- Name: Mesosaurus (Greek for "middle lizard"); pronounced MAY-so-SORE-us
- Habitat: Swamps of Africa and South America
- Historical Period: Early Permian (300 million years ago)
- Size and Weight: About three feet long and 10-20 pounds
- Diet: Plankton and small marine organisms
- Distinguishing Characteristics: Slender, crocodile-like body; long tail
Mesosaurus was the odd duck (if you'll excuse the mixed species metaphor) among its fellow prehistoric reptiles of the early Permian period. For one thing, this slender creature was an anapsid reptile, meaning it did not have any characteristic openings on the sides of its skull, rather than a more common synapsid (a category that embraced the pelycosaurs, archosaurs and therapsids that preceded the dinosaurs; today, the only living anapsids are turtles and tortoises). And for another, Mesosaurus was one of the first reptiles to return to a partially aquatic lifestyle from its fully terrestrial forebears, like the prehistoric amphibians that preceded it by tens of millions of years. Anatomically, though, Mesosaurus was pretty much plain vanilla, looking a bit like a small, prehistoric crocodile… that is, if you're willing the overlook the thin teeth in its jaws that seem to have been used to filter plankton.
Now that all that's been said, however, the most important thing about Mesosaurus is where it lived. The fossils of this prehistoric reptile have been discovered in eastern South America and southern Africa, and since Mesosaurus lived in freshwater lakes and rivers, it clearly couldn't have swum across the expanse of the southern Atlantic Ocean. For this reason, the existence of Mesosaurus helps support the theory of continental drift; that is, the now-well-attested fact that South America and Africa were joined together into the giant continent Gondwana 300 million years ago before the continental plates supporting them broke apart and drifted into their current positions.
Mesosaurus is important for yet another reason: this is the earliest identified animal to have left amniote embryos in the fossil record. It's widely believed that amniote animals existed a few million years before Mesosaurus, only recently evolved from the first tetrapods to climb up onto dry land, but we have yet to discern any conclusive fossil evidence for these very early amniote embryos.