A small, long-snouted reptile that lived in freshwater lakes is found only in South America and southern Africa, a famous clue that the continents were once joined.
The story
Mesosaurus was a slim aquatic reptile about a metre long, with a long tail, webbed feet and a narrow snout lined with fine, curved teeth. It lived about 290 million years ago in the Early Permian and is among the first known reptiles to have returned to the water. Its fossils are found in the Irati Formation of Brazil and in the Whitehill Formation of South Africa and Namibia, and nowhere else.
Paul Gervais described it in 1865. Alfred Wegener and later Alexander du Toit pointed out that a small freshwater animal could not have swum across the Atlantic, and that its distribution made sense if South America and Africa were then joined. Today the two continents lie thousands of kilometres apart. Mesosaurus is one of the best-known pieces of evidence for continental drift, together with Glossopteris and Lystrosaurus. It probably hunted small crustaceans and fish in shallow inland waters.
Why it mattered
Mesosaurus supplied some of the clearest fossil evidence for continental drift in Wegener's time.
It shows reptiles returned to water long before ichthyosaurs and mosasaurs.
Its deposits record Permian inland seas on Gondwana.