Historically, Australia has been important in the study of, and the development of hypotheses aimed at understanding, the evolution of viviparity in amniote vertebrates. Part of the importance of Australia in the field results from a rich fauna of skinks, including one of the broadest ranges of diversity of placental structures within one geographic region. During the last decade, we have focussed our studies on one lineage, the Eugongylus group of skinks of the subfamily Lygosominae because it contains oviparous species and some that exhibit complex placentae. Our specific objective has been to attempt to understand the fundamental steps required when viviparity, and ultimately complex placentae, evolve from oviparous ancestors. We have taken a three-prong approach: (1) detailed study of the morphology and ontogeny of the placentae of key species at the light microscope level; (2) study of changes in the uterus associated with pregnancy, or the plasma membrane transformation; and (3) measures of the net exchange of nutrients across the placenta or eggshell of key species. In turn, we have found that: (1) details of the morphology and ontogeny of placentae are more complex that originally envisaged, and that the early conclusions about a sequence in the evolution of complex placentae was naïve; (2) a plasma membrane transformation occurs in viviparous, but not oviparous lizards, and thus may be a fundamental feature of the evolution of viviparity in amniotes; and (3) species with more complex chorioallantoic placentae tend to transport more nutrients across the placenta during pregnancy than those with simpler chorioallantoic placentae but, because the correlation is not tight, the importance of the omphaloplacenta in transporting nutrients may have been overlooked. Also, the composition of yolk of highly matrotrophic species is broadly similar, but not identical, to the yolk of oviparous species. Some of the interpretation of our data within the context of our specific objective is not yet possible, pending the publication of a robust phylogeny of Eugongylus group skinks. Once such a phylogeny is available, we are in a position to propose specific hypotheses about the evolution of viviparity that can be tested using another lineage of amniotes, possibly Mabuya group skinks. © 2002 Elsevier Science Inc. All rights reserved.

Yolk supplies the majority of embryonic calcium in snakes. Oviparous and viviparous snakes also receive calcium late in development from the eggshell and placenta, respectively. The pattern and mechanism of calcium transport are partly understood for oviparous snakes. I studied a viviparous snake, Virginia striatula, to determine the pattern of embryonic calcium accumulation as well as the ontogenetic expression of calcium transporting proteins in extraembryonic tissues. The pattern of embryonic calcium uptake of V. striatula occurs late in development, during the phase of highest embryonic growth. Calbindin-D28k, Ca2+ ATPase, and carbonic anhydrase II are expressed in chorioallantoic membrane, while yolk sac only expresses calbindin-D28k, coincident with the timing of calcium transport in embryos of V. striatula. Thus, the pattern of embryonic calcium accumulation in V. striatula is similar to that of oviparous snakes. Although calbindin-D28k and Ca2+ ATPase are likely active in embryonic calcium transport, the role of carbonic anhydrase II remains less clear.