An international team of paleontologists has unearthed the oldest known fossil reptile embryos dating back about 280 million years.
The study, published in the journal Historical Biology, reports well-preserved amniotic embryos of mesosaur, the ancient aquatic reptile, from the Early Permian of Uruguay and Brazil. This discovery suggests that mesosaurs were either viviparous (pushing back this mode of reproduction by 60 million years) or that they laid eggs in advanced stages of development.
Although the oldest known adult amniote fossils date back some 315 million years, very few collections of fossil eggs and embryos are available to paleontologists.
In Brazil, the team unearthed a fossil specimen in gestation, which revealed that mesosaur embryos were retained in the uterus during most of their development. These reptiles, therefore, were probably viviparous.
The team also unearthed 26 adult mesosaur specimens in Uruguay, all of which were associated with embryos or very young individuals, dating from the same period as the Brazilian fossil.
Although these more or less disarticulated specimens are difficult to interpret, most of them are probably embryos in the uterus, thus backing up the hypothesis that mesosaurs were viviparous. The largest of these fossils may be young animals that were looked after by at least one of the parents, pointing to the existence of parental care. However, one isolated mesosaur egg was also found at the Uruguayan site.
This find casts doubt on the hypothesis of viviparity (which, in theory, excludes the laying of eggs). It suggests that the Uruguay mesosaurs laid eggs at an advanced stage of development, which then hatched shortly afterwards (several minutes to days later).
The study therefore reveals the oldest known fossil amniote embryos from the Paleozoic (543 to 250 million years) and the first examples of embryo retention (and perhaps viviparity), pushing back this reproductive mechanism by some 60 million years.