The Forelimb and Pectoral Girdle of Pachyrhinosaurus lakustai (Ceratopsia, Centrosaurinae)

Rebekah Marion Vice
Ceratopsidae is a group of non-avian dinosaurs known for their distinctive cranial ornamentation and horns. This ornamentation has become the focal point of ceratopsid research and therefore has led to a state of affairs in ceratopsid palaeontology in which taxa are diagnosed on the basis of cranial morphology and the postcranial skeleton is generally neglected and assumed to be unimportant. However, recent studies have begun to alter this situation by introducing descriptions of articulated or
more » ... s of articulated or associated ceratopsid skeletons. This thesis sets out to build on previous understandings of ceratopsid postcranial elements in order to detect, and distinguish between, intraspecific and interspecific variation within Ceratopsidae. This sample was drawn from the upper Campanian Pipestone Creek Bonebed in the Wapiti Formation near Grande Prairie, Alberta, where hundreds of Pachyrhinosaurus lakustai elements have been collected over the past few decades, making this species an ideal focal point for the present study. Morphological and allometric analyses of the forelimb and pectoral girdle of P. lakustai revealed multiple characters potentially unique to the species, as well as variation within the genus Pachyrhinosaurus. Other characteristics of the scapula and humerus that differentiate Centrosaurinae from Chasmosaurinae, including the ratio of the anterior and posterior widths of the scapula to the midshaft width, were identified. Such characters are key to identifying ceratopsid skeletons as accurately as possible when cranial material is not available. Furthermore, histological sampling was used in this study to determine the growth trajectory and age range of specimens from the Pipestone Creek Bonebed. This revealed an that juveniles were more abundant in the sample than subadults or adults. The youngest individual was determined to be under a year old, and the oldest at least 21 years old. The overall growth iii curve of P. lakustai is typical for non-avian dinosaurs in being sigmoidal, the rate of body mass increase greatest in subadults. Lastly, P. lakustai manual pathologies apparently resulting from stresses produced during normal locomotion were discovered and described. This is the first description of this pathology in a centrosaurine ceratopsid, but similar pathologies have been documented in Chasmosaurinae. CT analyses and morphological comparisons suggested that these pathologies were consistent with the unique step cycle described by Thompson and Holmes (2007) for Chasmosaurus irvinensis, therefore expanding the potential scope of this kinematic hypothesis to centrosaurines. Overall, this comparative study of the forelimb and pectoral girdle of ceratopsids revealed taxonomically significant characteristics that may be used in the identification of ceratopsid material in the absence of the cranium. These analyses have implications for understanding the behaviour and ecology of these animals, underlining the importance of studying the postcrania of ceratopsids and paving the way for future studies on this topic. iv
doi:10.7939/r3-814a-6j34 fatcat:n4b4wv4bvzba7dtkxs3jlqh4wy