Mesosaurus
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''Mesosaurus'' (meaning "middle lizard") is an
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
from the
Early Permian 01 or '01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), 01'' (Richard Müller album), 2001 * 01 (Son of Dave album), ''01'' (Son of Dave album), 2000 * 01 (Urban ...
of southern
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
and
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
. Along with it, the genera '' Brazilosaurus'' and ''
Stereosternum ''Stereosternum tumidum'' (meaning "rigid chest") (Stereos, Greek: “solid, firm”; Sternon, Greek: “chest, breastbone”) is an extinct genus of mesosaur marine reptile from the Early Permian of Brazil and also the Great Karoo Basin of Sout ...
'', it is a member of the
family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
Mesosauridae and the order
Mesosauria 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 known aquatic reptiles, having apparently returned to an aquatic life ...
. ''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 An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolve ...
, although Friedrich von Huene considered it to be a
synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
, and this hypothesis has been revived recently.


Discovery and naming

The
holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...
of ''M. tenuidens'', MNHN 1865-77, is nicknamed the "
Griqua Griqua may refer to: * Griqua people * Griqua language or Xiri language * Griquas (rugby) Griquas (known as the Windhoek Draught Griquas for sponsorship reasons since April 2022) are a South African rugby union team that participates in the an ...
''Mesosaurus''" and it was found in a Griqua hut in
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
, likely in
Kimberley, Northern Cape Kimberley is the 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 and Orange Rivers. The city has considerable historical significance due to it ...
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 palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
Paul Gervais Paul Gervais full name François Louis Paul Gervais (26 September 1816 – 10 February 1879) was a French palaeontologist and entomologist. Biography Gervais was born in Paris, where he obtained the diplomas of doctor of science and of medicine ...
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
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
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 mesosaurids have since been described, which are ''
Stereosternum ''Stereosternum tumidum'' (meaning "rigid chest") (Stereos, Greek: “solid, firm”; Sternon, Greek: “chest, breastbone”) is an extinct genus of mesosaur marine reptile from the Early Permian of Brazil and also the Great Karoo Basin of Sout ...
''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. The bones of the postcranial skeleton are thick, having undergone
pachyostosis Pachyostosis is a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification (osteosclerosis), reducing inner ca ...
. ''Mesosaurus'' is unusual among reptiles in that it possesses a
cleithrum The cleithrum (plural cleithra) is a membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = " key (lock)", by analogy with "cl ...
. A cleithrum is a type of
dermal bone A dermal bone or investing bone or membrane bone is a bony structure derived from intramembranous ossification forming components of the vertebrate skeleton including much of the skull, jaws, gill covers, shoulder girdle and fin spines rays ( le ...
that overlies the
scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
, and is usually found in more primitive
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...
and tetrapods. The head of the
interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In t ...
of ''Mesosaurus'' is triangular, unlike those of other early reptiles, which are diamond-shaped.


Palaeobiology

''Mesosaurus'' was one of the first reptiles known to have returned to the water after early
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
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. ''Mesosaurus'' had a small
skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...
with long
jaw The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...
s. The
nostril A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbi ...
s were located at the top, allowing the creature to breath 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 even more loosely to inclu ...
. The teeth were originally thought to have been straining devices for the
filter feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
of
plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
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
nekton Nekton or necton (from the ) refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms t ...
ic prey such as crustaceans. 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 densi ...
in the upper few meters of the
water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
. 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, loggerhead, ...
s when nesting on beaches. Clearly
amniote Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are disti ...
-type fossil
embryo An embryo is an initial stage of development of 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 spe ...
s of ''Mesosaurus'' in an advanced stage of development (i.e.
fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal deve ...
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 Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...
. Prior to their description, the oldest known amniote fetuses were from the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
. One isolated coiled fetus called FC-DPV 2504 is not surrounded by
calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...
eggshells, suggesting that the glands in the
oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
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 Egg fossils are the fossilized remains of eggs laid by ancient animals. As evidence of the physiological processes of an animal, egg fossils are considered a type of trace fossil. Under rare circumstances a fossil egg may preserve the remains of ...
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 The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi- ...
which is interpreted as a fetus ‘
in utero ''In Utero'' is the third and final studio album by American rock band Nirvana. It was released on September 21, 1993, by DGC Records. After breaking into the mainstream with their second album, ''Nevermind'' (1991), Nirvana hired Steve Albin ...
’, 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 Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...
in the fossil record. The isolated fetus FC-DPV 2504, however, rather points to an
ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop insi ...
reproduction strategy in ''Mesosaurus''. 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 Pachyostosis is a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification (osteosclerosis), reducing inner ca ...
and other adaptations for an aquatic lifestyle would have made foraging on land difficult.


Distribution

''Mesosaurus'' was significant in providing evidence for the
theory A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be s ...
of
continental drift Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pla ...
, because its remains were found in southern Africa, Whitehill Formation, and eastern South America ( Melo Formation, Uruguay and
Irati Formation 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 ...
, 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 oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
, this distribution indicated that the two continents used to be joined together.


Gallery

Mesosaurus tenuidens 1.jpg, Fossil from
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
Mesosaurus.jpg, Fossil in Milan Dual Mesosaurus.jpg, Fossil in Louisiana Mesosaurus BW.jpg, Restoration 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. Mesosaurus fetus Pineiro et al (2016) PeerJ 4-e2036 fig S1.png, Fossil of unhatched juvenile or fetus of ''Mesosaurus tenuidens'' (FC-DPV 2504) from Uruguay Mesosaurus Fossil bei Keetmanshoop, Namibia.jpg, Skeleton molds in whitish weathering shales of the Whitehill Formation, Keetmanshoop,
Namibia Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and ea ...
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


References


Further reading

* Parker, Steve. Dinosaurus: the complete guide to dinosaurs. Firefly Books Inc, 2003. Pg. 90 * * * * {{Taxonbar, from=Q132822 Prehistoric marine reptiles Prehistoric reptile genera Permian reptiles of Africa Fossils of Namibia Fossils of South Africa Permian reptiles of South America Permian Brazil Fossils of Brazil Permian Uruguay Fossils of Uruguay Paraná Basin Fossil taxa described in 1865 Taxa named by Paul Gervais