EAGER: Experimental Techniques for Discerning Female-specific Morphology in Non-avian Theropod Dinosaurs

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Birds include more species than in any other group of extant vertebrates, and were the only lineage of dinosaurs to survive the mass extinction at the end of the Cretaceous Period about 65 million years ago. The long-standing success of birds may be attributable in part to their unique reproductive physiology, which includes a specialized skeletal tissue deposited in the inner cavity of long bones known as medullary bone (MB). MB acts as a calcium reserve for the production of eggshells and was long thought to be a key reproductive adaptation unique to birds. However, the tissue has recently been purported to occur in a handful of extinct species, including both non-avian dinosaurs and dinosaurian relatives. Given the vast evolutionary distance across the groups in which MB has been tentatively identified (across lineages that diverged over 100 million years prior to the origin of birds), substantiation (or refutation) of MB in these taxa and a careful study of the chemical characteristics and microstructure of these skeletal tissues is a critical first step to determine the validity of these identifications and the evolutionary significance of this tissue. The proposed project will be transformative in developing an untested, independent proxy for chemically distinguishing MB in fossils, and a non-destructive means for identifying the microstructure of MB in the fossil record that can be used to explore reproductive life history and physiology of the bird lineage in deep time. Public outreach and education associated with this research will be conducted at the North Carolina Museum of Natural Sciences. Medullary bone (MB) is an ephemeral, estrogen dependent, endosteal tissue lining the marrow cavity in extant avialans (birds). This specialized reproductive tissue is highly vascular histologically, exhibits a woven bone matrix, and is more densely mineralized than other bone types. Although originally considered a key avian adaptation, MB has since been identified in non-avian theropod and ornithopod dinosaurs, far removed phylogenetically from the direct lineage of extant avialans, and most recently outside of Dinosauria, in one of the clade's closest extinct relatives Pterosauria (non-dinosaurian flying reptiles). To date, identifications of MB in fossils have relied exclusively on structural comparisons (location, texture, distribution, and microstructure) with MB in birds by means of histological (destructive) sampling. However, MB in living birds can also be identified by its unique molecular signature because the tissue incorporates more non-collagenous proteins than cortical bone, and utilizes more sulfated glycosaminoglycans, particularly keratan sulfate, which is not present in cortical bone. This project will provide new information on chemical and microstructural variation in MB within extant avialans and will use these data to generate novel µCT and histochemical techniques for identifying MB in the fossil record. This protocol will then be used to test the homology of published accounts of MB in extinct Archosauria, ultimately providing an informed phylogenetic context for the evolution of medullary bone and avian-specific reproductive physiology. This research is co-funded through the NSF Office of International Science and Engineering in support of research on fossils in China and Mongolia in this project.