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Title: Ontogenetic dietary shifts in Deinonychus antirrhopus (Theropoda; Dromaeosauridae): Insights into the ecology and social behavior of raptorial dinosaurs through stable isotope analysis
The image of the highly intelligent, pack-hunting raptor has become engrained in scientific literature and popular works alike. First proposed to explain the relatively common co-occurrence of the large-bodied iguanodontian Tenontosaurus tilletti and the wolf-sized Deinonychus antirrhopus from the Lower Cretaceous of NorthAmerica, a canid-like social hunting structure has become the standard depiction of dromaeosaurs in popular works over the last three decades. This reconstruction is, however, problematic largely due to the fact that highly coordinated hunting strategies are rarely observed in modern archosaurs. This has led to the alternative hypothesis that D. antirrhopus was more analogous to agonistic reptilian carnivores, like the Komodo dragon (Varanus komodoensis). Among the many differences between these two analogs is how social and asocial organisms rear their young, producing a diagnostic pattern based on the presence or absence of ontogenetic dietary changes. In order to test for dietary changes through growth, stable carbon and oxygen isotope (δ13C, δ18O)analysis was performed on tooth carbonate from small (<4.5 mm crown height) and large (>9 mm crown height) D. antirrhopus specimens from two microsites from the Lower Cretaceous Cloverly (Montana) and Antlers(Oklahoma) formations. Teeth from goniopholidid crocodylians and Tenontosaurus tilletti from the Cloverly Formation were also tested more » for comparison. The results show that the Cloverly goniopholidids, like their modern counterparts, went through a distinct transition in diet as they grew. The smallest teeth were the relatively most enriched in13C (mean = −9.32‰; n= 5), the medium-sized teeth were the most-depleted in13C(mean = −10.56‰; n = 5), and the largest teeth were intermediate (mean = −10.12‰; n= 6). These factors are characteristic of the dietary shifts seen in modern asocial reptiles. D. antirrhopus showed this same pattern in tooth samples collected from both rock units, with small teeth being the more enriched in13C (mean = −8.99‰; n= 10) and the large teeth being more depleted in13C (mean = −10.38‰; n = 10). These differences suggest that juvenile and adult D. antirrhopus from both formations likely consumed different prey. Hypothetical food sources, such as T. tilletti, are close to the13C isotopic signal of adult D. antirrhopus, consistent with the hypothesized trophic relationship (predator-prey) between these two species. Juvenile D. antirrhopus had a diet more enriched in13C, likely composed of smaller-bodied, but trophically higher species. Taken together, these data add to the growing evidence that D. antirrhopus was not a complex social hunter by modern mammalian standards « less
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Palaeogeography palaeoclimatology palaeoecology
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National Science Foundation
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  1. Abstract

    Dental microwear texture analysis (DMTA) is commonly used to assess the dietary ecology of modern and fossil taxa. In carnivorans, teeth with different functions record dietary behavior differently. Here, we assess DMTA variability along the tooth row of an extant carnivorous marsupial—the Tasmanian devil, Sarcophilus harrisii—which has multiple carnassial-like molars that may function and record diet similarly. We compared the complexity (Asfc), anisotropy (epLsar), and textural fill volume (Tfv) of the lower second, third, and fourth molars of Tasmanian devils to test the hypothesis that teeth with similar forms yield similar functions. Although third molars do have significantly higher epLsar values than fourth molars, all other DMTA attributes are indistinguishable from one another. These data suggest that teeth with comparable morphologies in the same taxon have similar functions and largely record diet similarly. In addition, we compared fossil and modern specimens of S. harrisii from Tasmania to assess dietary behavior over time. These analyses indicate that foods with similar textures have been consumed since the late Quaternary.

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