Oviraptorosauria: Morphology, Phylogeny, and Endocranial Evolution
Creators: Balanoff, Amy
Publisher: name: Columbia University
Year: 2011
Subjects: Paleontology

description: Oviraptorosauria, an extinct lineage of coelurosaurian dinosaurs from the Cretaceous of Asia and North America, includes some of the most morphologically distinctive theropod taxa yet known. Their bizarre appearance and numerous skeletal similarities with extant birds instantly made oviraptorosaurs the subject of considerable interest when first discovered in the early 20th Century by the American Museum of Natural History Central Asiatic Expeditions. Subsequent discoveries have only increased the potential of the group for informing the origin of modern birds and characters that make birds distinctive among living vertebrates, including the origin of flight. The current list of shared similarities between oviraptorosaurs and modern birds includes such striking features as loss of teeth, extreme pneumatization and ornamentation of the skull, an unusual sliding jaw articulation, reduction of the tail vertebrae to form a pygostyle, feathers of modern aspect, and the behavior of brooding eggs in the same stereotypical posture. Despite such an extended period of research and popular interest, some fundamental questions regarding oviraptorosaurs remain. First, what is the phylogenetic position of Oviraptorosauria within Coelurosauria? Recent analyses produce contentious results that disagree on whether oviraptorosaurs represent a clade of bird-like, non-avian coelurosaurs or whether they actually are nested within Avialae. Obviously, these disparate topologies pose disparate models of character evolution. For example, if oviraptorsaurs are avialans they represent the first evolution of flightlessness within that clade. Second, what are the phylogenetic relationships of the taxa comprising Oviraptorosauria? And lastly, what insight would a resolved tree topology provide the study of morphological evolution, both within Oviraptorosauria specifically and more generally within Coelurosauria? I analyzed 384 morphological characters and recovered two most parsimonious trees that resolve both the position of Oviraptorosauria within Coelurosauria as well as the interrelationships of species within Oviraptorosauria. Oviraptorosauria is found to have a sister group relationship with Therizinosauria, and this entire clade is positioned as the sister taxon to the clade formed by (Paraves + Alvarezsauridae). These findings support oviraptorosaurs as non-avian coelurosaurs and thus not avialans. The implication of this topology is that many of the avian-like characteristics expressed in the group are the product of homoplastic evolution between oviraptorids (a more exclusive clade within Oviraptorosauria) and avialans. These phylogenetic hypotheses subsequently are used to elucidate the evolutionary history of endocranial morphology in Oviraptorosauria and more broadly within Coelurosauria near the origin of avian flight. Using the relatively newly employed technology of computed tomography (CT), this study provides descriptive morphology of five coelurosaur endocasts (which approximate the shape of the brain in these taxa that effectively filled the endocranial space) and evaluates shared discreet morphological characters with respect to the aforementioned phylogeny. Diagnostic morphologies are found for Oviraptorosauria and the more exclusive clades, Maniraptora, Paraves, and crown birds. This study also is the first to use CT technology to divide the endocranial casts into six neuroanatomical partitions that correspond closely to the olfactory bulbs, cerebrum, pituitary space, optic lobes, cerebellum, and brain stem. These partitions are then used to evaluate how these different regions of the "brain" are evolving. The division of the endocranial cast into partitions is a novel approach to studying endocranial morphology. Previous analyses have been limited to surveying total endocranial volume and have not been able to distinguish between regions of the brain. Those earlier analyses established that crown birds possess a much larger endocranial space with respect to body size than more distantly related groups and that there is a general transition along the coelurosaur lineage towards an increased endocranial volume. This analysis distinguishes the expansion of the cerebrum as the primary driver of volumetric change within the entire endocranium and identifies three possible expansions of the cerebrum within the maniraptoran lineage. Unique volumetric morphologies are found for both Oviraptorosauria and Paraves. Most interestingly, the volumetric proportions of Archaeopteryx lithographica illustrate that this taxon shares a plesiomorphic morphology with other paravians, suggesting that non-avialan paravians such as Microraptor zhaoianus also possessed what has previously been referred to as a "flight-ready" brain that likely supported some type of volant activity.