| dc.description.abstract | This dissertation is broken into four chapters. In chapter I, I cover background literature and set the framework for the following chapters by describing the relationship between dinosaur ecology and ontogeny. In chapters 2 through 4 I cover different aspects of dinosaur paleoecology from the middle Cretaceous of North America.
In chapter II, the hypothetical ecologies are explored for multiple theropods derived from the Upper Cretaceous Mussentuchit Member of the Cedar Mountain Formation of Utah. The samples for this study come from six microsites, ranging in depositional setting from distal floodplain to channel lags. In total 866 teeth were analyzed, of which 309 were found to represent four unique tooth morphotypes: a large theropod, a medium-sized dromaeosaurid, a small dromaeosaurid, and a tooth-morph similar to the genus Richardoestesia. The four morphotypes vary significantly in mean size, from 15.1 mm in the large theropod (5.2–34.7 mm) to 3.7 mm in Richardoestesia (2.1–7.6 mm). Further, tooth representation from two of the best-sampled microsites show differing patterns. The large theropod teeth are about twice as common in the floodplain environment (V695, 15.6% of 135 teeth) as compared to the splay/channel (V794, 7.7% of 104 teeth); while medium-sized dromaeosaurid teeth are more than three times as common in the floodplain (12.5 to 41.5%). Small dromaeosaurid teeth show little difference between the sites (30.8 to 38.45%), but Richardoestesia teeth increase more than three-fold in abundance from the floodplain to the splay/channel (12.8 to 41.3%). Preliminary sedimentological and taphonomic data suggest that sorting bias is not responsible for the observed faunal differences. Stable isotope analysis (δ13CVPDB-LSVEC; δ18OVSMOW-SLAP) of tooth carbonate from the theropod morphotypes, goniopholidids (crocodylians), and matrix from V695 and V794 were also analyzed. Statistical differences between the matrix and theropod teeth for both δ13C and δ18O in V695 and for δ18O in V794, indicate that the samples have not been completely homogenized by diagenesis. The theropods show only small differences in δ13C values between each other, with morph 4 being the most depleted and most similar to goniopholidids in V695, as compared to the other theropods. In the sample fromV794 morph 2, which is the relatively most-enriched morphotype in V695, is the most-depleted morphotype in V794. In V695, morphs 1, 3, and 4 all show modest enrichment in δ13C indicating a level of dietary of plasticity not observed between the two different environmental samples for morph 2. Taken as a whole, we hypothesize that these data indicate that the Mussentuchit theropods had different habitat preferences, and presumably different niches within the predator guild. Most notably, morph 2, the medium-sized dromaeosaurid shows little difference in diet between sites indicating a stricter reliance on a single food source, presumably one not found near the channel. Richardoestesia (morph 4) conversely shows a wide-ranging diet depending on environment, but is more-often isotopically lighter than the other morphs. Given its small size, tooth shape, isotopic signature, and abundance near the channels, we interpret this animal as having an analogous diet to a modern wading bird. Finally, the large theropod (morph 1) and small dromaeosaurid (morph 3) appears to have had cosmopolitan tastes and wide-distribution. These lines of evidence all suggest plausible means by which ecospace was divided among the predatory dinosaurs of the Mussentuchit local fauna.
Chapter III focusses on the diet and ontogeny of the large herbivorous dinosaur, Tenontosaurus tilletti. T. tilletti was one of the most common herbivorous dinosaurs in North America during the Aptian-Albian stages of the Early Cretaceous. With a fossil record composed of hundreds of specimens, including multiple nearly complete individuals, its superior fossil record has made T. tilletti one of the best known ornithischian dinosaurs. Here, I investigate the consistency of multiple proxies for growth in this dinosaur, in order to determine the relative maturity of individual specimens of T. tilletti using a cladistic methodology. In total, 32 post-cranial characters were coded for eight of the most complete specimens. The resulting tree was highly consistent (CI=0.97) arranging the specimens into seven growth stages. In general, growth follows a pattern from gracile to more robust, with the largest changes appearing to track the transition to greater quadrupedality in the largest individuals. The arrangement of specimens is strongly correlated with size, histological, and bone textural changes. These data were then used to test the least and most mature specimens for dietary differences using stable isotope analysis. This follows the general line of reasoning that some herbivores tend to be more carnivorous when young to increase nutritional uptake during rapid stages of growth. Analysis of OMNH 10144 (least mature) and OMNH 10132 (most mature) show no statistical difference in either ��13C or ��18O, nor do they differ in tooth wear angle significantly, implying that by the time they reach the size of OMNH 10144, the diet of juveniles is not substantially different from that of adults.
Finally, chapter IV looks at one the most well-known examples of speculative dinosaur behavior. The image of the highly intelligent, pack-hunting raptor has become engrained in the scientific literature and popular works. First proposed to explain the relatively common co-occurrence of the large-bodied iguanodontian Tenontosaurus tilletti and the wolf-sized Deinonychus antirrhopus from the Early 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, however, is problematic, largely due to the fact that highly coordinated hunting strategies are rarely observed in modern archosaurs. This debate has led to the alternative hypothesis that Deinonychus was more analogous to agonistic reptilian carnivores, like the Komodo dragon (Varanus komodoensis, Squamata, Varanidae). 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 isotope (δ13C, VPDB-LSVEC) analysis was performed on tooth carbonate from small (<4.5 mm crown height) and large (>9 mm crown height) Deinonychus specimens from two microsites from the Early Cretaceous Cloverly and Antlers formations (in Montana and Oklahoma, respectively). Further, goniopholidid crocodylian teeth varying in size from the Cloverly Formation were also tested as a control. The results show that the Cloverly goniopholidids, like their modern counterparts, went through a distinct transition in diet as they grew, where the smallest teeth were the relatively most enriched (avg. -9.32‰; n=5), the medium-sized teeth were the most-depleted (avg. -10.56‰; n=5) , and the largest teeth were in-between (avg. -10.12‰; n=6); characteristic of the dietary shifts seen in modern asocial reptiles. Deinonychus showed this same pattern in both formations, with small teeth being the more enriched (avg. -8.99‰; n=10) and the large teeth being more depleted (avg. -10.38‰; n=10). These differences indicate that juvenile and adult Deinonychus from both formations likely consumed different prey. Hypothetical food sources, such as Tenontosaurus tilletti, are close to the 13C isotopic signal of adult Deinonychus, consistent with the hypothesized trophic relationship between these two species. Juvenile Deinonychus had a more enriched-diet, likely composed of smaller, but trophically-higher species. Taken together, these data add to the growing evidence that Deinonychus was not a complex social hunter, at least by modern mammalian standards. | en_US |
