Blackburn, D.G. (1992).
Convergent evolution of viviparity, matrotrophy, and specializations for
fetal nutrition in reptiles and other vertebrates. American Zoologist 32: 313-321.
Abstract: 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
seems 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.
Reprints: daniel.blackburn@mail.trincoll.edu