Parallel evolution of placentation in Australian scincid lizards.

Viviparity and placental nutrient provision have evolved on numerous occasions in squamate reptiles. Five lineages are substantially placentotrophic and two of these, the genera Pseudemoia and Niveoscincus, are closely related scincid lizards that have independently evolved viviparity and placentation. Histological comparison of placental ontogeny of placentotrophic species of these genera with development of the extraembryonic membranes of oviparous outgroups indicates a high level of homoplasy. The terminal placental stage of each lineage consists of an omphaloplacenta (yolk sac placenta) and a chorioallantoic placenta and each type of placentation consists of both identical and distinguishable characters. We infer three derived placental functions based on morphology and suggest that the level of structural similarity between lineages results from different evolutionary patterns. Functional characteristics include (1) respiratory exchange regions of chorioallantoic placentation, (2) nutritive exchange regions of the omphaloplacenta, and (3) localized sites of nutritive exchange in the chorioallantoic placenta. Structural similarity in the chorioallantoic placentae for respiration likely resulted from parallel evolutionary trajectories driven by selection to maintain functions characteristic of the eggs of oviparous ancestors. Structural similarity in omphaloplacentation is explainable by selection for nutritional transport developmentally constrained by formation of the isolated yolk mass complex, whereas nutritive sites in the chorioallantoic placentae evolved in the absence of genetic or developmental constraints and differ structurally.