Mesosaurus (meaning "middle lizard") is an extinct genus of aquatic reptile from the late Early Permian (Kungurian, ~275 million years ago) of southern Africa and South America. It is the only member of the family Mesosauridae and order Mesosauria. Two other genera of mesosaurs, Brazilosaurus and Stereosternum, were formerly recognised, but are now considered synonyms of Mesosaurus. Mesosaurus contains a single valid species, M. tenuidens. Mesosaurus represents one of the earliest lineages of aquatically adapted reptiles. It had many adaptations to a fully aquatic lifestyle. Mesosaurus lived around the shorelines of the Irati–Whitehill sea, an epicontinental sea that covered parts of southern Pangaea during the late Early Permian, likely feeding on small prey such as pygocephalomorph crustaceans. Mesosaurs have often been considered parareptiles, though the validity of "Parareptilia" has recently been brought into question. Recent studies regardless place mesosaurs as basal, non-diapsid reptiles.
Discovery and naming
The holotype of M. tenuidens, MNHN 1865–77, is nicknamed the "Griqua Mesosaurus" and it was found in a Griqua hut in South Africa, likely in Kimberley, Northern Cape around 1830 and was being used as a pot lid.Helm, Charles & Benoit, Julien. (2019). Geomythology in Southern Africa. ResearchGate 36. 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 the French palaeontologist Paul Gervais during the 1860s and he designated it as the holotype of a new genus and species he named Mesosaurus tenuidens in 1865. In 1889, the species Ditchrosaurus capensis was named by Georg Gürich based on remains found in South Africa,Gürich, G. (1889). Ditrochosaurus capensis-ein neuer Mesosaurier aus der Karooformation Süd-Afrikas. Zeitschrift der Deutschen Geologischen Gesellschaft, 64: 16–52. though this is now regarded as a synonym of M. tenuidens. Since then, Mesosaurus remains have also been identified from South America and were first identified in 1908 as belonging to a second species, M. brasiliensis, by J. H. MacGregor.MacGregor, J.H. (1908) Mesosaurus brasiliensis nov. sp. IN: White, I.C. (1908) Commission for Studies on Brazilian Coal Mines - Final Report; (Bilingual report, Portuguese & English), Imprensa Nacional, Rio de Janeiro, Brazil, 617 p.: Part II, pp. 301-336. Later studies have shown that M. brasiliensis is another synonym of M. tenuidens.
Two other species of mesosaurs have been described and historically considered valid; Stereosternum tumidumCope, E.D. (1885). A contribution to the vertebrate paleontology of Brazil. Proceedings of the American Philosophical Society 25, 7-15. and Brazilosaurus sanpauloensis. Stereosternum tumidum was named by Edward Drinker Cope in 1886, based on specimens he studied while Brazil, in the collections of the Museu Nacional (Rio de Janeiro).COPE, E.D. 1886. A contribution to the vertebrate paleontology of Brasil. Proceedings of the American Philosophical Society 25, 7-15. Brazilosaurus is known from specimen BSPG 1965 I 131, a single skeleton recovered from the Assistencia Member of the Irati Formation (Hanayama Farm, Tatuí, São Paulo), in the Paraná Basin. It was named by T. Shikama and H. Ozaki in 1966.
Stereosternum, Brazilosaurus & Mesosaurus were historically regarded as valid genera, distinguished by the shape of their teeth. However, recent studies have suggested that the teeth actually changed shape of the course of growth, and that only Mesosaurus is actually a valid genus, with the other two genera representing juvenile or fragmentary specimens of Mesosaurus, and that M. tenuidens represents the only valid mesosaur species.
Description
Mesosaurs have been described as small-medium-sized reptiles, with hatchlings around long, and young adult specimens around in length, with most collected specimens being around . The largest known individuals are estimated to have reached a length of , though such specimens are rare as fossils, perhaps because large individuals had different environmental preferences than younger individuals.
The bodies of mesosaurs are slender with an elongate tail, which was proportionally narrow from side to side. The bones exhibit pachyosteosclerosis (the property of being both thick and relatively dense). The limbs are paddle-like, with preserved soft tissue from some individuals indicating that the feet were webbed. Mesosaurus is unusual among reptiles in that it possesses a cleithrum, 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.
The rostrum (the front part of the skull) is relatively elongate and the jaws exhibit numerous slender, conical teeth, which underwent replacement, as well as palatal teeth at the roof of the mouth. Mesosaurus possessed a smooth enamel-dentine junction, reflecting its enamel being solely formed by cells of the ectoderm. The nostrils 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 modern crocodile. The back of the skull exhibits a single pair of lower temporal fenetestrae. Over the course of growth, the skull became proportionally shorter though the proportion made up by the snout increased due to elongation, the teeth became more elongate and the hindlimb and the forelimb hand (manus) became proportionally shorter.
Palaeobiology
Diet
Mesosaurus had a small skull with long jaws. The teeth were originally thought to have been straining devices for the filter feeding of planktonic organisms. However, this idea was based on the assumption that the teeth of Mesosaurus were numerous and close together in the jaws. Newly examined remains of Mesosaurus show that it had fewer teeth and that the dentition was suitable for catching small (under in length) nektonic prey such as crustaceans, specifically those belonging to the extinct order Pygocephalomorpha, which were abundant in the environment in which Mesosaurus lived (which are otherwise scarce in fossils) and have been found in coprolites (fossilised feces) attributed to Mesosaurus. Mesosaurus may also have engaged in cannibalism, based on remains of juvenile mesosaurs found in coprolites and regurgitates attributed to mesosaurs. These however may be a result of scavenging rather than active predation.
Locomotion Mesosaurus was one of the first reptiles known to have returned to the water after early tetrapods came to land in the Late Devonian or later in the Paleozoic. It was around 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. The pachyostosis seen in the bones of Mesosaurus may have enabled it to reach neutral buoyancy in the upper few meters of the water column. The additional weight may have stabilized the animal at the water's surface. Alternatively, it could have given Mesosaurus greater momentum when gliding underwater. Biomechanical analysis of its body suggests that Mesosaurus would have been a relatively slow swimmer, with its optimal swimming speed in both hypersaline water and seawater most likely being between 0.15 and 0.41 m/s.
While many features suggest a wholly aquatic lifestyle, 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 if Mesosaurus moved onto land, it would push itself forward in a similar way to living female sea turtles when nesting on beaches. A study on vertebral column proportions suggested that, while young Mesosaurus 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 their pachyostosis and other adaptations for an aquatic lifestyle would have made foraging on land difficult.Pablo Nuñez Demarco et al. Was Mesosaurus a Fully Aquatic Reptile? Front. Ecol. Evol, published online July 27, 2018; Other scholars have contested the idea that adult mesosaurs were more terrestrially adapted than juveniles, and have suggested that they may have spent more time in open water.
Reproduction
Clearly amniote-type fossil embryos of Mesosaurus 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 the Late Carboniferous. Prior to their description, the oldest known amniote fetuses were from the Triassic.
One isolated coiled fetus called FC-DPV 2504 is not surrounded by calcareous eggshells, suggesting that the glands in the oviduct of Mesosaurus and probably all Paleozoic amniotes were not able to secrete calcium carbonate, in contrast to post-paleozoic archosaurs. This would explain the scarcity of egg fossils in the paleozoic amniote fossil record.
One Mesosaurus specimen called MCN-PV 2214 comprises a medium-size adult with a small individual in its rib cage which is interpreted as a fetus 'in utero', even suggesting that Mesosaurus like many other marine reptiles, gave live birth. If this interpretation is correct, this specimen would represent the earliest known example of viviparity in the fossil record. The isolated fetus FC-DPV 2504, however, rather points to an ovoviviparous reproduction strategy in Mesosaurus.
Distribution and habitat
Mesosaurus was significant in providing evidence for the theory of continental drift, because its remains were found in southern Africa, Whitehill Formation, and eastern South America (Mangrullo Formation, Uruguay and Irati Formation, Brazil), two widely separated regions. Mesosaurus lived on the coastline of the Irati–Whitehill sea, an epicontinental sea that existed in the southern part of Gondwanan Pangaea covering parts of south and eastern South America and southern Africa during the Early Permian for a period of up to 4 million years during the late Kungurian age of the Early Permian, around 275 million years ago, before drying up, resulting the extinction of the mesosaurs. At least some mesosaurs, like those from the Mangrullo Formation, are suggested to have lived in saline lagoons with anoxic bottom waters that had a relatively depauperate fauna dominated by mesosaurs and pygocephalomorph crustaceans, with otherwise common aquatic animals like fish and temnospondyl amphibians being absent from the lagoons.
Relationships to other reptiles