Mesosaurus: Fossils & Facts

Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Mesosaurus may or may not have been a reptile. It may or may not have lived in the ocean. Is there anything about it we know for sure? Read on and find out.

Mesosaurus and Its Fossils

You may reasonably wonder why a 270 million-year-old lizard is such a big deal. Turns out, it just happened to live in the right place at the right time. And that made it possible to use its fossils as evidence of the biggest idea in geology: plate tectonics.

Mesosaurus, meaning middle lizard, is the name of a genus of extinct aquatic animals that lived in the southern hemisphere during early Permian time. Genus names are very often descriptive. In this case, the reason for the 'lizard' part of the name (-saurus) is obvious; its body shape resembles that of modern lizards. The middle part of the name might refer to its size, or to its physical characteristics (midway between primitive and more advanced species), or to its place in the evolutionary path of reptiles (about halfway between their first appearance and dominance); or all three, or something else.

On top of all of that, there is today some doubt about whether mesosaurus were reptiles at all. If that is true, it means they weren't lizards, either. Because of the type of rocks in which the original fossils were found, they were presumed to be marine (ocean-living) reptiles, or at the least land-dwelling reptiles that returned to living (at least part of the time) in an aquatic environment.

Many paleontologists still think they were a type of reptile, but recent research has called that conclusion into question. Some researchers classify them as having a common ancestor with reptiles (making them first cousins, in effect), and others have even suggested (based on skull features) they might have belonged to the same group of animals from which mammals evolved (known as the synapsids).

Interesting, huh? This lack of agreement is just the kind of debate that makes paleontological conferences so enthralling!

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Mesosaurus Facts

Most Mesosaurus fossils have been found in rocks that were deposited in shallow coastal waters, but a few fossils have come from rocks deposited in very salty water and even fresh-water lakes. It seems that the animal was suited for a wide range of aquatic environments.

Mesosaurus had a long tail, and its elongated skull, which seems a tad undersized for its body, had the nostrils on top. It might even have been able to float near the surface, like modern crocodiles. It had a mouth full of sharp, outwardly angled teeth, nicely adapted for snagging small, swimming prey.

Adults averaged around one meter long, tip to tail. Its webbed feet were probably used for propulsion, and the narrow body was just perfect for slicing through the water. But could it walk on land? Sort of, maybe. The bones in Mesosaurus elbows and ankles weren't designed for traipsing along the beach, but it might have been able to push itself forward with its back feet, rather like sea turtles do today.

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