Squamates have evolved from egg layers (the ancestral condition) to live bearers (a derived condition) tens to hundreds of times, in response to environmental changes.It is not at all unusual to find live bearing species in a clade of otherwise egg-laying species. Now Stewart et al. have documented the Australian Yellow-bellied Three-toed Skink, Saiphos equalis, has egg shell thickness that is inversely proportional to the length of time the eggs are retained in the female lizard. In an interview James Stewart, senior author at East Tennessee State University said, "By studying differences among populations that are in different stages of this process, you can begin to put together what looks like the transition from one [birth style] to the other." One of the question related to this is how do evolving species switch from delivering eggs to live babies provide nourishment to the embryos before birth. Some squamates have evolved a placenta, similar to mammals. In egg-laying species, the embryo gets nourishment from the yolk, but calcium absorbed from the porous shell is also an important nutrient source. The shells of these eggs thin dramatically so that the embryos can breathe, until live babies are born covered with only thin membranes—all that remains of the shells. However, a thinner shell has less calcium, which could cause deficiencies for the young reptiles. Stewart and colleagues decided to look for clues to the nutrient problem in the structure and chemistry of the yellow-bellied three-toed skink's uterus. He reports that, "Now we can see that the uterus secretes calcium that becomes incorporated into the embryo—it's basically the early stages of the evolution of a placenta in reptiles." Both birthing styles come with evolutionary tradeoffs: Eggs are more vulnerable to external threats, such as extreme weather and predators, but internal fetuses can be more taxing for the mother. For the skinks, females in balmier climates may opt to conserve their own bodies' resources by depositing eggs on the ground for the final week or so of development. Females in harsh mountain climates, by contrast, might find that it's more efficient to protect their young by keeping them longer inside their bodies. Overall, the results suggest the move from egg-laying to live birth in reptiles is fairly common—at least in historic terms—because it's relatively easy to make the switch, Stewart said. "We tend to think of this as a very complex transition but it's looking like it might be much simpler in some cases than we thought." Stewart. J. R. et al. 2010. Uterine and eggshell structure and histochemistry in a lizard with prolonged uterine egg retention (Lacertilia, Scincidae, Saiphos). Journal of Morphology, DOI: 10.1002/jmor.10877.
