| Mesosaurus | |
|---|---|
 | |
| Holotype ofMesosaurus tenuidens (specimen MNHN 1865-77) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Subclass: | †Parareptilia |
| Order: | †Mesosauria |
| Family: | †Mesosauridae |
| Genus: | †Mesosaurus Gervais, 1865[1] |
| Species: | †M. tenuidens |
| Binomial name | |
| †Mesosaurus tenuidens Gervais, 1865[1] | |
| Synonyms | |
List
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 | It has been suggested thatMesosaur,Stereosternum andBrazilosaurus bemerged into this article. (Discuss) Proposed since December 2025. |
Mesosaurus (meaning "middle lizard") is anextinctgenus ofreptile from theEarly Permian of southernAfrica andSouth America. Along with it, the generaBrazilosaurus andStereosternum, it is a member of thefamilyMesosauridae and theorderMesosauria.Mesosaurus was long thought to have been one of the firstmarine reptiles, although new data suggests that at least those of Uruguay inhabited a hypersaline water body, rather than a typical marine environment.[3] In any case, it had many adaptations to a fully aquatic lifestyle. It is usually considered to have beenanapsid, althoughFriedrich von Huene considered it to be asynapsid.[4] Recent study of Mesosauridae phylogeny places the group as either the basal most clade withinParareptilia or the basal most clade withinSauropsida (with the latter being the less supported position)[5] despite the skull ofMesosaurus possessing the "Synapsid condition" of onetemporal fenestra.[6][7]
Theholotype ofM. tenuidens, MNHN 1865-77, is nicknamed the "GriquaMesosaurus" and it was found in a Griqua hut inSouth Africa, likely inKimberley, Northern Cape around 1830 and was being used as a pot lid.[1][8] The circumstances of its discovery and how it was taken from its previous owners in South Africa are unknown, but what is known is that the specimen eventually surfaced in the collection of theFrenchpalaeontologistPaul Gervais during the 1860s and he designated it as the holotype of a new genus and species he namedMesosaurus tenuidens in 1865.[1]
Since then,Mesosaurus remains have also been identified fromSouth America and were first identified in 1908 as belonging to a second species,M. brasiliensis, by J. H. MacGregor.[9] Later studies have shown thatM. brasiliensis was the same animal asM. tenuidens, which remains as the single valid species ofMesosaurus to this day.
Two other species ofmesosaurids have since been described, which areStereosternum[10] andBrazilosaurus,[11] which are also considered to be synonyms ofMesosaurus tenuidens according to Piñeiroet al. (2021).[2]
Mesosaurus had a long skull that was larger than that ofStereosternum and had longer teeth. The teeth are angled outwards, especially those at the tips of the jaws.[12] Like other mesosaurids,Mesosaurus possessed a smooth enamel-dentine junction, reflecting its enamel being solely formed by cells of theectoderm.[13]
The bones of the postcranial skeleton are thick, having undergonepachyostosis.Mesosaurus is unusual among reptiles in that it possesses acleithrum, usually found in more primitivebony fish and tetrapods.[14] The head of theinterclavicle ofMesosaurus is triangular, unlike those of other early reptiles, which are diamond-shaped.[15]
Thenostrils were located at the top, allowing the creature to breathe with only the upper side of its head breaking the surface, in a similar manner to a moderncrocodile.[16]
Mesosaurus had a smallskull with longjaws. The teeth were originally thought to have been straining devices for thefilter feeding ofplanktonic organisms.[16] However, this idea was based on the assumption that the teeth ofMesosaurus were numerous and close together in the jaws. Newly examined remains ofMesosaurus show that it had fewer teeth and that the dentition was suitable for catching smallnektonic prey such as crustaceans.[12]
Mesosaurus was one of the first reptiles known to have returned to the water after earlytetrapods came to land in theLate Devonian or later in the Paleozoic.[17] 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 thickenedribs, which would have prevented it from twisting its body.[16] The pachyostosis seen in the bones ofMesosaurus may have enabled it to reachneutral buoyancy in the upper few meters of thewater column. The additional weight may have stabilized the animal at the water's surface. Alternatively, it could have givenMesosaurus greater momentum when gliding underwater.[18] Biomechanical analysis of its body suggests thatMesosaurus would have been a relatively slow swimmer, with its optimal swimming speed in both hypersaline water and seawater most likely being between 0.15 to 0.41 m/s.[19]
While many features suggest a wholly aquatic lifestyle,[18]Mesosaurus may have been able to move onto land for short periods of time. Its elbows and ankles were restricted in their movement, making walking appear impossible. It is more likely that ifMesosaurus moved onto land, it would push itself forward in a similar way to living femalesea turtles when nesting on beaches.[14] A study on vertebral column proportions suggested that, while youngMesosaurus might have been fully aquatic, adult animals spent some time on land. This is supported by the rarity of adult animals in aquatic settings, and a coprolite possessing drying fractures. However, how terrestrial these animals were is difficult to say, as theirpachyostosis and other adaptations for an aquatic lifestyle would have made foraging on land difficult.[20]
_PeerJ_4-e2036_fig_S1.png)
Clearlyamniote-type fossilembryos ofMesosaurus in an advanced stage of development (i.e.fetuses) have been discovered in Uruguay and Brazil. These fossils are the earliest record of amniote fetuses, although amniotes are inferred to have had their typical reproductive strategy since their first appearance in theLate Carboniferous. Prior to their description, the oldest known amniote fetuses were from theTriassic.[21]
One isolated coiled fetus called FC-DPV 2504 is not surrounded bycalcareous eggshells, suggesting that the glands in theoviduct ofMesosaurus and probably all Paleozoic amniotes were not able to secretecalcium carbonate, in contrast to post-paleozoicarchosaurs. This would explain the scarcity ofegg fossils in the paleozoic amniote fossil record.[21]
OneMesosaurus specimen called MCN-PV 2214 comprises a medium-size adult with a small individual in itsrib cage which is interpreted as a fetus ‘in utero’, even suggesting thatMesosaurus like many othermarine reptiles, gave live birth. If this interpretation is correct, this specimen would represent the earliest known example ofviviparity in the fossil record. The isolated fetus FC-DPV 2504, however, rather points to anovoviviparous reproduction strategy inMesosaurus.[21]
Mesosaurus was significant in providing evidence for thetheory ofcontinental drift, because its remains were found in southern Africa,Whitehill Formation, and eastern South America (Melo Formation, Uruguay andIrati Formation, Brazil), two widely separated regions.[22][23] AsMesosaurus was a coastal animal, and therefore less likely to have crossed theAtlantic Ocean, this distribution indicated that the two continents used to be joined together.
_Naturhistorisches_Museum_Wenen_7-06-2023_10-28-52.jpg)
![Early reconstruction of the skeleton of M. brasiliensis showing many small teeth in the jaws (MacGregor, 1908).[9]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.m.wikipedia.org%2fwiki%2fFile%3aMesosaurus.png)
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