Chorioallantoic placentation in squamate reptiles: Structure, function, development, and evolution

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Abstract
Allantoplacentae in lizards and snakes form during the evolution of viviparity through apposition of the chorioallantois and a vestige of the shell membrane to the uterine lining. Generalized squamate allantoplacentae are epitheliochorial, diffuse, adeciduate, and highly vascular, accomplish maternal‐fetal gas exchange, and possibly transfer small quantities of organic and inorganic nutrients. Placental gas exchange presumably is enhanced by the thinning of tissues lying between fetal and maternal capillaries, a progressive increase in placental vascularity, and in some species, by differences in oxygen affinity of fetal and maternal blood. A few saurian genera (e.g., Chalcides, Mabuya, and Pseudemoia) include species with specialized placentae that transfer large quantities of nutrients. Specializations of the allantoplacentae of these lizards include interdigitating, hypertrophied uterine and chorioallantoic tissues, and enlarged absorptive chorionic epithelia. South American Mabuya are further specialized by the presence of chorionic areolae and a distinctive placentome. Weekes' 1935 classification of placental morphotypes is reviewed, and a few minor modifications are proposed, in addition to recognition of a fourth morphotype. Phylogenetic analyses suggest that placental organs have originated on more than 100 occasions among squamate reptiles, and indicate that three separate lineages have converged on substantial placentotrophy through the evolution of specialized histotrophic placentae.