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 North America, 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 (delta C-13, delta O-18) 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 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 in C-13 (mean = -9.32 parts per thousand; n = 5), the medium-sized teeth were the most-depleted in C-13 (mean = -10.56 parts per thousand; n = 5), and the largest teeth were intermediate (mean = -10.12 parts per thousand; 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 in C-13 (mean = -8.99 parts per thousand; n = 10) and the large teeth being more depleted in C-13 (mean = -10.38 parts per thousand; 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 the C-13 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 in C-13, 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.