Date of Award
Fall 2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Geology and Geophysics
First Advisor
Bhullar, Bhart-Anjan
Abstract
Compared with their Mesozoic theropod ancestors, extant birds (Aves) exhibit extensively modified skulls and associated soft tissue, with massively enlarged brains and reduced temporal musculature, an inner ear with dorsally elongated semicircular canals, and cranial kinesis allowing independent beak movements that permit it to be a dexterous manipulatory organ. This kinetic system in turn occurs in two distinct forms—palaeognathous and neognathous, the latter having a novel joint in the palate. Through µCT scanning and confocal microscopy, the evolution and development of these skull modifications can now be explored in three dimensions. I digitally reconstructed 3D skulls of two Cretaceous toothed birds that diverged just prior to the extant Avian radiation: Ichthyornis and Hesperornis. Geometric morphometric analyses using the well-preserved inner ear of Hesperornis show correlations between inner ear shape and behavioral changes along the archosaurian lineage leading to birds. With contrast-stained µCT scans of modern birds and non-avian reptiles, I identify osteological correlates with soft tissue to reconstruct the cranial musculature of Hesperornis, and use contrast- and immunostaining to identify developmental differences between crocodilians, palaeognaths, and neognaths. Finally, I demonstrate experimentally that development of the pterygoid-palatine joint unique to neognathous birds depends upon the action of the cranial muscles.
Recommended Citation
Hanson, Michael, "The Origin of the Modern Bird Skull, with a Focus on the Evolution and Embryonic Development of Musculoskeletal Structures in the Temporal and Otic Regions" (2022). Yale Graduate School of Arts and Sciences Dissertations. 816.
https://elischolar.library.yale.edu/gsas_dissertations/816