3D architecture of medullary bone in extinct and extant Avemetatarsalia: testing controversial reports of avian reproductive tissues in non-

Medullary bone (MB) is a female-specific tissue used to form the eggshell in extant birds and hypothesized to have originated within extinct Avemetatarsalia. The identification of MB in extinct species would provide an objective means to recognize gravid females. However, reports of fossil MB-like tissues are contentious because MB resembles avian bone pathologies (BP). Recently, our team made progress establishing criteria unique to MB, with the goal of discriminating it from BP. We studied MB and BP in over 60 bird species and found that i) MB skeletal distribution is linked to pneumaticity, differing from BP; ii) MB microstructure is uniform among neognaths and small paleognaths, but may differ in large ratites, the most ancestral bird species; iii) some avian BP have a similar molecular composition as MB. In light of our discoveries, our project aims at reassessing the nature of purported MB-like tissues in pterosaurs and non-avian dinosaurs using Synchrotron X-ray imaging.