Mesosaur: Discovering the Secrets of the Oldest Semi-Aquatic Reptile

Travel back in time to the early 1980s on the Hastings household’s cattle ranch in Uruguay. Welcome to El Baron Ranch, nearly 3,000 acres of rolling hills, meandering streams, and native trees, where the household raises free-range livestock.

During this time, the household was busy developing a 1,600-foot dam on the land to water rice. Little did they know, their project would lead to the remarkable discovery of a group of fossils that were almost 280 million years old, hidden within an outcrop of rock concealed in a hill. What they found inside was a creature measuring about a meter (about 3 feet) in length, with a long snout and tail, and thin rows of fragile teeth—a Mesosaur.

(Credit: Graciela Piñeiro) Outcrop in El Baron Ranch

What Is a Mesosaur?

A Mesosaur is an extinct semi-aquatic reptile. As one of the earliest known reptiles to have returned to a water lifestyle, Mesosaurs have provided important insights into the early adaptations of reptiles to different environments.

When Did Mesosaurs Live?

Xavier Jenkins, a college student who studies ancient reptiles at Idaho State University, says Mesosaurs lived during the early Permian, a time when there were still glaciers on the poles.

“It lived really close to the poles,” said Jenkins. “In a cool, damp environment along with mainly fish and shellfish in the water.” Meanwhile, significant reptiles like Dimetrodon, a back-sailed predator, would have roamed the land. “Mesosaur was like a marine iguana today,” Jenkins explained.

What Did Mesosaurs Eat?

With its interlocking jaws and ample stomach scales, Mesosaurs were ideally equipped for catching and nibbling shellfish and fish.

How Did Mesosaurs Reproduce?

Perhaps most notably, the fossils told a story of reptile reproduction that had never been told before, says Graciela Piñeiro, the Uruguayan biologist and paleontologist who led the discovery of Mesosaur fossils in Uruguay.

Her work at El Baron Ranch led to the finding of the first preserved Mesosaur embryo. “This discovery is the only evidence of amniote reproduction during the Paleozoic,” she says.

(Credit: Javier Calvelo) Almost complete juvenile Mesosaurus from Uruguay

Were Mesosaurs Aquatic?

Marking the earliest known reptiles to transition from land to water and become semi-aquatic, the Mesosaur fossils have provided unprecedented insights. “They are the only known vertebrates from Gondwana in the Early Permian, and they are represented by numerous intact and almost complete skeletons,” wrote the study authors in Frontiers in Earth Science

While we have more fossil remains of Mesosaur than most other reptiles from the Permian era, they are only found in South America and Africa, nowhere else on earth. “They are really restricted to lake deposits within the Gondwana region,” says Jenkins. This is likely because reptiles had not yet become semi-aquatic in other parts of the world.

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