Mesosaurus
| Mesosaurus | ||||||||||||
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Skeleton in limestone of the Irati Formation ( Paraná Basin , Brazil) in the Museo Geominero in Madrid |
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| Temporal occurrence | ||||||||||||
| Unterperm ( Artinskium ) | ||||||||||||
| 290 to 279 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Mesosaurus | ||||||||||||
| Gervais , 1865 | ||||||||||||
| Art | ||||||||||||
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Mesosaurus is an extinct reptiles - kind from the time of the early Permian ( Artinskian , before 290-279 million years ago). Mesosaurus was aquatic and probably couldn't move around very well on land. He lived on and in an extensive lake or inland sea that stretched across what is now southern Africa and eastern South America .
The reptile family in which Mesosaurus is placed, the mesosaurids, are the first amniotic , ie. In other words , fertile land vertebrates that are in principle independent of waters and that returned to an almost purely aquatic way of life - a direction of development that followed numerous other lines of amniotes in the further course of the Earth's history (including sauropterygians , thalattosaurs , penguins , seals ). They are also the only secondary aquatic amniotes of the early Permian. It was not until the late Permian , about 20 million years after the mesosaurs, that some representatives of the Younginiformes (e.g. Acerosodontosaurus and Hovasaurus ), a possibly paraphyletic group of early diapsids , appear the geologically next younger forms in the fossil record .
etymology
The name "Mesosaurus" is composed of the ancient Greek words μέσος mésos 'medium' and σαῦρος sauros 'lizard', consequently means something like "medium lizard". The name was coined in 1865 by the French naturalist Paul Gervais , who wanted to express that he found individual features of the Mesosaurus skeleton in numerous fossil and recent reptiles that were already known at the time, and that the genus therefore occupies an intermediate position.
features
Mesosaurus was a relatively small, slender reptile that reached a length of up to one meter. Most of the specimens found, however, are only about 40 centimeters long. The jaws are very narrow and very elongated, much like today's gavials . They are with a variety of needle-shaped and z. Sometimes very long teeth are equipped, the tips of which were pointed relatively strongly outward during lifetime, so that the teeth were inclined in the jaw. The neck has 12 vertebrae and is elongated, which presumably increased the freedom of movement of the head.
As an adaptation to the aquatic way of life, they had a flattened oar tail for propulsion in the water, similar to that of today's crocodiles . Since the limbs were not primarily used to carry the trunk on land, as in the case of rural dwellers, the bones in the carpal and tarsus of the mesosaurids are slightly reduced. In addition, the hands and feet are designed like paddles and the feet are relatively large and were probably also used for propulsion. Probably tensed between the fingers and toes webbed , possibly the soft tissues formed by hand and foot even outright "fins", similar to today's sea lions .
The ribs are thickened compared to similarly sized land-dwelling reptiles - a variation known as pachyostosis . Pachyostosis also occurs as a characteristic element in many other secondary aquatic vertebrates (e.g. particularly evident in today's manatees ). The thicker and therefore heavier bones reduce buoyancy in the water.
Temple window
Many extinct and living groups of reptiles are characterized by characteristic openings in the rear, upper and side parts of the roof of the skull, so-called temporal or temple windows . Such temporal windows were also described in Mesosaurus in the first half of the 20th century. However, it has been established in the course of later investigations that the skull bones of Mesosaurus are very thin-walled and fragile in this area, and that earlier observations of temporal windows were based on the fact that these thin-walled parts of the skull were damaged during fossilization, i.e. Mesosaurus did not have any temporal windows. Recently, however, individual Mesosaurus specimens were described again that are said to have a temporal window.
Way of life
nutrition
Mesosaurus was a hunter who actively pursued its prey in the open water, similar to the hunting behavior of seals or toothed whales today . The needle-shaped teeth were probably not used to impale prey, but rather as a trap to catch individual, small, free-swimming crustaceans and fish that were held in the mouth when the animal closed its jaws and pressed the water out of its mouth between the interlocking teeth . The main diet of Mesosaurus probably consisted of krill- like small crustaceans of the extinct family Notocarididae.
Reproduction
A very small individual Mesosaurus with a largely ossified skeleton that was discovered within the rib cage of an adult animal shows no signs of gastric acid- related dissolution symptoms . Therefore, it is probably an embryo or fetus in the womb. This would lead to the conclusion that Mesosaurus , like some marine reptiles of the Mesozoic Era , was viviparous . It is no less likely, however, that he laid eggs on the beach, much like today's sea turtles do. In contrast to the turtles, the Mesosaurus embryo was already relatively well developed at the time of oviposition, i.e. it was largely hatched in the mother animal (so-called ovoviviparia ). Since these eggs were quite large compared to the adult, a female could only carry one or two eggs, which is why only a few offspring resulted from a reproductive cycle. Therefore, it cannot be ruled out that Mesosaurus operated something like brood care to improve the chances of survival of these few offspring.
Systematics
Due to different interpretations of the skull structure with regard to the presence of temporal windows, Mesosaurus was first identified in the 20th century as a diapsis and thus as a relative of most of the reptiles and dinosaurs living today, then as a synapsis and thus as a distant relative of the mammals and more recently as an " anapside " (see classic concept of Anapsida according to Caroll, 1988 ). The latter group and the underlying structural concept, however, do not correctly reflect the actual course of evolution and thus the actual relationships of the amniotes, or the importance of the absence of temporal openings for the systematics of amniotes has decreased. More recent, cladistic studies have shown that Mesosaurus is apparently related to a number of other, predominantly Permian and now completely extinct reptiles, which, including the mesosaurids, are called parareptiles . The body structure that Mesosaurus has in common with both extinct and now living aquatic reptiles, such as the thalattosaurs or crocodiles, due to its aquatic way of life , is the result of a convergent evolution.
The closest relatives of Mesosaurus are the other genera in the Mesosauridae family: Brazilosaurus and Stereosternum . From Mesosaurus even two species have been described: M. tenuidens and M. brasiliensis . A third species, M. capensis , was originally described as a member of its own genus ( Ditrochosaurus ). Today M. (= "Ditrochosaurus") capensis , and M. brasiliensis are regarded as younger synonyms of M. tenuidens . Sometimes Stereosternum tumidum , the only species of this genus, is considered a Mesosaurus species.
The holotype of the species Mesosaurus tenuidens , which in turn is a type species of the genus Mesosaurus , was in the 1840s in the "Griqualand" (very likely it was the West Griqualand - today part of the North Cape Province of South Africa) by the brother of French naturalist Édouard Verreaux discovered in a Griqua hut. According to him, the rock plate with the fossil served the residents as a lid for their cooking pot.
Occurrence and fossil conservation
Mesosaurus fossils occur in southern Africa predominantly in the sub-Permian Whitehill Formation of the Karoo system and in Brazil and Uruguay predominantly in the likewise sub-Permian Irati or Mangrullo Formation of the Paraná Basin . You occur in these layers z. Sometimes in large numbers, mostly in the form of whole or partial, flattened skeletons.
The Whitehill, Irati, and Mangrullo Formations are made up of dark clay stones . The skeletons of Mesosaurus have been transformed into a very soft material in these claystones and are often already largely weathered out, so that only prints remain in the rock. Some sections of the Irati Formation (Assistência Subformation) consist of yellowish-white limestone. In these limestones, the fossil record of the bone substance is significantly better. However, not Mesosaurus , but the two other Mesosaur genera Stereosternum and Brazilosaurus are typical representatives of the limestone facies.
The clay and limestones of the Whitehill, Irati and Mangrullo Formation are the deposits of an extensive lake or inland sea (" Mesosaurus Sea"), with the limestones representing shallow waters near the coast and the mudstones representing deeper waters further away from the coast .
Mesosaurus and plate tectonics
The distribution of the genus Mesosaurus in the regions of eastern South America and southern Africa on both sides of the South Atlantic , which are now separated by thousands of kilometers of open ocean, was already known to the German natural scientist Alfred Wegener . It served him as an important argument in support of the theory of continental drift he proposed in 1915 . The distribution of the Mesosaurus fossils on both sides of the Atlantic and the similarity of the rocks they contain are a clear indication that Africa and South America were united in the ancient continent Gondwana during the lifetime of Mesosaurus . Gondwana broke apart only after the mesosaurs became extinct as a result of the movements of the lithospheric plates caused by the mechanisms of plate tectonics , and Africa and South America have been drifting apart since then.
literature
- Martin Sander: Reptiles. 220 individual representations (= Haeckel library. Vol. 3). Ferdinand Enke Verlag, Stuttgart 1994, ISBN 3-432-26021-0 .
- Michael J. Benton: Vertebrate Palaeontology. 3. Edition. Wiley-Blackwell Publishing, Malden MA 2005, ISBN 0-632-05637-1 .
Individual evidence
- ^ A b c d Paul Gervais: Description du Mesosaurus tenudiens . Reptile fossile de l'Afrique australe. Académie des Sciences et Lettres de Montpellier. Mémoires de la Section des Sciences. 6th Volume, Part II, 1865, pp. 169-175 ( MDZ Reader ).
- ^ Samuel Wendell Williston: Water Reptiles of the Past and Present. The University of Chicago Press, Chicago IL 1914, p. 128 ( archive.org ).
- ↑ a b Graciela Piñeiro, Jorge Ferigolo, Alejandro Ramos, Michel Laurin: Cranial morphology of the Early Permian mesosaurid Mesosaurus tenuidens and the evolution of the lower temporal fenestration reassessed. Comptes Rendus Palevol. Vol. 11, No. 5, 2012, pp. 379-391, doi : 10.1016 / j.crpv.2012.02.001 .
- ↑ a b c d Sean Patrick Modesto: The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens : implications for relationships and palaeobiology. Zoological Journal of the Linnean Society. Vol. 146, No. 3, 2006, pp. 345-368, doi : 10.1111 / j.1096-3642.2006.00205.x .
- ↑ The information in the entire paragraph comes from: Graciela Piñeiro, Jorge Ferigolo, Melitta Meneghel, Michel Laurin: The oldest known amniotic embryos suggest viviparity in mesosaurs. Historical Biology. Vol. 24, No. 6, 2012, pp. 620-630, doi : 10.1080 / 08912963.2012.662230 .
- ↑ a b Linda A. Tsuji, Johannes Müller: Assembling the history of the Parareptilia: phylogeny, diversification, and a new definition of the clade. Fossil Record. Vol. 12, No. 1, 2009, pp. 71-81, doi : 10.1002 / mmng.200800011 .
- ↑ Hans-Volker Karl, Elke Gröning, Carsten Brauckmann: The Mesosauria in the collections of Göttingen and Clausthal: implications for a modified classification. Clausthal geosciences. Vol. 6, 2007, ISSN 1611-0609 , pp. 63-78.
- ^ A b c Burger W. Oelofsen, Dina C. Araújo: Mesosaurus tenuidens and Stereosternum tumidum from the Permian Gondwana of both Southern Africa and South America. South African Journal of Science. Vol. 83, No. 6, 1987, ISSN 0038-2353 , pp. 370-372.
- ↑ a b c Sean Patrick Modesto: The Postcranial Skeleton of the Aquatic Parareptile Mesosaurus tenuidens from the Gondwanan Permian. Journal of Vertebrate Paleontology. Vol. 30, No. 5, 2010, pp. 1378-1395, doi : 10.1080 / 02724634.2010.501443 .
Web links
- Cladogram of the mesosaurids and other parareptiles (Mikko's Phylogeny Archive)