 In the latter group, many species, including all boas and most vipers, utilize this mode of reproduction.The degree of viviparity varies; some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a structure similar to the mammalian placenta.The earliest documented case of viviparity in reptiles is the Early Permian mesosaurs, although some individuals or taxa in that clade may also have been oviparous because a putative isolated egg has also been found.