Histology and Ontogeny of Pachyrhinosaurus Nasal Bosses

Elizabeth A Kruk
Pachyrhinosaurus is a peculiar ceratopsian known only from Upper Cretaceous strata of Alberta and the North Slope of Alaska. The genus consists of three described species Pachyrhinosaurus canadensis, Pachyrhinosaurus lakustai, and Pachyrhinosaurus perotorum that are distinguishable by cranial characteristics, including parietal horn shape and orientation, absence/presence of a rostral comb, median parietal bar horns, and profile of the nasal boss. A fourth species of Pachyrhinosaurus is
more » ... nosaurus is described herein and placed into its phylogenetic context within Centrosaurinae. This new species forms a polytomy at the crown with Pachyrhinosaurus canadensis and Pachyrhinosaurus perotorum, with Pachyrhinosaurus lakustai falling basal to that polytomy. The diagnostic features of this new species are an apomorphic, laterally curved Process 3 horns and a thick longitudinal ridge separating the supraorbital bosses. Another focus is investigating the ontogeny of Pachyrhinosaurus nasal bosses in a histological context. Previously, little work has been done on cranial histology in ceratopsians, focusing instead on potential integumentary structures, the parietals of Triceratops, and how surface texture relates to underlying histological structures. An ontogenetic series is established for the nasal bosses of Pachyrhinosaurus at both relative (subadult versus adult) and fine scale (Stages 1-5). It was demonstrated that histology alone can indicate relative ontogenetic level, but not stages of a finer scale. Through Pachyrhinosaurus ontogeny the nasal boss undergoes increased vascularity and secondary remodeling with a reduction in osteocyte lacunar density. A histological study of ceratopsian cranial elaborations was also performed to better understand the functional and developmental implications of these structures, as well to place the nasal boss of Pachyrhinosaurus into the context of ceratopsian elaborations. In centrosaurines, parietal spikes and hooks, postorbital horncores, bosses, and nasal horns are formed as iii outgrowths of the dermatocranium. Although the bone tissue type remains fibrolamellar across any given specimen, organization of the bone varies (cortex versus core, compact versus spongy) across cranial elaborations. However, epoccipitals (epiparietals and episquamosals) form via metaplasia. This may also be the case for epinasals in chasmosaurines, which do not form from the nasal bone, but instead fuse onto the nasal-rostral complex later in life, indicating that they are not outgrowths of the dermatocranium. Historically, cranial histology is a poorly studied component of paleohistology and is expanded upon in this thesis. The ontogeny of Pachyrhinosaurus nasal bosses is explored in a histological perspective, which gives deeper understanding to how these atypical nasal ornamentations form. New fossil reports have expanded our understanding of Pachyrhinosaurus diversity, although their relationship to each other is not entirely clear. This increased understanding has revealed interesting evolutionary patterns, such as the replacement of nasal horns with nasal bosses by the last of the centrosaurines, the Pachyrostra (Achelousaurus + Pachyrhinosaurus). Here I provide new phylogenetic, historical, and histological research that informs these topics. iv Preface This thesis is an original work by Elizabeth Kruk. No part of this thesis has been previously published. v Acknowledgements I would like to thank Dr. Philip Currie for supervising my thesis and Dr. Michael Caldwell for all the help and feedback they have given me over the years. I want to thank Brandon Strilisky and Don Brinkman from the Royal Tyrrell Museum of Palaeontology for allowing me to visit their collections and thin section material. Also, Dr. Jordan Mallon, Margaret Currie, and Kieran Shepard from the Canadian Museum of Nature for allowing me to visit collections. Dr. David Evans and all the graduate students associated with paleontology at the University of Toronto, Toronto-Mississauga, and the Royal Ontario Museum for hosting a paleohistology workshop. Thanks to Dr. Martin Sander and associated students at Universität Bonn for hosting a histology workshop. John Horner and Ellen-Therese Lamm from the Museum of the Rockies for guidance on my thin sections and allowing observation of their thin sectioning techniques. Dr. Tobin Hieronymus for pachyrhinosaur guidance and sending me histological slides to use for my studies. I would like to thank all the graduate students of the University of Alberta Laboratory for Vertebrate Paleontology, both past and present, for their guidance and friendship over the years; especially Michael Burns who taught me the basics of histology and thin sectioning for without him this project would not have been entirely possible. Also, thanks to my friends who helped me even from far away, from bouncing around ideas to editing my
doi:10.7939/r3zp3w66x fatcat:2g2gtm6gvbhrljabhsfu4ywkhu