These finds, published in the journal Historical Biology, were revealed by an international team including Michel Laurin, CNRS senior researcher at the Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements (CNRS/Museum national d’histoire naturelle/UPMC).
Although the oldest known adult amniote[2] fossils date back some 315 million years, very few collections of fossil eggs and embryos are available to paleontologists. The discovery by an international team including Michel Laurin, from the Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements (CNRS/Museum national d’histoire naturelle/UPMC), of fossilized embryos of mesosaurs, ancient aquatic reptiles that lived ca. 280 million years ago, sheds light on these animals' reproductive mechanism.
In Brazil, the team uncovered 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 viviparous1.
In addition, the same researchers 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 (see photograph below) 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).
Read more at Science Daily
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