Mesosaurs may have spent time on land

by Mary Caperton Morton
Friday, December 21, 2018

Mesosaurs are famous for being the earliest-known fully aquatic reptiles. With their whip-like tails, webbed feet and nostrils on top of their heads, the 2-meter-long reptiles appear to have been well adapted for life in the water. But in a new study, scientists have found fossil evidence that mesosaurs may have spent some of their adult lives on land.

Studying mesosaur fossils pulled from the 280-million-year-old Mangrullo Formation of Uruguay, researchers noticed that fossils of larger, mature mesosaurs were often found disarticulated and more weathered than juvenile fossils, suggesting that the adults may have died on land, where fossils are not usually as well preserved. To test this hypothesis, the researchers turned to digital morphometrics to analyze individual bones from 40 mesosaurs ranging from juveniles to adults. They compared mesosaur features to modern species of aquatic and semi-aquatic animals, identifying subtle similarities that suggest mesosaurs might have spent time on land.

Ankle bones in adult mesosaurs suggest “a more terrestrial or amphibious locomotion rather than a fully aquatic behavior as widely suggested before,” said lead author Pablo Nuñez Demarco of the University of the Republic in Uruguay in a statement. “The tail bones also showed similarities to semi-aquatic and terrestrial animals,” Demarco said.

How much time adult mesosaurs may have spent on land remains unknown. “It is impossible to know if mesosaurids came onto land only to bask, like seals or crocodiles, or if they were a bit more agile,” the team wrote in Frontiers in Ecology and Evolution.