation of larger animals and more durable skeletal elements, errors in age-dating imposed by uncertainties in correlation and paleomagnetic timescale calibrations, and uneven taxonomic treatment across groups. We attempt to control for them primarily by using a relative-abundance model to estimate limits for the first and last appearances from the occurrence data. This model also incorporates uncertainties in age estimates. Because of sampling limitations inherent in the terrestrial fossil record, our 100-Kyr temporal resolution may approach the finest possible level of resolution for studies of vertebrate faunal changes over periods of millions of years. Approximately 40,000 specimens from surface and screenwash collections made at 555 localities form the basis of our study. Sixty percent of the localities have maximum and minimum age estimates differing by 100 Kyr or less, 82% by 200 Kyr or less. The fossils represent 115 mammalian species or lineages of ten orders: Insectivora, Scandentia, Primates, Tubulidentata, Proboscidea, Pholidota, Lagomorpha, Perissodactyla, Artiodactyla, and Rodentia. Important taxa omitted from this study include Carnivora, Elephantoidea, and Rhinocerotidae. Because different collecting methods were used for large and small species, they are treated separately in analyses. Small species include insectivores, tree shrews, rodents, lagomorphs, and small primates. They generally weigh less than 5 kg. The sediments of the study interval were deposited by coexisting fluvial systems, with the larger emergent Nagri system being displaced between 10.1 and 9.0 Ma by an interfan Dhok Pathan system. In comparison to Nagri floodplains, Dhok Pathan floodplains were less well drained, with smaller rivers having more seasonally variable flow and more frequent avulsions. Paleosol sequences indicate reorganization of topography and drainage accompanying a transition to a more seasonal climate. A few paleosols may have formed under waterlogged, grassy woodlands, but most formed under drier conditions and more closed vegetation. The oxygen isotopic record also indicates significant change in the patterns of precipitation beginning at 9.2 Ma, in what may have been a shift to a drier and more seasonal climate. The carbon isotope record demonstrates that after 8.1 Ma significant amounts of C4 grasses began to appear and that by 6.8 Ma floodplain habitats included extensive C4 grasslands. Plant communities with predominantly C3 plants were greatly diminished after 7.0 Ma, and those with predominantly C4 plants, which would have been open woodlands or grassy woodlands, appeared as early as 7.4 Ma. Inferred first and last appearances show a constant, low level of faunal turnover throughout the interval 10.7–5.7-Ma, with three short periods of elevated turnover at 10.3, 7.8, and 7.3–7.0 Ma. The three pulses account for nearly 44% of all turnover. Throughout the late Miocene, species richness declined steadily, and diversity and richness indices together with data on body size imply that community ecological structure changed abruptly just after 10 Ma, and then again at 7.8 Ma. Between 10 and 7.8 Ma the large-mammal assemblages were strongly dominated by equids, with more balanced faunas before and after. The pattern of appearance and disappearance is selective with respect to inferred habits of the animals. Species appearing after 9.0 Ma are grazers or typical of more open habitats, whereas many species that disappear can be linked to more closed vegetation. We presume exceptions to this pattern were animals of the mixed C3/C4 communities or the wetter parts of the floodplain that did not persist into the latest Miocene. The pace of extinction accelerates once there is C4 vegetation on the floodplain. The 10.3 Ma event primarily comprises disappearance of taxa that were both common and of long duration. The event does not correlate to any obvious local environmental or climatic event, and the pattern of species disappearance and appearance suggests that biotic interactions may have been more important than environmental change. The 7.8 Ma event is characterized solely by appearances, and that at 7.3 Ma by a combination of appearances and disappearances. These two latest Miocene events include more taxa that were shorter ranging and less common, a difference of mode that developed between approximately 9.0 and 8.5 Ma when many short-ranging and rare species began to make appearances. Both events also show a close temporal correlation to changes in floodplain deposition and vegetation. The 7.8 Ma event follows the widespread appearance of C4 vegetation and is coincident with the shift from equid-dominated to more evenly balanced large-mammal assemblages. The 7.3 to 7.0 Ma event starts with the first occurrence of C4-dominated floras and ends with the last occurrence of C3-dominated vegetation. Absence of a consistent relationship between depositional facies and the composition of faunal assemblages leads us to reject fluvial system dynamics as a major cause of faunal change. The close correlation of latest Miocene species turnover and ecological change to expansion of C4 plants on the floodplain, in association with oxygen isotopic and sedimentological evidence for increasingly drier and more seasonal climates, causes us to favor explanations based on climatic change for both latest Miocene pulses. The Siwalik record supports neither “coordinated stasis” nor “turnover pulse” evolutionary models. The brief, irregularly spaced pulses of high turnover are characteristic of both the stasis and pulse models, but the high level of background turnover that eliminates 65–70% of the initial species shows there is no stasis in the Siwalik record. In addition, the steadily declining species richness and abrupt, uncoordinated changes in diversity do not fit either model.

Vertebrate preservation in fluvial channels

Abstract 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, unidentifiable 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 vertebrate 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.

Isotopic Tracking of Change in Diet and Habitat Use in African Elephants

The carbon, nitrogen, and strontium isotope compositions of elephants in Amboseli Park, Kenya, were measured to examine changes in diet and habitat use since the 1960s. Carbon isotope ratios, which reflect the photosynthetic pathway of food plants, record a shift in diet from trees and shrubs to grass. Strontium isotope ratios, which reflect the geologic age of bedrock, document the concentration of elephants within the park. The high isotopic variability produced by behavioral and ecological shifts, if it is representative of other East African elephant populations, may complicate the use of isotopes as indicators of the source region of ivory.

Taphonomy's Contributions To Paleobiology

Taphonomy established itself in paleontology primarily as a subdiscipline of paleoecology, but it has evolved into a much broader study of the ways in which preservation affects the fossil record. The past decade has seen a change in emphasis from descriptive taphonomic studies of fossil assemblages to more experimental, process-oriented investigations of necrolysis, stratification, and diagenesis of organic remains in modern environments. These actualistic studies are increasing the sophistication of taphonomic analysis in the fossil record by sharpening the diagnosis of bias in paleontological data and by providing a baseline for quantitative modeling of preservational patterns. The analysis of bias is also expanding into the evaluation of temporal resolution in the fossil record (sample acuity, stratigraphic completeness), and taphonomic research is thus contributing to broad-scale problems in evolution, biogeography, and bio- stratigraphy. In addition, taphonomic studies are providing new insights into paleoenvironmental recon- struction and into the direct paleobiological significance of post mortem processes such as the behavior of scavengers and the role of dead hardparts in structuring benthic communities. One of taphonomys most promising new frontiers is comparative analysis applied to different taxonomic groups within assemblages and across environments, tectonic settings, and dimatic regimes. All of this currently active research is contributing to a better understanding of the fossil record as the result of a dynamic, evolving, integrated system of biological and sedimentological processes that have both limited and enhanced knowledge of Earth history.

New perspectives in vertebrate paleoecology from a recent bone assemblage

Interpretations of vertebrate paleoecology depend on knowledge of taphonomical processes which alter the composition of the preserved fossil assemblage from that of the original community. Study of the potential fossil record of a recent mammal community in Amboseli National Park, Kenya, shows the effects of some of these biasing processes and demonstrates how a bone assemblage on a modern land surface can be a source of past and present ecological information. In the bone assemblage, species presence or absence and relative abundance differ from recorded living species occurrences and population sizes: only 74% of the extant species in the basin are identified in the bone sample, and carcass abundances vary significantly from known population sizes of the major herbivore species. Both biases appear to be strongly correlated to body size, and this results from greater destruction of bones of smaller animals within the weight range from about 1-1000 kg. This size-biasing against small species appears to be due primarily to the greater susceptibility of small bones to destruction by carnivore mastication, breakage through bioturbation (trampling), and physical and chemical processes of weathering. Size- biasing resulting from such primary processes can thus be inherited by buried bone assemblages whatever their final mode of deposition. The bone assemblage also provides information on the spatial distributions of the major herbivore species over six major habitats. Patterns of strong habitat specificity are accurately represented in the bone assemblage. However, the record for certain species is affected by their seasonal and diurnal habitat shifts so that their bone distributions do not match live census data. The Amboseli bone assemblage provides a modern analogue for taphonomical processes which may have affected fossil assemblages derived from paleo-land surfaces prior to fluvial transport. It also helps to define limits of resolution in interpreting paleoecological information from such fossil assemblages.

FxJj50: An early Pleistocene site in northern Kenya

Abstract Excavation in the Upper Member of the Koobi Fora Formation in Kenya has revealed a cluster of stone artefacts and broken up bones which accumulated 1–5 million years ago on the banks of a water course. The assemblage had been preserved by layers of silt. The stone artefacts consist of flakes and flake fragments plus simple flaked cobbles. It has been possible to conjoin individual pieces linking about 10 per cent of the artefacts and 4 per cent of the identifiable bones in pairs or sets. In some cases it seems likely that the specimens were fractured on the spot. Some of the fracture patterns on the bones suggest breakage with hammers, and apparent cut marks have also been found on some bones. There are signs of the presence of scavenging carnivores as well as of tool‐making hominids, and both could have contributed to the workings of a complex input‐output system. Whether the site was a home‐base camp or simply a locality used for meat‐eating and tool‐making remains uncertain. Experimental work ...

Early Hominin Foot Morphology Based on 1.5-Million-Year-Old Footprints from Ileret, Kenya

Hominin footprints offer evidence about gait and foot shape, but their scarcity, combined with an inadequate hominin fossil record, hampers research on the evolution of the human gait. Here, we report hominin footprints in two sedimentary layers dated at 1.51 to 1.53 million years ago (Ma) at Ileret, Kenya, providing the oldest evidence of an essentially modern human–like foot anatomy, with a relatively adducted hallux, medial longitudinal arch, and medial weight transfer before push-off. The size of the Ileret footprints is consistent with stature and body mass estimates for Homo ergaster/erectus, and these prints are also morphologically distinct from the 3.75-million-year-old footprints at Laetoli, Tanzania. The Ileret prints show that by 1.5 Ma, hominins had evolved an essentially modern human foot function and style of bipedal locomotion.