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bioRxiv2018

A Reassessment of the Taxonomic Position of Mesosaurs Based on Two Data Matrices

1The Early Permian mesosaurs are the oldest known primarily aquatic amniotes. Despite the interest that they have generated over time, their affinities remain controversial. Recently, two hypotheses have been supported, in which mesosaurs are either the sister-group of all other sauropsids, or the sister-group of other parareptiles. We recently upheld the former hypothesis, but in the latest study on mesosaur affinities, MacDougall et al. published a study highly critical of our work, while upholding the hypothesis that mesosaurs are basal parareptiles. We expect that the debate about mesosaur affinities will continue in the foreseeable future, but we wish to respond to the two central comments published by MacDougall et al. in 2018, who argue that variability in the temporal fenestration of early sauropsids, combined with the omission of several recently-described parareptile taxa, explain the differences in topologies between their study and ours. Reanalyzing our data matrix and theirs without characters linked with temporal fenestration, and removing from their matrix the parareptile taxa that they added (and that we omitted) does not alter the resulting topologies. Thus, their main conclusions are false; the differences in taxonomic position of mesosaurs must result from character choice and scoring differences.

paleontology↗

bioRxiv2018

Kinematics of wings from Caudipteryx to modern birds

This study seeks to better quantify the parameters that drove the evolution of flight from nonvolant winged dinosaurs to modern birds. In order to explore this issue, we used fossil data to model the feathered forelimb of Caudipteryx, the most basal non-volant maniraptoran dinosaur with elongate pennaceous feathers that could be described as forming proto-wings. In order to quantify the limiting flight factors, we created three hypothetical wing profiles for Caudipteryx representing incrementally larger wingspans, which we compared to the actual wing morphology as what revealed through fossils. These four models were analyzed under varying air speed, wing beat amplitude, and wing beat frequency to determine lift, thrust potential and metabolic requirements. We tested these models using theoretical equations in order to mathematically describe the evolutionary changes observed during the evolution of modern birds from a winged terrestrial theropod like Caudipteryx. Caudipteryx could not fly, but this research indicates that with a large enough wing span Caudipteryx-like animal could have flown, the morphology of the shoulder girdle would not actually accommodate the necessary flapping angle and metabolic demands would be much too high to be functional. The results of these analyses mathematically confirm that during the evolution of energetically efficient powered flight in derived maniraptorans, body weight had to decrease and wing area/wing profile needed to increase together with the flapping angle and surface area for the attachment of the flight muscles. This study quantifies the morphological changes that we observe in the pennaraptoran fossil record in the overall decrease in body size in paravians, the increased wing surface area in Archaeopteryx relative to Caudipteryx, and changes observed in the morphology of the thoracic girdle, namely the orientation of the glenoid and the enlargement of the sternum.

paleontology↗

bioRxiv2018

Life Inside A Dinosaur Bone: A Thriving Microbiome

Fossils were long thought to lack original organic material, but the discovery of organic molecules in fossils and sub-fossils, thousands to millions of years old, has demonstrated the potential of fossil organics to provide radical new insights into the fossil record. How long different organics can persist remains unclear, however. Non-avian dinosaur bone has been hypothesised to preserve endogenous organics including collagen, osteocytes, and blood vessels, but proteins and labile lipids are unstable during diagenesis or over long periods of time. Furthermore, bone is porous and an open system, allowing microbial and organic flux. Some of these organics within fossil bone have therefore been identified as either contamination or microbial biofilm, rather than original organics. Here, we use biological and chemical analyses of Late Cretaceous dinosaur bones and sediment matrix to show that dinosaur bone hosts a diverse microbiome. Fossils and matrix were freshly-excavated, aseptically-acquired, and then analysed using microscopy, spectroscopy, chromatography, spectrometry, DNA extraction, and 16S rRNA amplicon sequencing. The fossil organics differ from modern bone collagen chemically and structurally. A key finding is that 16S rRNA amplicon sequencing reveals that the subterranean fossil bones host a unique, living microbiome distinct from that of the surrounding sediment. Even in the subsurface, dinosaur bone is biologically active and behaves as an open system, attracting microbes that might alter original organics or complicate the identification of original organics. These results suggest caution regarding claims of dinosaur bone soft tissue preservation and illustrate a potential role for microbial communities in post-burial taphonomy.

paleontology↗

bioRxiv2018

The multi-peak adaptive landscape of crocodylomorph body size evolution

Background Little is known about the long-term patterns of body size evolution in Crocodylomorpha, the > 200-million-year-old group that includes living crocodylians and their extinct relatives. Extant crocodylians are mostly large-bodied (3–7 m) predators. However, extinct crocodylomorphs exhibit a wider range of phenotypes, and many of the earliest taxa were much smaller (< 1.2 m). This suggests a pattern of size increase through time that could be caused by multi-lineage evolutionary trends of size increase or by selective extinction of small-bodied species. In this study, we characterise patterns of crocodylomorph body size evolution using a model fitting-approach (with cranial measurements serving as proxies). We also estimate body size disparity through time and quantitatively test hypotheses of biotic and abiotic factors as potential drivers of crocodylomorph body size evolution.Results Crocodylomorphs reached an early peak in body size disparity during the Late Jurassic, and underwent essentially continually decreases in disparity since then. A multi-peak Ornstein-Uhlenbeck model outperforms all other evolutionary models fitted to our data (including both uniform and non-uniform), indicating that the macroevolutionary dynamics of crocodylomorph body size are better described within the concept of an adaptive landscape, with most body size variation emerging after shifts to new macroevolutionary regimes (analogous to adaptive zones). We did not find support for a consistent evolutionary trend towards larger sizes among lineages (i.e., Cope’s rule), or strong correlations of body size with climate. Instead, the intermediate to large body sizes of some crocodylomorphs are better explained by group-specific adaptations. In particular, the evolution of a more aquatic lifestyle (especially marine) correlates with increases in average body size, though not without exceptions.Conclusions Shifts between macroevolutionary regimes provide a better explanation of crocodylomorph body size evolution than do climatic factors, suggesting a central role for lineage-specific adaptations rather than climatic forcing. Shifts leading to larger body sizes occurred in most aquatic and semi-aquatic groups. This, combined with extinctions of groups occupying smaller body size regimes (particularly during the Late Cretaceous and Cenozoic), gave rise to the upward-shifted body size distribution of extant crocodylomorphs compared to their smaller-bodied terrestrial ancestors.AbbreviationsCranial measurementsDCLdorsal cranial lengthODCLorbito-cranial lengthEvolutionary modelsBMBrownian motionEBEarly burstOUOrnstein-UhlenbeckBMSmulti-regime BM model that allows parameter σ2 to varyOUMVmulti-regime OU model that allows θ and σ2 to varyOUMAmulti-regime OU model in which θ and α can varyOUMVAOU model in which all three parameters (θ, α and σ2) can varyModel parametersθtrait optimum of OU-based modelsαattraction parameter of OU-based modelsσ2Brownian variance or rate parameter of BM or OU-based modelsμevolutionary trend parameter of BM-based modelsZ0estimated trait value at the root of the tree of OU-based modelsOptimality criteriaAICAkaike’s information criterionAICcAkaike’s information criterion for finite sample sizesBICBayesian information criterionpBICphylogenetic Bayesian information criterionView Full Text

paleontology↗

bioRxiv2018

New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA

Cheloniellids comprise a small, distinctive group of Paleozoic arthropods of whose phylogenetic relationships within the Arthropoda remain unresolved. A new form, Xus yus, n. gen, n. sp. is reported from the Waukesha Lagerstatte in the Brandon Bridge Formation (Silurian: Telychian), near Waukesha, Wisconsin, USA. Exceptionally preserved specimens show previously poorly known features including biramous appendages; this is the first cheloniellid to show large, anterior raptorial appendages. We emend the diagnosis of Cheloniellida; cephalic appendages are uniramous and may include raptorial appendages; trunk appendages are biramous.

paleontology↗

bioRxiv2018

Reversing Time Averaging and Reconstructing Extinction Rates with Approaches from Image Processing

In this paper, the relation between the extinction rate and the rate of last fossil occurrences as well as the relation between the fossil occurrence rate and the time averaged fossil occurrence rate is examined. Both relations are described by the same mathematical operation. This operation is commonly used in image processing, where it generates a blurring effect. Therefore the rate of last fossil occurrences can be taken as a blurred version of the extinction rate, and the time averaged fossil occurrence rate as a blurred version of the fossil occurrence rate. This connection has different applications. It allows to study the patterns different types of time averaging generate or the patterns of last fossil occurrences generated by different extinction rates. More importantly, it opens the possibility to use algorithms from image processing that reverse blurring effects for geological applications. This can be used to reverse the effects of time averaging or to reconstruct extinction rates from the rate of last fossil occurrences.

paleontology↗

bioRxiv2018

Surface sediment samples from early age of seafloor exploration can provide a late 19th century baseline of the marine environment

Ocean-floor sediment samples collected up to 150 years ago represent an important historical archive to benchmark global changes in the seafloor environment, such as species range shifts and invasions and pollution trends. Such benchmarking requires that the historical sediment samples represent the state of the environment at or shortly before the time of collection. However, early oceanographic expeditions sampled the ocean floor using devices like the sounding tube or a dredge, which potentially disturb the sediment surface and recover a mix of Holocene (surface) and deeper, Pleistocene sediments. Here we use climate-sensitive microfossils as a fast biometric method to assess if historical seafloor samples contain a mixture of modern and glacial sediments. Our assessment is based on comparing the composition of planktonic foraminifera (PF) assemblages in historical samples with Holocene and Last Glacial Maximum (LGM) global reference datasets. We show that eight out of the nine historical samples contain PF assemblages more similar to the Holocene than to the LGM PF assemblages, but the comparisons are only significant when there is a high local species temporal turnover (from the LGM to the Holocene). When analysing temporal turnover globally, we show that upwelling and temperate regions had greatest species turnover, which are areas where our methodology would be most diagnostic. Our results suggest that sediment samples from historical collections can provide a baseline of the state of marine ecosystems in the late 19th century, and thus be used to assess ocean global change trends.

paleontology↗

bioRxiv2018

The R package divDyn for quantifying diversity dynamics using fossil sampling data

O_LIUnbiased time series of diversity dynamics are vital for quantifying the grand history of life. Applications include identifying ancient mass extinctions and inferring both biotic and abiotic controls on diversification rates.\nC_LIO_LIWe introduce divDyn, a new R package that facilitates the calculation of taxonomic richness, extinction and origination rates from time-binned fossil sampling data. State-of-the-art sampling completeness metrics, counting protocols, and sampling standardisation functions permit the reconstruction of biologically meaningful time series. Additional functions permit the partitioning of turnover rates by trait and environmental affinity.\nC_LIO_LIWe display Phanerozoic-scale diversity dynamics of marine invertebrates using the divDyn package. Using the core function and standard subsampling options, we revisit earlier assessments of declining extinction and origination rates over time and of equilibrial diversity dynamics to assess their methodological dependency.\nC_LIO_LIThe modular and fast implementation of published methods ensures traceability, reproducibility, and comparability of future studies.\nC_LI

paleontology↗

bioRxiv2018

Illustrating phylogenetic placement of fossils using RoguePlots: An example from ichneumonid parasitoid wasps (Hymenoptera, Ichneumonidae) and an extensive morphological matrix

The fossil record constitutes the primary source of information about the evolutionary history of extant and extinct groups, and many analyses of macroevolution rely on fossils that are accurately placed within phylogenies. To avoid misinterpretation of the fossil record, especially by non-palaeontologists, the proper assessment and communication of uncertainty in fossil placement is crucial. We here use Bayesian morphological phylogenetics to evaluate the classifications of fossil parasitoid wasps (Hymenoptera, Ichneumonidae) and introduce RoguePlots to illustrate placement uncertainty on the phylogeny of extant taxa. Based on an extensive, newly constructed morphological matrix of 222 characters in 24 fossil and 103 extant taxa, we test three different aspects of models of morphological evolution. We find that a model that includes ordered characters, among-character rate variation, and a state-space restricted to observed states achieves the highest marginal likelihoods. The individual RoguePlots reveal large differences in confidence in the placement of the different fossils and allow some refinements to their classification: Polyhelictes bipolarus and Ichninsum appendicrassum are moved from an uncertain subfamily placement to Pimplinae, Plectiscidea lanhami is transferred to Allomacrus in Cylloceriinae (Allomacrus lanhami, comb. nov.), Lithotorus cressoni is moved from Diplazontinae to Orthocentrinae, and we note uncertainty in the generic placement of Xanthopimpla? messelensis. We discuss potential artefacts that might result in biased posterior probabilities in Bayesian morphological phylogenetic analyses, pertaining to character and taxon sampling, fossilization biases, and model misspecification. Finally, we suggest future directions both in ichneumonid palaeontology, in the modelling of morphological evolution, and in the way Bayesian phylogenetics can improve both assessment and representation of fossil placement uncertainty.

paleontology↗

bioRxiv2018

The ecomorphology of southern African rodent incisors: Potential applications to the hominin fossil record

The taxonomic identification of mammalian fauna within fossil assemblages is a well-established component of paleoenvironmental reconstructions. However, many fragmentary specimens recovered from fossil sites are often disregarded as they can be difficult to identify with the precision required for taxonomic methods. For this reason, the large numbers of isolated rodent incisors that are often recovered from hominin fossil bearing sites have generally been seen as offering little interpretive value. Ecomorphological analysis, often referred to as a \"taxon-free\" method, can potentially circumvent this problem by focusing on the adaptive, rather than the taxonomic significance of rodent incisor morphology. Here, we determine if the morphology of the upper incisors of modern southern African rodents reflects dietary behavior using discriminant function analysis. Our model suggests that a strong ecomorphological signal exists in our modern sample and we apply these results to two samples of isolated incisors from the hominin fossil bearing sites, Sterkfontein and Swartkrans.

paleontology↗

bioRxiv2018

Absence of general rules governing molluscan body-size response to climatic fluctuation during Cenozoic

Body size is a key factor in dictating the fate of interaction between an organism and its surrounding environment. A negative temperature-size relationship (TSR) has been suggested as one of the universal responses to climatic warming. It is also predicted that groups with narrow latitudinal range, tropical affinity and higher body size, would show higher sensitivity to climatic fluctuation. Moreover, because of the difference in thermal sensitivity, it is also expected that the response to climatic fluctuation would be different between epifaunal and infaunal groups. To confirm the generality of these relationship among marine families, we compiled the relationship between body-size and global temperature trends over Cenozoic using a database of marine benthic molluscs of class gastropoda and bivalvia resolved to temporal stages. We evaluated the dependence of climate induced body-size response to the existing size and latitudinal spread via correlating the first-difference correlation coefficient of temperature-size ({rho}1st (size-temp)) with maximum size and latitudinal spread of family respectively. Cenozoic record of this highly diverse group does not show any signature of TSR for molluscan class or for any other regional, ecological groups during the past 66My long climatic fluctuation. We did not find any evidence supporting heightened response to climatic fluctuation in groups with limited latitudinal spread or with large body-size. The tropical species did not show significant difference in their body-size response in comparison to temperate species. It also shows lack of any difference in response between ecological groups of molluscs with varying substrate relationship and hence, refutes the predicted variation due to difference in thermal specialization. Although a negative correlation between maximum latitudinal spread and {rho}1st (size-temp) is observed for infaunal families, it is not statistically significant. Our results highlight the limited validity of \"universal rules\" in explaining the climate induced morphological response of marine communities in deep time and underscores the complexity in generalizing the biotic outcome of future climatic fluctuation.

paleontology↗

bioRxiv2018

A new method for enamel amino acid racemization dating: a closed system approach

Analysis of the predictable breakdown of proteins and amino acids in ancient biominerals enables age estimation over the Quaternary. We postulate that enamel is a suitable biomineral for the long-term survival of endogenous amino acids. Analysis of multiple amino acids for geochronological studies is typically achieved using a RP-HPLC method. However, the low concentrations of amino acids coupled with high concentrations of inorganic species make accurate determination of amino concentrations challenging. We have developed a method for the routine preparation of multiple enamel samples using biphasic separation. Furthermore, we have shown that amino acids that exhibit effectively closed system behaviour can be isolated from enamel through an exposure time of 72 h to bleach. Elevated temperature experiments investigating the processes of intra-crystalline protein degradation (IcPD) do not appear to match the patterns from fossil samples, reinforcing the need for a comprehensive understanding of the underlying mechanisms of protein degradation. This novel preparative method isolates intra-crystalline amino acids suitable for the development of mammalian geochronologies based on enamel protein degradation. The lower rates of racemisation in enamel (cf. Bithynia opercula) suggest that the enamel AAR may be able to be used as a relative dating technique over time scales > 2.8 Ma. Enamel AAR has the potential to estimate the age of mammalian remains past the limit of all other current direct dating methods, providing an invaluable tool for geochronological studies.

paleontology↗

bioRxiv2018

Ignoring stratigraphic age uncertainty leads to erroneous estimates of species divergence times under the fossilized birth-death process

Fossil information is essential for estimating species divergence times, and can be integrated into Bayesian phylogenetic inference using the fossilized birth-death (FBD) process. An important aspect of palaeontological data is the uncertainty surrounding specimen ages, which can be handled in different ways during inference. The most common approach is to fix fossil ages to a point estimate within the known age interval. Alternatively, age uncertainty can be incorporated by using priors, and fossil ages are then directly sampled as part of the inference. This study presents a comparison of alternative approaches for handling fossil age uncertainty in analysis using the FBD process. Based on simulations, we find that fixing fossil ages to the midpoint or a random point drawn from within the stratigraphic age range leads to biases in divergence time estimates, while sampling fossil ages leads to estimates that are similar to inferences that employ the correct ages of fossils. Second, we show a comparison using an empirical dataset of extant and fossil cetaceans, which confirms that different methods of handling fossil age uncertainty lead to large differences in estimated node ages. Stratigraphic age uncertainty should thus not be ignored in divergence time estimation and instead should be incorporated explicitly.

paleontology↗

bioRxiv2018

Functional Anatomy, Biomechanical Performance Capabilities and Potential Niche of StW 573: an Australopithecus Skeleton (circa 3.67 Ma) From Sterkfontein Member 2, and its significance for The Last Common Ancestor of the African Apes and for Hominin Origins

Abstract (300 words)StW 573, from Sterkfontein Member 2, dated ca 3.67 Ma, is by far the most complete skeleton of an australopith to date. Joint morphology is in many cases closely matched in available elements of Australopithecus anamensis (eg. proximal and distal tibial and humeral joint-surfaces) and there are also close similarities to features of the scapula, in particular, of KSD-VP-1/1 A. afarensis from Woranso-Mille. The closest similarities are, however, to the partial skeleton of StW 431 from Sterkfontein Member 4. When considered together, both StW 573 and StW 431 express an hip joint morphology quite distinct from that of A. africanus Sts14, and a proximal femur of a presumed A. africanus from Jacovec Cavern at Sterkfontein, StW 598. This, and other evidence presented herein, suggests there are two pelvic girdle morphs at Sterkfontein, supporting Clarke (2013) in his recognition of a second species, A. prometheus, containing StW 573 and StW 431. StW 573 is the first hominid skeleton where limb proportions are known unequivocally. It demonstrates that some early hominins, at the time of formation of the Laetoli footprints (3.6 Ma), were large-bodied. with hindlimbs longer than forelimbs. Modelling studies on extant primates indicate that the intermembral index (IMI) of StW 573, low for a non-human great ape, would have substantially enhanced economy of bipedal walking over medium-to-long distances, but that it was still too high for effective walking while load-carrying. It would, however, have somewhat reduced the economy of horizontal climbing, but made Gorilla-like embracing of large tree-trunks less possible. Consideration of both ethnographic evidence from modern indigenous arboreal foragers and modern degeneracy theory cautions against prescriptive interpretations of hand- and foot-function, by confirming that both human-like upright bipedalism and functional capabilities of the hand and foot can be effective in short-distance arboreal locomotion.

paleontology↗

bioRxiv2018

A multiscale stratigraphic investigation of the context of StW 573 Little Foot and Member 2, Sterkfontein Caves, South Africa

The Sterkfontein Caves has an 80 year history of yielding remarkable evidence of hominin evolution and is currently the worlds richest Australopithecus-beafing site. Included in Sterkfonteins hominin assemblage is StW 573 ( Little Foot). Discovered in the Member 2 deposit in the Silberberg Grotto, StW 573 represents the most complete Australopithecus skeleton yet found. Because of its importance to the fossil hominin record, the geological age of Little Foot has been the subject of significant debate. Two main hypotheses have been proposed regarding the formation and age of Member 2 and by association StW 573. The first, proposes that Member 2 formed relatively rapidly, starting to accumulate at around 2.8 million years ago and that the unit is isolated to the Silberberg Grotto - the underlying chambers and passages forming later. The second proposes that Member 2 formed slowly, its accumulation starting before 3.67 million years ago and that the deposit extends into the Milner Hall and close to the base of the cave system. Both assume a primary association between StW 573 and Member 2, although which sediments in the Silberberg Grotto are associated with Member 2 has also been questioned. Recently a third alternative hypothesis questioning the association of StW 573 to Member 2 sediments proposed a two-stage burial scenario in which sediments associated with StW 573 represent a secondary and mixed-age deposit reworked from a higher cave. The stratigraphic and sedimentological implications of these hypotheses are tested here through the application of a multiscale investigation of Member 2, with reference to the taphonomy of the Little Foot skeleton. The complete infilling sequence of Member 2 is described and depositional units are tracked across all exposures of the deposit in the Silberberg Grotto and into the Milner Hall. Facies development follows patterns characteristic of colluvially accumulated taluses with 30-40{degrees} angles of repose developing coarser distal facies. Sediments are generally stratified and conformably deposited in a sequence of silty sands eroded from well-developed lateritic soils on the landscape surface. Voids, clasts and bioclasts are organized consistently across and through Member 2 according to the underlying deposit geometry, indicating a gradual deposit accretion with no distinct collapse facies evident, no successive debris flow accumulation, and only localized intra-unit post- depositional modification. The stratigraphy and sedimentology of Member 2 supports a simple single-stage accumulation process through which Member 2 partially fills the Silberberg Grotto and extends into the deeper chambers and passages of the Sterkfontein Caves. Through this work we demonstrate at multiple scales the primary association between the sediments of Member 2 and the StW 573 Little Foot skeleton.

paleontology↗

bioRxiv2018

Integrative isotopic Paleoecology (δ13C, δ18O) of a Late Pleistocene vertebrate community from Sergipe, NE Brazil

Isotopes are one of the best tools to reconstruct the Paleoecology of extinct taxa, yielding insights about their diet (through carbon; C3 and C4 plants), niche breadth (BA) and the environment in which they lived. In the present work we go deeper in the use of isotopes and explore a mathematical mixing model with the stable isotopes of two elements (carbon and oxygen) to (1) suggest the relative contribution of four types of food resources (leaves, fruits, roots and C4 grass) for meso- and megaherbivores (weight > 100 kg) that lived in the Late Pleistocene of Poco Redondo, Sergipe, Brasil, and (2) evaluate which of these herbivores could be the potential prey for the carnivores Smilodon populator and Caiman latirostris. To explore the intra/interspecific competition of these fauna, we generate weight estimation, standardized niche breadth (BA) for the meso-megamammals from Sergipe and compare with data from the meso-megaherbivores from Africa, concluding that Eremotherium laurillardi and Toxodon platensis were the best resource competitors in the Late Pleistocene of Sergipe, and reinforcing their importance as key species in this extinct community. Finally, we reconstructed the paleoenvironment in which the vertebrate community of Sergipe lived, estimating Mean Annual Temperature ({degrees}C), Mean Annual Precipitation, Biomass and Energy Expendidure, noting that environments in the Late Pleistocene of Sergipe were similar to those of Africa nowadays, but hotter and with more energy expenditure for these meso-megamammals.

paleontology↗

bioRxiv2018

The skull of StW 573, a 3.67 Ma Australopithecus skeleton from Sterkfontein Caves, South Africa

Here we present the first full anatomical description of the 3.67 million-year-old Australopithecus skull StW 573 that was recovered with its skeleton from the Sterkfontein Member 2 breccia in the Silberberg Grotto. Analysis demonstrates that it is most similar in multiple key morphological characters to a group of fossils from Sterkfontein Member 4 and Makapansgat that are here distinguished morphologically as A. prometheus. This taxon contrasts with another group of fossils from those sites assigned to A. africanus. The anatomical reasons for why these groupings should not be lumped together (as is frequently done for the South African fossils) are discussed in detail. In support of this classification, we also present for the first time a palate (StW 576 from Sterkfontein Member 4) newly reconstructed by RJC, which has a uniquely complete adult dentition of an A. africanus. The StW 573 skull also has certain similarities with other earlier Australopithecus fossils in East Africa, A. afarensis and A. anamensis, which are discussed. One of its most interesting features is a pattern of very heavy anterior dental wear unlike that found in A. africanus but resembling that found in A. anamensis at 4.17 Ma. While StW 573 is the only hominid fossil in Sterkfontein Member 2, we conclude that competitive exclusion probably accounts for the synchronous and sympatric presence of two species of Australopithecus in the younger deposits at Makapansgat and Sterkfontein Member 4. Because the StW 573 skull is associated with a near-complete skeleton that is also described for the first time in this special issue, we are now able to use this individual to improve our understanding of more fragmentary finds in the South African fossil record of Australopithecus.

paleontology↗

bioRxiv2018

Bilateral Asymmetry of the Forearm Bones as Possible Evidence of Antemortem Trauma in the StW 573 Australopithecus Skeleton from Sterkfontein Member 2 (South Africa)

The 3.67-million-year-old StW 573 Australopithecus skeleton is important for the light it sheds on the paleobiology of South African species of that genus, including, as discussed here, how the possible pathology of the specimen informs our understanding of Australopithecus behavior. The StW 573 antebrachium exhibits bilateral asymmetry, with significantly more longitudinally curved left forearm bones than right. Arguing from a comparative perspective, we hypothesize that these curvatures resulted from a fall onto a hyperextended, outstretched hand. It is unlikely that the fall was from a significant height and might have occurred when the StW 573 individual was a juvenile. This type of plastic deformation of the forearm bones is well-documented in modern human clinical studies, especially among children between the ages of four and ten years who tumble from bicycles or suffer other common, relatively low-impact accidents. Left untreated, such injuries impinge normal supination and pronation of the hand, a condition that could have had significant behavioral impact on the StW 573 individual.View Full Text

paleontology↗