The Oldest Evidence Of Reptilian Live Birth

A composite photo showing the size differences
between the mesosaurs and the embryos

Many biologists will tell you that live birth is a feature unique to mammals, with the exception of the platypus and echidna which lay eggs. Yet that is not technically correct. Mammals are unique in the sense that they give birth to live young which have developed in special organs within the body, namely the uterus. However some species of reptile and fish can also give birth to live young.

A few species keep the eggs of their young inside their bodies and allow them to fully mature and then hatch, giving birth to the live young in a process known as ovoviviparity, which is the proper term for the retention and maturity of eggs inside the body.The oldest known creature which displays this characteristic is the late Devonian Materpiscus. Yet this form of live birth is quite rare: the more common version is true live birth or viviparity.

The fossil embryo and a drawing of it, making
the position of the bones a little clearer

It was thought that the oldest viviparous reptiles were the pachypleurosaurs, which lived in the Triassic lakes of China. However a team of palaeontologists, led by Graciela Piñeiro, from the University of the Republic in Uruguay have found evidence that ovoviviparity in reptiles is even older than previously thought. They discovered two well preserved mesosaurs, which were the first ever marine reptiles on Earth.

Nearly all the bones were present with some traces of the soft parts. Close analysis revealed something remarkable in between the abdominal ribs. In the end the researchers concluded that the tiny 0.75 to 1.5 centimetre long structures containing clusters of tiny bones were mesosaur embryos. The fossils were undoubtedly ovoviviparous. When the fossils were dated, they came in at 280 million years old, making them the oldest reptiles to bare live young on Earth.

Gypsum crystals in the rock showed that the mesosaurs were living in a very low oxygen environment which was probably why the embryos survived. A feature which the researchers noted was that one of the well developed embryos was found outside of the body cavity of the parent mesosaur. It is uncertain, however, whether this was as a result of a miscarriage or if the embryos were ejected from the body when they had reached a certain level of maturity.

A lack of preserved eggshell suggests the former. 'Despite their age and their delicate nature, they remained in the rocks all that long time almost perfectly preserved' said Piñeiro. 'With the discovery of the mesosaur embryos, we may now have direct evidences that embryo retention or viviparity were strategies developed by early amniotes.' What remains unclear is when terrestrial vertebrates developed forms of live birth.

Evolutionary traits rarely develop twice across different groups. Therefore it seems likely that the ancestors of both marine reptiles and other non-dinosaurs, such as snakes or tortoises, evolved viviparity at some point in the Earth's history. All palaeontologists have to do is find the fossils. Yet these mesosaur fossils, however, have pushed back the date by around 60 to 70 million years.