Mesosaur Embryos Suspected To Be 280 Million Years Old

Embryos of fossil reptiles, from ca. 280 million years ago, have been discovered by a team of international palaeontologists. Their research article, published in the journal Historical Biology, contains reports of well-preserved mesosaur amniotic embryos. Mesosaurs are an ancient group of aquatic reptiles that lived during the Early Permian in Brazil and Paraguay. The discovery of the embryos points to mesosaurs either laying their eggs towards the advanced stages of development, or it means they were viviparous, a situation that could push back this way of reproduction by 60 million years.

Mesosaur Embryos Indicate In Uterine Development

Adult amniote fossils date as far back as 315 million years; yet despite this there are not many fossil egg and embryonic fossils available for palaeontological study. The recent find was from Brazil, where the scientific team uncovered a fossil specimen in gestation. This showed clearly that the embryos of mesosaurs are retained inside the uterus during the bulk of development, which suggests they were likely to be viviparous.

Mesosaur Embryos Are Difficult to Interpret

In Uruguay another 26 specimens of adult mesosaurs were discovered from a similar period to that of the fossil from Brazil. In all cases the adults were associated with embryos or with very young individuals. These specimens are not easy to interpret since they are mostly disarticulated, but it seems that they are likely to have embryos in the uterus, which backs up the viviparity hypothesis.

Mesosaurs May Have Been Caring Parents

There is also evidence of young animals that may have received parental care from at least one parent, but in contrast the site from Uruguay yielded one mesosaur egg that was isolated. The latter makes the viviparous hypothesis doubtful, since viviparity excludes the option of egg-laying. It rather suggests that mesosaurs in Uruguay were egg-layers that only laid their eggs once a more advanced developmental stage had been reached, with hatching occurring soon after (within a few minutes to a few days).

The Impact of These Mesosaur Embryos

In summary, this study shows the most ancient amniote embryos to occur in fossils from 543 – 250 million years ago (during the Paleozoic); it also reveals the first examples of retention of embryos, and possible viviparity, by pushing back the occurrence of this reproductive strategy by ca. 60 million years. In short, these mesosaur embryos could change our understanding of reptile reproduction as we know it.