Title

Developmental link between the brain and skull shapes the evolution of the reptilian head

Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Bhullar, Bhart-Anjan

Abstract

Modern birds are among the most specious clades of tetrapods, with more than 10,000 species and remarkable ecomorphological diversity. They are distinguished by over thirty unique derived characteristics. Among these are an enlarged brain and an expanded braincase surrounding it. Given their peculiar anatomy, it is not surprising that their phylogenetic affinities have been problematic for centuries. It was only with the discovery of remarkable fossil remains from North America and Asia that birds were resolved as living descendants of non-avian dinosaurs. The fossil record permits tracing of the stepwise character acquisitions leading to the phenotype of modern birds. Each such acquisition ultimately derives from an alteration to embryonic development. Developmental pathways influence the evolutionary response of phenotype to selection: organisms are composed of multiple interacting systems whose integration depends on their ontogenetic association. This thesis aims to clarify the relationship between the brain and skull across Archosauria with an evo-devo approach, to understand how the brain affects the skull during development and how this influenced the evolution of the archosaur cranium through time. The third and fourth chapters are focused on tracking the evolution of the braincase and brain among archosaurs using data from the fossil record. μCT scan and shape analyses were used to quantify changes in these two structures. Euparkeria, a taxon closely related to crown Archosauria, shows the first identifiable instance of an “archosaur type” endocast with considerable cerebral flexure. Major innovation of the brain appears in theropods on the line to birds, with an enlargement of the forebrain and cerebellum and latero-ventralization of the optic lobes. The elaboration of the brain in theropods led to considerable modification of the surrounding bones, especially in the temporal region. In contrast, in sauropodomorph dinosaurs, the temporal region is significantly modified although the brain is not dramatically different from that of other archosaurs. We find that the highly modified sauropod skull is a product of early reshaping of cranial elements. Early sauropodomorphs close to Sauropoda acquire these modifications during postnatal ontogeny, whereas sauropods already display them as embryos. The fifth and sixth chapters aim to reveal the previously unknown unclear early morphogenesis of the braincase and the relationship between braincase and brain among reptiles. High-resolution imaging techniques —μCT scan and confocal microscopy — reveal that the brain has a primacy in development and that mesenchymal cells condense early in organogenesis between the regions of the developing brain. A remarkably conserved morphotype of the pre-cartilaginous condensations forming the braincase is shared among reptiles during early organogenesis. Anatomical innovation coinciding with clade-specific evolutionary novelty appears later in development, supporting the phylotypic hypothesis. Similar developmental pathways of mesenchymal condensation and ossification among reptilian taxa imply that patterns of skull morphogenesis are not entirely responsible for the morphological diversity observed across Diapsida. Three-dimensional geometric morphometrics performed on the brain and skull of extant and extinct reptiles, including their developmental series, show a one-to-one correlation between the frontal and the forebrain on the one hand, and the parietal and midbrain on the other. This implies a character non-independence between the skull vault and the brain, both from an evolutionary and developmental perspectives.

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