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]
Eutherian-like reproductive specializations in a viviparous reptile by Daniel G. Blackburn and others. Abstract and observations of egg size, embryonic growth and placental morphology
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.
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