Convergent Evolution of Viviparity, Matrotrophy, and Specializations for Fetal Nutrition in Reptiles and Other Vertebrates

Quantitative analyses based upon the superimposition of phylogeneticand reproductive data have revealed that viviparity has originatedon at least 132 independent occasions among vertebrates, with98 of these origins having occurred among reptiles. The viviparouslineages have given rise to at least 24 matrotrophic clades,all but four of which are anamniotes. Traditional scenariosassume progressive, gradualistic evolution from oviparity tolecithotrophic viviparity to matrotrophic viviparity. However,mammalian evidence indicates that matrotrophy can precede theevolution of viviparity. Moreover, data on reptiles seem tobe consistent with a punctuated equilibrium model for viviparityand a saltatory model for incipient matrotrophy and placentation. Among the specializations for fetal nutrition, strong convergenceis evident at organismal, organological, and cytological levels.Examples include yolk sac placentation, trophotaeniae, and adaptationsfor embryonic cannibalism. Certain lizards of the genera Mabuyaand Chalcides have converged strongly on eutherian mammals withrespect to morphology of the chorioallantoic placenta. Placentalspecializations that have evolved independently in some eutheriansand matrotrophic lizards include placentomes, giant binucleatecells, deciduate maternal tissue, and chorionic areolae.