The Birth of Mammals: Uncovering the Origins of Viviparity

This project makes use of new fossil material from Scotland, and the latest techniques in digital palaeo-histology, to pinpoint the origin of live birth in mammals for the first time. I will characterize the histological pattern for today’s viviparous (live birth) and oviparous (egg-laying) mammals using micro computed and synchrotron tomography (micro CT and SSRCT) and apply these findings to the fossil record. These techniques have not been widely applied to study mammal histology before – fossil or extant. Monotreme histology in particular remains poorly characterised, and therefore, my dataset will constitute one of the first of its kind. The non-destructive nature of these tomographic approaches means that rare and fragile fossil samples can be analysed without damage, providing insights previously unavailable using traditional techniques. Most mammals alive today are viviparous, making this a key trait of the group as a whole. Being viviparous is thought to have contributed to mammal success, even aiding them in surviving the K-Pg mass extinction that wiped out many other groups. We now live in the ‘age of mammals’ (Cenozoic), so-called because of this group’s wide range of body mass (2 g - 200 tons), ecological and morphological diversity, and global distribution (Nowak 1999). The two largest extant mammal clades are viviparous, but exhibit differing patterns of gestation and early growth that may have shaped their current taxonomic and ecological diversity (Zachos and Asher, 2018). Placental mammals – which comprise over 90% of all mammal species today - undergo prolonged gestation, birthing more developed neonates that have a shorter suckling period. Marsupials on the other hand, have shorter gestations, birthing less physically developed neonates that complete their growth during a protracted suckling period. These patterns of early development and care are reflected in the deposition and resorption of bone tissues, making it possible to differentiate the ‘marsupial-like’ pattern from the ‘placental-like’ pattern in the bone structure (histology) of living mammals, and their fossil counterparts (Weaver et al. 2022). Platypuses and echidnas are the only oviparous living mammalians, having retained this mode of birth from their ancestors. While the nature of mammal oviparity and viviparity has been studied extensively for over two centuries, the timing of the emergence of viviparity during the evolutionary history of mammaliaforms (the group that includes mammals) remains poorly understood. I will obtain histological samples from extant and fossil mammaliaforms spanning the Late Triassic to the present day, comparing them to establish a signature for oviparity/viviparity. I will obtain fossil material from around the world, including key specimens found in recently during my fieldwork on the Isle of Skye. This will constrain the emergence of viviparity in the mammal tree both temporally, and phylogenetically – resolving the ‘when and where’ of mammal live birth. The data generated will have many applications beyond the core scope of the project. Within palaeomammalogy they will influence research on the emergence of other major innovations in the clade, such as endothermy (warm-bloodedness), milk production, ecological complexity, and myriad life history traits. They provide calibration points to anchor molecular datasets. The dataset of living monotreme histology will have applications in zoology, veterinary science and conservation, and the analytical techniques have applications across the vertebrate tree. This research will uncover a pivotal moment in mammal evolution, answering a major question at the heart of evolutionary biology