Quantitative analyses based upon the superimposition of phylogenetic and reproductive data have revealed that viviparity has originated on at least 132 independent occasions among vertebrates, with 98 of these origins having occurred among reptiles. The viviparous lineages have given rise to at least 24 matrotrophic clades, all but four of which are anamniotes. Traditional scenarios assume progressive, gradualistic evolution from oviparity to lecithotrophic viviparity to matrotrophic viviparity. However, mammalian evidence indicates that matrotrophy can precede the evolution of viviparity. Moreover, data on reptiles seem to be consistent with a punctuated equilibrium model for viviparity and a saltatory model for incipient matrotrophy and placentation.

Among the specializations for fetal nutrition, strong convergence is evident at organismal, organological, and cytological levels. Examples include yolk sac placentation, trophotaeniae, and adaptations for embryonic cannibalism. Certain lizards of the genera Mabuya and Chalcides have converged strongly on eutherian mammals with respect to morphology of the chorioallantoic placenta. Placental specializations that have evolved independently in some eutherians and matrotrophic lizards include placentomes, giant binucleate cells, deciduate maternal tissue, and chorionic areolae.

Author notes

1From the Symposium on Evolution of Viviparity in Vertebrates presented at the Annual Meeting of the American Society of Zoologists, 27–30 December 1990, at San Antonio, Texas.