Early origin of viviparity in the mammalian lineage

Abstract

Viviparity is a reproductive mode in which the embryo completes their development inside the female body. Present-day mammals, with the exception of monotremes, are viviparous, as opposed to the egg-laying reproductive mode of most vertebrates. Non-therian cynodonts are regarded as oviparous although direct evidence on the reproductive modes in extinct taxa is lacking. We identified, in a palaeohistological study, a neonatal line in two bones of the early probainognathian Chiniquodon theotonicus from the Late Triassic of South America. The body mass at birth is comparable to that observed in living placentals, and notably higher than the one registered for monotremes, marsupials, lepidosaurs, crocodiles, and birds. Our results reveal that viviparity is not exclusive of therians among synapsids, pushing back its recognition in the clade ~90–95Myr.