Understanding variation in monotreme long-bone microstructures, and the emergence of marsupial reproduction

A sub-micron resolution synchrotron radiation-based X-ray computed tomography (SRCT) study of histological microstructures in the femora and humeri of extant monotremes and extinct Cretaceous mammals will pinpoint histological signatures of oviparity (egg-laying) vs. marsupial-like viviparity (live-birth) in the fossil record. We will test hypotheses that emerged from our previous experiment (LS-3244), in which we examined largely unknown bone-tissue microstructures of extant platypus and echidna and a variety of Mesozoic mammals. We discovered hitherto unrecognised histological variation within monotremes, and we did not detect a marsupial-like histological pattern in our Mesozoic mammal sample (counter to expectations). We propose to broaden our monotreme sample to understand the source of their histological variation (e.g., sex, ontogeny), and sample Mesozoic mammals from nearer the Cretaceous-Paleogene (K-Pg) boundary (66 Ma) when marsupial-like reproduction likely emerged.