Mesosaurus
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Mesosaurus Temporal range: Cisuralian, 299–280 Ma |
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| Mesosaurus tenuidens | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | †Parareptilia |
| Order: | †Mesosauria |
| Family: | †Mesosauridae |
| Genus: |
†Mesosaurus Gervais, 1864-66 |
| Type species | |
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†Mesosaurus tenuidens Gervais, 1864-66 |
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| Synonyms | |
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Mesosaurus (meaning "middle lizard") is an extinct genus of reptile from the Early Permian of southern Africa and South America. Along with the genera Brazilosaurus and Stereosternum, it is a member of the family Mesosauridae and the order Mesosauria. Mesosaurus was long thought to have been one of the first marine reptiles, although new data suggests that at least those of Uruguay inhabited a hypersaline water body, rather than a typical marine environment. In any case, it had many adaptations to a fully aquatic lifestyle. It is usually considered to have been anapsid, although Friedrich von Huene considered that it was synapsid, and this hypothesis has been revived recently.
Mesosaurus had a long skull that was larger than that of Stereosternum and had longer teeth. The teeth are angled outwards, especially those at the tips of the jaws.
The bones of the postcranial skeleton are thick, having undergone pachyostosis. Mesosaurus is unusual among reptiles in that it possesses a cleithrum. A cleithrum is a type of dermal bone that overlies the scapula, and is usually found in more primitive bony fish and tetrapods. The head of the interclavicle of Mesosaurus is triangular, unlike those of other early reptiles, which are diamond-shaped.
Mesosaurus was one of the first reptiles to return to the water after early tetrapods came to land in the Late Devonian or later in the Paleozoic. It was around 1 metre (3.3 ft) in length, with webbed feet, a streamlined body, and a long tail that may have supported a fin. It probably propelled itself through the water with its long hind legs and flexible tail. Its body was also flexible and could easily move sideways, but it had heavily thickened ribs, which would have prevented it from twisting its body.
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