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Daniel G. Blackburn studied the evolution of viviparity and maternal provisioning in reptiles. A 1984 paper described the skink Mabuya heathi, whose very small eggs and specialized placenta indicated extensive nutrient transfer during pregnancy. Most embryonic growth occurred late in pregnancy, when placental specialization was pronounced.^[ref]

The comparison is useful because live birth and the source of embryonic nutrition are different traits. An embryo can be retained in its mother while relying mainly on yolk; this reptile instead exhibited substantial maternal provisioning. Similar physiological problems can therefore favor comparable reproductive specializations in separate evolutionary lineages.

A live birth is delivery of offspring showing signs of life after separation from the mother. In human vital statistics, breathing or a beating heart can provide such evidence; being born alive and surviving the subsequent neonatal period are distinct outcomes.^[ref]

In comparative reproduction, delivery of living young is associated with viviparity, whereas oviposition releases eggs whose embryonic development continues outside the maternal body. The evolutionary question concerns where and how the developing offspring is supported, rather than whether the mother produces ova.

Mabuya heathi is the historical scientific name of a Brazilian skink now listed as Brasiliscincus heathi. The name used in older reproductive studies identifies the same species under an earlier generic classification.^[ref]

The species is important in the study of viviparity and placental matrotrophy. In a 1984 study, Daniel G. Blackburn and colleagues compared the small ovulated eggs with developing offspring and examined the reproductive tissues. They reported eggs about one millimetre across and estimated that placental transfer supplied more than $99\%$ of fetal dry mass near birth. The estimate concerns dry material accumulated during development, rather than simply uptake of water.^[ref]

The comparison demonstrates that live birth can involve extensive maternal nutrient supply after ovulation. Its similarity to nutritional support in placental mammals is an example of convergent evolution; it does not place the skink within the mammalian lineage.

Placental matrotrophy is nourishment of developing offspring by transfer of maternal material across a placenta. It supplements or replaces reliance on reserves packaged in the ovum, such as yolk. Viviparity specifies retention of developing offspring and live birth; placental matrotrophy additionally specifies a route of nutrient supply.

A striking example comes from the lizard described as Mabuya heathi in a 1984 study. Its very small ovum could not account for the large increase in offspring dry mass; the authors estimated that placental transfer supplied more than 99 percent of the near-term fetus's dry mass. Examination of the placenta linked the growth measurements with an anatomical route of exchange.^[ref]

Dry mass matters because uptake of water alone can greatly increase wet mass. In a synthetic comparison, an initial 1 mg of dry mass and a final 10 mg imply a net addition of 9 mg of dry material. Measuring that increase supports transfer beyond the original reserves; determining the particular nutrients and transport mechanisms requires further measurements. The lizard example also makes placental matrotrophy a comparative character across vertebrates, rather than a synonym for being a mammal.

Viviparity is reproduction in which offspring develop within the maternal body and are delivered alive. It contrasts with oviposition, in which an egg is laid before development is completed. Viviparity describes retention and birth; it does not by itself specify how much nourishment is supplied during development.

Some live-bearing animals rely largely on reserves packaged in the ovum, while others receive substantial additional maternal nutrients. A study of the lizard Mabuya heathi compared egg size, fetal growth and placenta structure and found extensive maternal contribution to the developing offspring.^[ref] The comparison illustrates why live birth and the mode of nourishment are separate biological characters, and why a placenta is not exclusive to mammals.