Mesosaur

Mesosaurs Temporal range: Cisuralian, 299–280 Ma PreЄ Є O S D C P T J K Pg N
Mesosaurus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Subclass:	Anapsida
Order:	†Mesosauria Seeley, 1892
Family:	†Mesosauridae Baur, 1889
Genera
†Brazilosaurus †Stereosternum †Mesosaurus

Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period, roughly 299 to 270 million years ago. Mesosaurs were the first aquatic reptiles, having apparently returned to an aquatic lifestyle from more terrestrial ancestors. However, just how terrestrial mesosaur ancestors had become remains uncertain; recent research cannot establish with confidence if the first amniotes were fully terrestrial, or only amphibious.^[1] Most authors consider mesosaurs to have been aquatic,^[2][1] although at least some of them may have been amphibious, rather than completely aquatic, as indicated by their moderate skeletal adaptations to an aquatic lifestyle.^[3] They seem to have been coastal forms and probably inhabited relatively shallow water.^[4]Similarly, their affinities are uncertain; they may be either the most basal sauropsids,^[5] or among the most basal parareptiles.^[6]

Mesosaurus

The best-known mesosaur is Mesosaurus itself. Fossils of the animal were found in Namibia, Africa [1] and parts of South America, such as Brazil, Uruguay, and Paraguay[2].^[4][6][2][3] The widespread distribution of the fossil - particularly in those areas of Africa and South America which, when viewed on a map of the Earth, appear to 'interlock' - helped to reinforce the idea of continental drift.[3]

Mesosaurus vaguely resembled a small alligator, although the most common specimens are only 40 centimetres (16 in) in length. Nonetheless, the largest specimen is almost 2 metres (6.6 ft) in length, and Mesosaurus was probably the largest marine reptile of its time. It is believed to have fed on crustaceans^[2], by using its teeth as a sieve, rather like modern-day whales.

References

1. ^ ^{a b} Canoville, A. and M. Laurin. 2010. Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on paleobiological inferences. Biological Journal of the Linnean Society 100:384–406.
2. ^ ^{a b c} Modesto, S. 2006. The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society 146:345-368.
3. ^ ^{a b} Piñeiro, G. 2008. Los mesosaurios y otros fosiles de fines del Paleozoico; pp. 179–205 in D. Perera (ed.), Fósiles de Uruguay. DIRAC, Montevideo.
4. ^ ^{a b} Oelofsen, B. and D. C. Araújo. 1983. Palaeoecological implications of the distribution of mesosaurid reptiles in the Permian Irati sea (Paraná basin), South America. Revista Brasileira de Geociências 13:1–6.
5. ^ Laurin, M. and R. R. Reisz. 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113:165-223.
6. ^ ^{a b} Modesto, S. P. 1999. Observations on the structure of the Early Permian reptile Stereosternum temidum Cope. Palaeontologia Africana 35:7-19.