The Tyrants Aisle Dinosaur Tracksite: An Upper Cretaceous Ichnofauna from Unit 4 of the Wapiti Formation (Upper Campanian), Alberta, Canada

Abstract

<p>Dinosaur tracksites provide a wide variety of palaeobiological data that are complementary to the skeletal record. However, the occurrence and documentation of <i>in situ</i> tracksites in parts of the world—Canada included—is sporadic, hindering their use in broader studies of dinosaur diversity, palaeoecology, evolution, ontogeny and palaeobiogeography. The first chapter of this thesis explores the dinosaur track record from Canada as a whole, which is especially strong throughout the Cretaceous, and provides a crucial supplement to the temporally and spatially restricted skeletal record. Within northern Alberta, the Late Cretaceous Wapiti Formation represents a geographic link between partly coeval Laramidian dinosaur faunas in Alaska and those in southern Alberta, the contiguous U.S.A. and Mexico. Documentation of the Wapiti palaeofaunas therefore aids a more synthetic understanding of dinosaurs across Laramidia, as it contributes to the necessary latitudinal framework required to test a broad range of biogeographical hypotheses. Fossil tracks from the Wapiti Formation have been increasingly documented in recent years and represent one of the most diverse vertebrate ichnofaunas from the Cretaceous of North America. However, most finds constitute isolated and/or<i> ex situ</i> specimens recovered on fallen blocks alongside modern river margins, which contribute only a restricted view of a particular ichnoassemblage and its broader significance. In contrast, this thesis explores a new locality, Tyrants Aisle—the largest <i>in situ</i> tracksite known from the Wapiti Formation to date—in order to provide an evaluation of its ichnofaunas and the important palaeobiological data they contain. </p> <p>The second chapter of this thesis explores the application of tracks in studies of dinosaur growth. Tracks have only rarely been used to test ontogenetic hypotheses due to the difficulty in separating anatomical influences on track morphology from those related to sediment properties, kinematics and preservation. A collection of well-preserved tyrannosaurid tracks from Tyrants Aisle, and elsewhere within Unit 4 of the Wapiti Formation, is analysed using a geometric morphometric approach to test for possible ontogenetic trajectories, providing the first investigation of tyrannosaurid growth based on footprints. The results are consistent with prior suggestions of greater locomotory capability in juvenile tyrannosaurids, and indicate that once non-anatomical controls on track morphology have been accounted for, footprints can provide unique perspectives on dinosaur growth.</p> <p>The presence of didactyl tracks at Tyrants Aisle (attributed to probable troodontids) is discussed within the third chapter. Evidence of deinonychosaurs from the Wapiti Formation is rare and largely restricted to isolated teeth. The new tracks represent the first didactyl theropod tracks from Canada and indicate that considerable morphological diversity exists within North American deinonychosaur tracks, mirroring similar observations within Asia.</p> <p>Finally, the overall diversity, palaeoecology and behavioural insights learned from Tyrants Aisle are evaluated in the fourth chapter. Evidence of social behaviour is documented in some of the hadrosaurid and small theropod trackmakers, which adds to a growing body of literature that paints dinosaurs as highly dynamic and social animals. Collectively, the site demonstrates a palaeoecological dominance of hadrosaurids that is in agreement with the skeletal record from the Wapiti Formation. This faunal signal is conspicuously widespread across the late Campanian of Laramidia, as hadrosaurids dominate almost every major ichnofauna from Alaska through to Mexico. Thus, at higher taxonomic levels (i.e., family and above), major dinosaurian faunal components in both high and low latitude deposits across Laramidia are relatively conserved, although distinct palaeoenvironmental preferences may partly explain this.</p> <p>This work ultimately demonstrates the utility of vertebrate ichnoassemblages for exploring a wide range of hypotheses, and significantly advances the use of tracks in studies of dinosaur ontogeny.</p>