According to new research published in the journal Frontiers in Ecology and Evolution, mesosaurs — the oldest known reptiles that developed aquatic adaptations — spent part of their life on land.

“Despite being considered the oldest-known fully aquatic reptile, mesosaurs share several anatomical features with terrestrial species,” said Professor Graciela Piñeiro, from the Universidad de la República in Uruguay.

“Our comprehensive analysis of the vertebrae and limbs of these ancient reptiles suggests they lived in the water during the earliest stages of their development, whereas mature adults spent more time on land.”

Since the discovery of Mesosaurus bones in the Mangrullo Formation of Uruguay, Professor Piñeiro and co-authors wondered why the larger, presumably adult specimens (around 6.5 feet, or 2 m, in length) were not as abundant as smaller mesosaur skeletons (around 3 feet, or 0.9 m, in length).

“The larger specimens, at least twice the length of the more commonly reported Mesosaurus fossils, could just be exceptionally big individuals,” Professor Piñeiro said.

“However, the environmental conditions of the Mangrullo lagoon of where they lived were harsh, making it difficult for the occasional mesosaur to reach such a relatively large size and age.”

“We then realized that in comparison to the smaller, better-preserved specimens, larger Mesosaurus fossils were almost always disarticulated, very weathered and badly preserved. This suggested these larger specimens had extended exposure to the air when they died.”

Terrestrial, semi-aquatic and aquatic animals show a clear difference in bone profiles, so Professor Piñeiro and colleagues used morphometrics to analyze the shape of the fossilized mesosaur bones.

They examined 40 juveniles and adult specimens of Mesosaurus tenuidens and compared their bone profiles to those of similar reptiles known to be aquatic or semi-aquatic, such as crocodiles and marine iguanas.

“The adult mesosaur tarsus — a cluster of bones in the ankle region — suggests a more terrestrial or amphibious locomotion rather than a fully aquatic behavior as widely suggested before,” said Dr. Pablo Núñez, also from the Universidad de la República.

“Their caudal vertebrae, the tail bones, also showed similarities to semi-aquatic and terrestrial animals.”

“This supports the hypothesis that the oldest and largest mesosaurs spent more time on land, where fossil preservation is not as good as in the subaquatic domain.”

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Pablo Nuñez Demarco et al. Was Mesosaurus a Fully Aquatic Reptile? Front. Ecol. Evol, published online July 27, 2018; doi: 10.3389/fevo.2018.00109