Mitochondrial DNA sequence data derived from two genes were used to infer phylogeographical relationships between 13 Agama atra populations. Three distinct geographical assemblages were found among the lizard populations. The first occurs in southern Namibia, the second is restricted to the western dry arid regions of South Africa, whereas the third is distributed throughout the more mesic southern and eastern parts of the subcontinent. Geographically structured differences among populations within Agama clades are probably the result of dispersal and historic isolations among populations. At the broader scale, there were marked congruences between the Agama genetic discontinuities and those described previously in other rock-dwelling vertebrates such as Pronolagus rupestris and Pachydactylus rugosus. This suggests vicariance, probably as a response to natural climatic changes during the past three million years.

Current studies on fetal membranes of reptiles are providing insight into three major historical transformations: evolution of the amniote egg, evolution of viviparity, and evolution of placentotrophy. Squamates (lizards and snakes) are ideal for such studies because their fetal membranes sustain embryos in oviparous species and contribute to placentas in viviparous species. Ultrastructure of the fetal membranes in oviparous corn snakes (Pituophis guttatus) shows that the chorioallantois is specialized for gas exchange and the omphalopleure, for water absorption. Transmission and scanning electron microscopic studies of viviparous thamnophine snakes (Thamnophis, Storeria) have revealed morphological specializations for gas exchange and absorption in the intra-uterine environment that represent modifications of features found in oviparous species. Thus, fetal membranes in oviparous species show morphological differentiation for distinct functions that have been recruited and enhanced under viviparous conditions. The ultimate in specialization of fetal membranes is found in viviparous skinks of South America (Mabuya) and Africa (Trachylepis, Eumecia), in which placentotrophy accounts for nearly all of the nutrients for development. Ongoing research on these lizards has revealed morphological specializations of the chorioallantoic placenta through which nutrient transfer is accomplished. In addition, African Trachylepis show an invasive form of implantation, in which uterine epithelium is replaced by invading chorionic cells. Ongoing analysis of these lizards shows how integration of multiple lines of evidence can provide insight into the evolution of developmental and reproductive specializations once thought to be confined to eutherian mammals.

In the viviparous lizard Trachylepis ivensi (Scincidae) of central Africa, reproducing females ovulate tiny ∼1 mm eggs and supply the nutrients for development by placental means. Histological study shows that this species has evolved an extraordinary placental pattern long thought to be confined to mammals, in which fetal tissues invade the uterine lining to contact maternal blood vessels. The vestigial shell membrane disappears very early in development, allowing the egg to absorb uterine secretions. The yolk is enveloped precocially by the trilaminar yolk sac and no isolated yolk mass or yolk cleft develops. Early placentas are formed from the chorion and choriovitelline membranes during the neurula through pharyngula stages. During implantation, cells of the chorionic ectoderm penetrate between uterine epithelial cells. The penetrating tissue undergoes hypertrophy and hyperplasia, giving rise to sheets of epithelial tissue that invade beneath the uterine epithelium, stripping it away. As a result, fetal epithelium entirely replaces the uterine epithelium, and lies in direct contact with maternal capillaries and connective tissue. Placentation is endotheliochorial and fundamentally different from that of all other viviparous reptiles known. Further, the pattern of fetal membrane development (with successive loss and re-establishment of an extensive choriovitelline membrane) is unique among vertebrates. T. ivensi represents a new extreme in placental specializations of reptiles, and is the most striking case of convergence on the developmental features of viviparous mammals known.

Phylogenetic information offers an important resource in analyses of reproductive diversity, including interpretations of fetal membrane evolution. In this paper, we draw upon ongoing studies of South American and African lizards to consider the value of combining phylogenetic and reproductive evidence in the construction of evolutionary interpretations. South American lizards of the genus Mabuya exhibit several reproductive specializations that are convergent on those of eutherian mammals, including viviparity, long gestation periods, ovulation of tiny eggs, and placental supply of the nutrients for development. Studies of placental morphology and development indicate that New World Mabuya share several other derived features, including chorionic areolae and a "Type IV" epitheliochorial placenta with a villous, mesometrial placentome. Some characteristics of these lizards are shared by two African skinks, M. ivensii and Eumecia anchietae, including minuscule eggs, placentotrophy, an absorptive chorioallantois, and features of the yolk sac. Available evidence is consistent with two explanations: (1) placentotrophy originated in Africa, predating a trans-Atlantic colonization by Mabuya of the New World; and (2) placentotrophy arose two or three separate times among these closely related skinks. As illustrated by analysis of these animals, not only can data on fetal membrane morphology yield phylogenetic information, but phylogenetic evidence in turn provides a valuable way to reconstruct the evolution of fetal membranes in a biogeographic context. When appropriately interpreted, morphological and phylogenetic evidence can be combined to yield robust evolutionary conclusions that avoid the pitfalls of circular reasoning.

The viviparous African skink, Eumecia anchietae, exhibits a matrotrophic fetal nutritional pattern. Until well after the limb bud stage, extravitelline nutritional provision is in the form of holocrine secretion originating from the stratified uterine epithelium of the uterine incubation chambers. Uterine secretions are absorbed by a specialized yolk sac ectoderm and chorioallantois through histotrophy. The yolk sac is not in close contact with the uterine lining from the limb bud stage onwards. The yolk sac ectoderm forms invaginations filled with uterine secretion and consists of a single layer of vacuolated hypertrophied cells bearing microvilli. The chorioallantois at the limb bud stage is extensive, well‐vascularized, and not intimately associated with the uterine epithelium. Where the uterus is folded, the chorioallantois may interdigitate loosely. Chorionic cells are low to high columnar, clearly vacuolated, and bear microvilli. The allantoic layer consists primarily of squamous cells exhibiting villous projections. By the time embryos have well‐defined digits, the specialized yolk sac ectoderm has regressed and the yolk sac lumen has been invaded by vitelline cells. The chorioallantois is very extensive and in areas greatly folded. Where it contacts the uterine epithelium, a proper chorioallantoic placenta is formed. Cell layers of the chorioallantois and uterine epithelium are thin and cuboidal to squamous in appearance. The chorioallantoic placenta is simple in structure, occurs throughout the incubation chamber, and is epitheliochorial in arrangement. It is unknown whether the placentome observed in other highly matrotrophic scincids is formed in late stage embryos of this species. J. Morphol. 247:264–287, 2001. © 2001 Wiley‐Liss, Inc.