Phylogenetic analyses indicate that viviparity (live-bearing reproduction) has originated independently in more than 150 vertebrate lineages, including a minimum of 115 clades of extant squamate reptiles. Other evolutionary origins of viviparity include 13 origins among bony fishes, nine among chondrichthyans, eight in amphibians, one in Paleozoic placoderms, six among extinct reptiles, and one in mammals. The origins of viviparity range geologically from the mid-Paleozoic through the Mesozoic to the Pleistocene. Substantial matrotrophy (maternal provision of nutrients to embryos during pregnancy) has arisen at least 33 times in these viviparous clades, with most (26) of these origins having occurred among fishes and amphibians. Convergent evolution in patterns of matrotrophy is widespread, as reflected by multiple independent origins of placentotrophy, histotrophy, oophagy, and embryophagy. Specializations for nutrient transfer to embryos are discontinuously distributed, reflecting the roles of phylogenetic inertia, exaptation (preadaptation), and constraint. Ancestral features that function in gas exchange and nutrition repeatedly and convergently have been co-opted for nutrient transfer, often through minor modification of their components and changes in the timing of their expression (heterochrony). Studies on functional and evolutionary morphology continue to play a central role in our attempts to understand viviparity and mechanisms of fetal nutrition.

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, Mubuya, 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. o 1993 Wiley-Liss, Inc.

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In lizards and snakes, the oviducts function in fertilization, sperm storage, egg transport, eggshell deposition, maintenance of the early embryo, and expulsion of the egg or fetus. In viviparous forms they also contribute to placentae responsible for gas exchange and nutrient provision to the fetus. Dissections of species of 30 genera coupled with data from the literature indicate that squamate oviducts vary interspecifically in seven macroscopic features, including the extent and nature of regional differentiation, vascular supply, topographic asymmetry, number of oviducts, vaginal pouches, and relationship to the cloaca. The uterus, infundibulum, and vagina differ histologically in their epithelia, glands, and myometrial layers. Season cyclicity occurs in all three oviductal regions, most prominently in the uterus, and is under endocrinological control. Regional and cytological specializations reflect the diverse functions performed by the oviduct. Definitive evidence for oviductal albumen production and egg resorption is lacking. In viviparous squamates, three uterine specializations may facilitate maternal-fetal gas exchange: an attenuated epithelium, reduced uterine glands (and a reduced shell membrane), and increased vascularization. Contrary to previous reports, pregnant uteri show no epithelial erosion or capillary exposure. Specializations for nutrient provision to the fetus include mucosal hypertrophy, enlarged glandular epithelia, and multicellular glands whose secretions are absorbed by the chorioallantois. Comparisons with other amniotes indicate that squamates inherited the oviduct as an organ with capabilities for egg uptake and transport, fertilization, eggshell deposition, and oviposition. Other features have evolved convergently among squamates: infundibular sperm receptacles, unilateral oviduct loss, uterine gestation, placentation, and specializations for placentotrophy. Cladistic analysis indicates that oviductal features associated with deposition of tertiary egg investments in reptiles reflect evolutionary convergence as well as secondary simplification, rather than a unidirectional trend towards increased specialization.

Reproductive mode data were extracted piecemeal from the literature and superimposed over currently accepted phylogenies to permit estimation of the minimum frequencies with which viviparity (live-bearing) has evolved in lizards, aswell as to facilitateanalysisoffactors hypothesizedto inlluencethis evolution. Viviparity has arisen on at least 45 separate occasions in the Sauria. Each ofthese origins is pinpointed phylogeneticallyas far as is now possible. Ofthese origins, 22 have occurred in the Scincidae, ten in the Iguanidae, five in the Anguidae, two each in the Lacertidae and Gekkonidae, and one each in the Chamaeleontidae, Xantusiidae, Agamidae, and Cordylidae. Further origins may be detected in the Scincidae, Iguanidae, and Diploglossa as phylogenetic relationships are elucidated. Over 19 % of the saurian species are live-bearing, and about 2/3 of the viviparous species are skinks. Most of the sub-generic saurian origins ofviviparity have occurred in cold climates, possibly as an adaptation to facilitate maternal thermoregulation of the developing embryos. Phylogenetic distributions of these origins are consistent with hypotheses that genetic sex-determination of the male-heterogametic type as weil as a tendency towards egg'retention preadapt a lineage for viviparity. Evolution of the live-bearing mode may be constrained by temperature-dependent sex determination, female heterogamety, and formation of highly calcified eggshells.