''Mesosaurus'' (meaning "middle lizard") is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

from the Early Permian of southern

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

and

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

. Along with it, the genera '' Brazilosaurus'' and '' Stereosternum'', it is a member of the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Mesosauridae and the order

Mesosauria Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period (Cisuralian), roughly Geologic time scale, 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having appar ...

. ''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 it to be a synapsid. Recent study of Mesosauridae phylogeny places the group as either the basal most clade within

Parareptilia Parareptilia ("near-reptiles") is an extinct group of basal sauropsids (" reptiles"), traditionally considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near the en ...

or the basal most clade within

Sauropsida Sauropsida (Greek language, Greek for "lizard faces") is a clade of amniotes, broadly equivalent to the Class (biology), class Reptile, Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern repti ...

(with the latter being the less supported position) despite the skull of ''Mesosaurus'' possessing the "Synapsid condition" of one temporal fenestra.

Discovery and naming

The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

of ''M. tenuidens'', MNHN 1865-77, is nicknamed the " Griqua ''Mesosaurus''" and it was found in a Griqua hut in

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

, likely in

Kimberley, Northern Cape Kimberley is the Capital city, capital and largest city of the Northern Cape province of South Africa. It is located approximately 110 km east of the confluence of the Vaal River, Vaal and Orange Rivers. The city has considerable historica ...

around 1830 and was being used as a pot lid. 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 Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

Paul Gervais during the 1860s and he designated it as the holotype of a new genus and species he named ''Mesosaurus tenuidens'' in 1865. Since then, ''Mesosaurus'' remains have also been identified from

and were first identified in 1908 as belonging to a second species, ''M. brasiliensis'', by J. H. MacGregor. Later studies have shown that ''M. brasiliensis'' was the same animal as ''M. tenuidens'', which remains as the single valid species of ''Mesosaurus'' to this day. Two other species of

mesosaur Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period ( Cisuralian), roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to a ...

ids have since been described, which are '' Stereosternum''Cope, E.D. (1885). A contribution to the vertebrate paleontology of Brazil. ''Proceedings of the American Philosophical Society'' 25, 7-15. and '' Brazilosaurus'', which are also considered to be synonyms of ''Mesosaurus tenuidens'' according to Piñeiro ''et al.'' (2021).

Description

''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. Mesosaurus tenuidens 1

The bones of the postcranial skeleton are thick, having undergone pachyostosis. ''Mesosaurus'' is unusual among reptiles in that it possesses a cleithrum, usually found in more primitive

bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...

and tetrapods. The head of the interclavicle of ''Mesosaurus'' is triangular, unlike those of other early reptiles, which are diamond-shaped. Mesosaurus Scale

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 Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include ...

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

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

ic 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 nektonic prey such as crustaceans.

Locomotion

''Mesosaurus'' was one of the first reptiles known to have returned to the water after early

tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...

s 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 Neutral buoyancy occurs when an object's average density is equal to the density of the fluid in which it is immersed, resulting in the buoyant force balancing the force of gravity that would otherwise cause the object to sink (if the body's de ...

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. 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 turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerh ...

s 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.

Reproduction

Clearly

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

-type fossil

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

s of ''Mesosaurus'' in an advanced stage of development (i.e.

fetus A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic development, embryonic stage, the fetal stage of development takes place. Pren ...

es) 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 The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

. 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 Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

, in contrast to post-paleozoic archosaurs. This would explain the scarcity of

egg fossils Egg fossils are the fossilized remains of eggs laid by Geologic time, ancient animals. As evidence of the Physiology, physiological processes of an animal, egg fossils are considered a type of trace fossil. Under rare circumstances a fossil egg ma ...

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 animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...

in the fossil record. The isolated fetus FC-DPV 2504, however, rather points to an ovoviviparous reproduction strategy in ''Mesosaurus''.

Distribution

''Mesosaurus'' was significant in providing evidence for the

theory A theory is a systematic and rational form of abstract thinking about a phenomenon, or the conclusions derived from such thinking. It involves contemplative and logical reasoning, often supported by processes such as observation, experimentation, ...

of continental drift, because its remains were found in southern Africa, Whitehill Formation, and eastern South America ( Melo Formation, Uruguay and Irati Formation, Brazil), two widely separated regions. As ''Mesosaurus'' was a coastal animal, and therefore less likely to have crossed the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

, this distribution indicated that the two continents used to be joined together.

Gallery

File:Mesosaurus brasiliensis NTM I02048.jpg, Fossil from Brazil File:Dual Mesosaurus.jpg, Dual fossil in Louisiana File:Mesosaurus BW.jpg, Restoration File:Mesosaurus tenuidens (Brazilië - Vroeg Perm) Naturhistorisches Museum Wenen 7-06-2023 10-28-52.jpg, From Brazil, at the Natural History Museum, Vienna File:Mesosaurus.png, Early reconstruction of the skeleton of ''M. brasiliensis'' showing many small teeth in the jaws (MacGregor, 1908).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. File:Mesosaurus Fossil bei Keetmanshoop, Namibia.jpg, Skeleton molds in whitish weathering shales of the Whitehill Formation,

Keetmanshoop Keetmanshoop is a town in the ǁKaras Region of southern Namibia. It is named after , a German Empire, German industrialist and benefactor of the city. Keetmanshoop had a population of 27,862 people in 2023. History Before the colonial era, ...

Namibia Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...

File:Wegener fossils-mapped.png, Distribution of four Permian and Triassic fossil groups used as biogeographic evidence for continental drift, and land bridging. Location of ''Mesosaurus'' remains shown by green squares