Mesosaurus

Mesosaurus; Temporal range: Cisuralian PreꞒ Ꞓ O S D C P T J K Pg N
	Holotype of Mesosaurus tenuidens (specimen MNHN 1865-77)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	† Parareptilia
Order:	† Mesosauria
Family:	† Mesosauridae
Genus:	† Mesosaurus ; Gervais, 1865
Species:	†M. tenuidens
Binomial name
	†Mesosaurus tenuidens; Gervais, 1865
Synonyms
	List Mesosaurus brasiliensisMcGregor, 1908; Ditrochosaurus capensisGurich, 1889; Mesosaurus capensis(Gurich, 1889); Brazilosaurus sanpauloensis?Shikama & Ozaki, 1966; Stereosternum tumidum?Cope, 1885; Mesosaurus tumidum?Cope, 1885;

Last updated October 12, 2024

Mesosaurus (meaning "middle lizard") is an extinct genus of reptile from the Early Permian of southern Africa and South America. Along with it, 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.^[3] 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.^[4] Recent study of Mesosauridae phylogeny places the group as either the basal most clade within Parareptilia or the basal most clade within Sauropsida (with the latter being the less supported position)^[5] despite the skull of Mesosaurus possessing the "Synapsid condition" of one temporal fenestra.^[6]^[7]

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.^[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 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.^[1]

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.^[9] 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 mesosaurids have since been described, which are Stereosternum ^[10] and Brazilosaurus ,^[11] which are also considered to be synonyms of Mesosaurus tenuidens according to Piñeiro et al. (2021).^[2]

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.^[12]

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 and tetrapods.^[13] The head of the interclavicle of Mesosaurus is triangular, unlike those of other early reptiles, which are diamond-shaped.^[14]

Size comparison with a human.

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.^[15]

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.^[15] 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.^[12]

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.^[16] 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.^[15] 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.^[17]

While many features suggest a wholly aquatic lifestyle,^[17]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.^[13] 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.^[18]

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.^[19]

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.^[19]

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.^[19]

Distribution

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 (Melo Formation, Uruguay and Irati Formation, Brazil), two widely separated regions.^[20]^[21] As Mesosaurus was a coastal animal, and therefore less likely to have crossed the Atlantic Ocean, this distribution indicated that the two continents used to be joined together.

Gallery

Fossil from Brazil
Dual fossil in Louisiana
Restoration
From Brazil, at the Natural History Museum, Vienna
Early reconstruction of the skeleton of M. brasiliensis showing many small teeth in the jaws (MacGregor, 1908).^[9]
Skeleton molds in whitish weathering shales of the Whitehill Formation, Keetmanshoop, Namibia
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

Related Research Articles

An anapsid is an amniote whose skull lacks one or more skull openings near the temples. Traditionally, the Anapsida are considered the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolved, making anapsids paraphyletic. It is, however, doubtful that all anapsids lack temporal fenestra as a primitive trait, and that all the groups traditionally seen as anapsids truly lacked fenestra.

Diapsids are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeoscelidians, appeared about three hundred million years ago during the late Carboniferous period. All diapsids other than the most primitive ones in the clade Araeoscelidia are often placed into the clade Neodiapsida. The diapsids are extremely diverse, and include birds and all modern reptile groups, including turtles, which were historically thought to lie outside the group. All modern reptiles and birds are placed within the neodiapsid subclade Sauria. Although some diapsids have lost either one hole (lizards), or both holes, or have a heavily restructured skull, they are still classified as diapsids based on their ancestry. At least 17,084 species of diapsid animals are extant: 9,159 birds, and 7,925 snakes, lizards, tuatara, turtles, and crocodiles.

Sauropsida is a clade of amniotes, broadly equivalent to the class Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern reptiles and birds. The most popular definition states that Sauropsida is the sibling taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early synapsids have historically been referred to as "mammal-like reptiles", all synapsids are more closely related to mammals than to any modern reptile. Sauropsids, on the other hand, include all amniotes more closely related to modern reptiles than to mammals. This includes Aves (birds), which are recognized as a subgroup of archosaurian reptiles despite originally being named as a separate class in Linnaean taxonomy.

Several groups of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least part of the time in water. These animals are called "secondarily aquatic" because although their ancestors lived on land for hundreds of millions of years, they all originally descended from aquatic animals. These ancestral tetrapods had never left the water, and were thus primarily aquatic, like modern fishes. Secondary aquatic adaptations tend to develop in early speciation as the animal ventures into water in order to find available food. As successive generations spend more time in the water, natural selection causes the acquisition of more adaptations. Animals of later generations may spend most their life in the water, coming ashore for mating. Finally, fully adapted animals may take to mating and birthing in water or ice.

<span class="mw-page-title-main">Marine reptile</span> Aquatically secondarily adapted reptiles

Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. Only about 100 of the 12,000 extant reptile species and subspecies are classed as marine reptiles, including marine iguanas, sea snakes, sea turtles and saltwater crocodiles.

Mesosaurs 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 an aquatic lifestyle from more terrestrial ancestors. It is uncertain which and how many terrestrial traits these ancestors displayed; recent research cannot establish with confidence if the first amniotes were fully terrestrial, or only amphibious. Most authors consider mesosaurs to have been aquatic, although adult animals may have been amphibious, rather than completely aquatic, as indicated by their moderate skeletal adaptations to a semiaquatic lifestyle. Similarly, their affinities are uncertain; they may have been among the most basal sauropsids or among the most basal parareptiles.

<span class="mw-page-title-main">Labyrinthodontia</span> Paraphyletic group of tetrapodomorphs

"Labyrinthodontia" is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras. Traditionally considered a subclass of the class Amphibia, modern classification systems recognize that labyrinthodonts are not a formal natural group (clade) exclusive of other tetrapods. Instead, they consistute an evolutionary grade, ancestral to living tetrapods such as lissamphibians and amniotes. "Labyrinthodont"-grade vertebrates evolved from lobe-finned fishes in the Devonian, though a formal boundary between fish and amphibian is difficult to define at this point in time.

Friedrich von Huene born Friedrich Richard Freiherr von Hoyningen-Huene was a German nobleman paleontologist who described a large number of dinosaurs, more than anyone else in 20th century Europe. He studied a range of Permo-Carboniferous tetrapods. He worked at the collections of the institute and museum for geology and paleontology at the University of Tübingen.

<span class="mw-page-title-main">Reptiliomorpha</span> Clade of reptile-like animals

Reptiliomorpha is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians). It was defined by Michel Laurin (2001) and Vallin and Laurin (2004) as the largest clade that includes Homo sapiens, but not Ascaphus truei. Laurin and Reisz (2020) defined Pan-Amniota as the largest total clade containing Homo sapiens, but not Pipa pipa, Caecilia tentaculata, and Siren lacertina.

Varanopidae is an extinct family of amniotes known from the Late Carboniferous to Middle Permian that resembled monitor lizards and may have filled a similar niche. Typically, they are considered to be relatively basal synapsids, although some studies from the late 2010s recovered them being taxonomically closer to diapsid reptiles, recent studies from the early 2020s support their traditional placement as synapsids on the basis of high degree of bone labyrinth ossification, maxillary canal morphology and phylogenetic analyses. A varanopid from the latest Middle Permian Pristerognathus Assemblage Zone is the youngest known varanopid and the last member of the "pelycosaur" group of synapsids.

Ophiacodon is an extinct genus of synapsid belonging to the family Ophiacodontidae that lived from the Late Carboniferous to the Early Permian in North America and Europe. The genus was named along with its type species O. mirus by paleontologist Othniel Charles Marsh in 1878 and currently includes five other species. As an ophiacodontid, Ophiacodon is one of the most basal synapsids and is close to the evolutionary line leading to mammals.

Labidosaurus is an extinct genus of captorhinid tetrapods from the Permian period of North America. Fossils have been discovered in Texas.

Claudiosaurus is an extinct genus of diapsid reptiles from the Late Permian Sakamena Formation of the Morondava Basin, Madagascar. It has been suggested to be semi-aquatic.

Marine vertebrates are vertebrates that live in marine environments. These are the marine fish and the marine tetrapods. Vertebrates are a subphylum of chordates that have a vertebral column (backbone). The vertebral column provides the central support structure for an internal skeleton. The internal skeleton gives shape, support, and protection to the body and can provide a means of anchoring fins or limbs to the body. The vertebral column also serves to house and protect the spinal cord that lies within the column.

Stereosternum tumidum is an extinct genus of mesosaur marine reptile from the Early Permian of Brazil and also the Great Karoo Basin of South Africa. The taxon mesosaur is a monophyletic group containing Brazilosaurus sanpauloensis and Mesosaurus tenuidens.

Captorhinus is an extinct genus of captorhinid reptiles that lived during the Permian period. Its remains are known from North America and possibly South America.

Colobomycter is an extinct genus of acleistorhinid parareptile known from the Early Permian of Oklahoma.

Irati Formation is the name of a geological formation of the Paraná Basin in Brazil. It has previously been dated as Late Permian using palynomorphs, but is now dated as Early Permian using zircon ages obtained from bentonite layers. The base of the formation has been dated at 278.4 ± 2.2 Ma. Exposures of the Irati Formation are to be found in the South, southeastern Brazil and in the states of Goiás, Mato Grosso, São Paulo, Paraná, Santa Catarina, Rio Grande do Sul and Mato Grosso do Sul. The formation is part of the Passa Dios Group, underlying the Serra Alta Formation and overlying the Palermo Formation. The formation has been deposited in a restricted marine environment. The Irati Formation, with a maximum thickness of 80 metres (260 ft), was defined and named by White in 1908.

Reptiles arose about 320 million years ago during the Carboniferous period. Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms. So defined, the group is paraphyletic, excluding endothermic animals like birds that are descended from early traditionally-defined reptiles. A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida.

The Mangrullo Formation is an Early Permian (Artinskian) fossiliferous geological formation in northeastern Uruguay. Some authors alternatively group it together with the Paso Aguiar Formation and the Frayle Muerto Formation as the three subdivisions of the Melo Formation, in which case it is referred to as the Mangrullo Member. Like the correlated formations of Irati and Whitehill, it is known for its abundant mesosaur fossils. It also contains the oldest known Konservat-Lagerstätte in South America, as well as the oldest known fossils of amniote embryos.

References

1 2 3 4 Gervais, P. (1865). Description du Mesosaurus tenuidens. Reptile fossile de l'Afrique australe. Académie des Sciences et Lettres de Montpellier. Mémoires de la Section des Sciences 6(2):169-175
1 2 3 4 Piñeiro, G.; Ferigolo, J.; Mones, A.; Núñez Demarco, P. (2021). "Mesosaur taxonomy reappraisal: are Stereosternum and Brazilosaurus valid taxa?". Revista Brasileira de Paleontologia. 24 (3): 205–235. doi: 10.4072/rbp.2021.3.04 . hdl: 11336/175571 . S2CID 244188443.
↑ Piñeiro, G.; Ramos, A.; Goso, C.; Scarabino, F.; Laurin, M. (2012). "Unusual environmental conditions preserve a Permian mesosaur-bearing Konservat-Lagerstätte from Uruguay". Acta Palaeontologica Polonica. 57 (2): 299–318. doi: 10.4202/app.2010.0113 .
↑ Huene, F. von (1940). "Osteologie und systematische Stellung von Mesosaurus". Palaeontographica Abteilung A. 92: 45–58.
↑ Laurin, Michael; Piñeiro, Graciela (2017-11-02). "A Reassessment of the Taxonomic Position of Mesosaurs, and a Surprising Phylogeny of Early Amniotes". Frontiers in Earth Science. 5: 88. Bibcode:2017FrEaS...5...88L. doi: 10.3389/feart.2017.00088 . hdl: 20.500.12008/33548 .
↑ Piñeiro, Graciela (2008). "Los mesosaurios y otros fosiles de fines del Paleozoico". In D. Perera (ed.). Fósiles de Uruguay. DIRAC, Montevideo.
↑ Piñeiro, G.; Ferigolo, J.; Ramos, A.; Laurin, M. (2012). "Cranial morphology of the Early Permian mesosaurid Mesosaurus tenuidens and the evolution of the lower temporal fenestration reassessed". Comptes Rendus Palevol. 11 (5): 379–391. Bibcode:2012CRPal..11..379P. doi:10.1016/j.crpv.2012.02.001.
↑ Helm, Charles & Benoit, Julien. (2019). Geomythology in Southern Africa. ResearchGate 36.
1 2 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.
↑ Cope, E.D. (1885). A contribution to the vertebrate paleontology of Brazil. Proceedings of the American Philosophical Society 25, 7-15.
↑ T. Shikama and H. Ozaki (1966). "On a Reptilian Skeleton from the Palaeozoic Formation of San Paulo, Brazil". Transactions and Proceedings of the Palaeontological Society of Japan. New Series. 64: 351–358.
1 2 Modesto, S.P. (2006). "The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology". Zoological Journal of the Linnean Society. 146 (3): 345–368. doi: 10.1111/j.1096-3642.2006.00205.x .
1 2 Modesto 2010, pp. 1392–1393.
↑ Modesto 2010, p. 1387.
1 2 3 Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 65. ISBN 978-1-84028-152-1.
↑ Laurin, Michel (2010). How Vertebrates left the Water (illustrated ed.). University of California Press. pp. xv + 199. ISBN 978-0-520-26647-6.
1 2 Canoville, Aurore; Michel Laurin (2010). "Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on paleobiological inferences". Biological Journal of the Linnean Society. 100 (2): 384–406. doi: 10.1111/j.1095-8312.2010.01431.x .
↑ Pablo Nuñez Demarco et al. Was Mesosaurus a Fully Aquatic Reptile? Front. Ecol. Evol, published online July 27, 2018; doi : 10.3389/fevo.2018.00109
1 2 3 Piñeiro, G.; Ferigolo, J.; Meneghel, M.; Laurin, M. (2012). "The oldest known amniotic embryos suggest viviparity in mesosaurs". Historical Biology. 24 (6): 620–630. Bibcode:2012HBio...24..620P. doi:10.1080/08912963.2012.662230. S2CID 59475679.
↑ Piñeiro, Graciela (2008). D. Perera (ed.). Fósiles de Uruguay. DIRAC, Montevideoy.
↑ Trewick, Steve (2016). "Plate Tectonics in Biogeography". International Encyclopedia of Geography: People, the Earth, Environment and Technology. John Wiley & Sons, Ltd. pp. 1–9. doi:10.1002/9781118786352.wbieg0638. ISBN 9781118786352.