A new paper by Pyron and Burbrink (2013) combines lizard viviparity and lizard phylogeny and finds multiple origins for viviparity…and, multiple reversals to oviparity. The paper also suggests that the basal condition in lizards was oviparity. Only living taxa were tested.
Earlier, Wang and Evans (2011) produced fossils of a Cretaceous lizard and her embryos (Fig. 1), all 15 of them!
From the Wang and Evans abstract:
“Although viviparity is most often associated with mammals, roughly one fifth of extant squamate reptiles give birth to live young. Phylogenetic analyses indicate that the trait evolved more than 100 times within Squamata, a frequency greater than that of all other vertebrate clades combined. However, there is debate as to the antiquity of the trait and, until now, the only direct fossil evidence of squamate viviparity was in Late Cretaceous mosasauroids, specialised marine lizards without modern equivalents. Here, we document viviparity in a specimen of a more generalised lizard, Yabeinosaurus, from the Early Cretaceous of China. The gravid female contains more than 15 young at a level of skeletal development corresponding to that of late embryos of living viviparous lizards. This specimen documents the first occurrence of viviparity in a fossil reptile that was largely terrestrial in life, and extends the temporal distribution of the trait in squamates by at least 30 Ma. As Yabeinosaurus occupies a relatively basal position within crown-group squamates, it suggests that the anatomical and physiological preconditions for viviparity arose early within Squamata.”
I would hasten add: perhaps not early in pylogeny, but often. Note these yabeinosaurs (Fig. 1) are beneath the rib cage close to the humerus. Moreover, the orientation is not head first toward the cloaca. Evidently it all works out.
We’ve also looked a possible viviparity in mesosaurs. Ichthyosaurs and plesiosaurs are also notable live-bearers. Pterosaurs maintained embryos within the mother until shortly before hatching took place, based on the extreme thinness of the leathery eggshells and the degree of development of known embryos.
Pyron RA and Burbrink FT 2013. Early origin of viviparity and multiple reversions to oviparity in squamate reptiles. Ecology letters. doi: 10.1111/ele.12168.
Wang Y and Evans SE 2011. A gravid lizard from the Cretaceous of China and the early history of squamate viviparity. Naturwissenschaften Sept 98(9):739-43.