Lystrosaurus

Lystrosaurus

Taxobox
name = "Lystrosaurus"



image_width = 250px
image_caption = "Lystrosaurus murrayi" skeleton, Muséum national d'histoire naturelle, Paris
fossil_range = Early Triassic
regnum = Animalia
phylum = Chordata
classis = Synapsida
ordo = Therapsida
subordo = Anomodontia
infraordo = Dicynodontia
familia = Lystrosauridae
genus = "Lystrosaurus"
subdivision_ranks = Species
subdivision =
* "Lystrosaurus curvatus"
* "Lystrosaurus declivus"
* "Lystrosaurus georgi"
* "Lystrosaurus mccaigi"
* "Lystrosaurus murrayi"
* "Lystrosaurus oviceps"
* "Lystrosaurus platyceps"

"Lystrosaurus" (meaning 'shovel lizard', pronunciation in IPAEng|ˌlɪstrɒˈsɔrəs) was a genus of Late Permian and Early Triassic Period dicynodont therapsids, which lived around 250 million years ago in what is now Antarctica, India and South Africa. At present 4 to 6 species are recognized, although from the 1930s to 1970s the number of species was thought to be much higher.

Being a dicynodont, "Lystrosaurus" had only two teeth, a pair of tusk-like canines, and is thought to have had a horny beak that was used for biting off pieces of vegetation. "Lystrosaurus" was a heavily-built, herbivorous animal, approximately the size of a pig. The structure of its shoulders and hip joints suggest that "Lystrosaurus" moved with a semi-sprawling gait. The forelimbs were even more robust than the hindlimbs, and the animal is thought to have been a powerful digger that nested in burrows.

"Lystrosaurus" was by far the most common terrestrial vertebrate of the Early Triassic, accounting for as many as 95% of the total individuals in some fossil beds. It has often been suggested that it had anatomical features that enabled it to adapt better than most animals to the atmospheric conditions that were created by the Permian–Triassic extinction event and which persisted through the Early Triassic — low concentrations of oxygen and high concentrations of carbon dioxide. However recent research suggests that these features were no more pronounced in "Lystrosaurus" than in genera that perished in the extinction or genera that survived but were much less abundant than "Lystrosaurus".

Description

"Lystrosaurus" was a pig-sized dicynodont therapsid, typically about convert|3|ft|m long and weighing about convert|200|lb|kg. [ cite web | title="Lystrosaurus" | url=http://dinosaurs.about.com/od/herbivorousdinosaurs/p/lystrosaurus.htm | accessdate=2008-08-07 ] Unlike other therapsids, dicynodonts had very short snouts and no teeth except for the tusk-like upper canines. It is generally assumed that dicynodonts had horny beaks like those of turtles, for shearing off pieces of vegetation which were then ground on a horny secondary palate when the mouth was closed. The jaw joint was weak and moved backwards and forwards with a shearing action, instead of sideways or up and down. It is thought that the jaw muscles were attached unusually far forward on the skull and took up a lot of space on the top and back of the skull. As a result the eyes were set high and well forward on the skull, and the face was short. [ cite book | title=The History of Life | author=Cowen, R. | edition=3rd | date=2000 | publisher=Blackwell Scientific | pages=167-168 | isbn=0-632-04444-6 ] Features of the skeleton indicate that "Lystrosaurus" moved with a semi-sprawling gait:
*The lower rear corner of the scapula (shoulder blade) was strongly ossified (built of strong bone), which suggests that movement of the scapula contributed to the stride length of the forelimbs and reduced sideways flexing of the body.
*The 5 sacral vertebrae were massive but not fused to each other and to the pelvis, making the back more rigid and reducing sideways flexing while the animal was walking. Therapsids with fewer than 5 sacral vertebrae are thought to have had sprawling limbs, like those of modern lizards. In dinosaurs and mammals, which have erect limbs, the sacral vertebrae are fused to each other and to the pelvis. [cite book |last=Benton |first=Michael J. |authorlink=Michael J. Benton |editor=Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.)|title=The Dinosauria |edition=2nd |year=2004|publisher=University of California Press |location=Berkeley |isbn=0-520-24209-2 |pages=7-19 |chapter=Origin and relationships of Dinosauria]
*A buttress above each acetabulum (hip socket) is thought to have prevented dislocation of the femur (thigh bone) while "Lystrosaurus" was walking with a semi-sprawling gait.

The forelimbs were massive. "Lystrosaurus" is thought to have been a powerful burrower.

Distribution and species

"Lystrosaurus" fossils have been found in many of the late Permian and Early Triassic terrestrial fossil beds, most abundantly in Africa but also including some in India, China and Antarctica (which was not over the South Pole at the time) and a few in Mongolia and the European part of Russia. cite journal | journal=Journal of Vertebrate Paleontology | title="Lystrosaurus georgi", a dicynodont from the Lower Triassic of Russia | author=Surkov, M.V., Kalandadze, N.N., and Benton, M.J. | volume=25 | issue=2 | pages=402–413 | date=June 2005 | url=http://palaeo.gly.bris.ac.uk/benton/reprints/2005lystrosaurus.pdf | accessdate=2008-08-07 | doi= 10.1671/0272-4634(2005)025 [0402:LGADFT] 2.0.CO;2 ]

pecies found in Africa

The majority of "Lystrosaurus" fossils have been found in the Karoo region, which is mostly in South Africa, and the Karoo specimens offer the best prospects of identifying species because they are the most numerous and have been studied for the longest time. As often with fossils there is debate about exactly how many species have been found in the Karoo:
*Studies from the 1930s to 1970s suggested a large number (23 in one case). cite journal | author=Botha, J., and Smith, R.M.H. | date=2007 | title=Lystrosaurus species composition across the Permo–Triassic boundary in the Karoo Basin of South Africa | journal=Lethaia | volume=40 | pages=125-137 | url=http://www3.interscience.wiley.com/journal/117996985/abstract?CRETRY=1&SRETRY=0 | accessdate=2008-07-02 | doi=10.1111/j.1502-3931.2007.00011.x Full version online at cite web | url=http://www.nasmus.co.za/PALAEO/jbotha/pdfs/Botha%20and%20Smith%202007.pdf | title=Lystrosaurus species composition across the Permo–Triassic boundary in the Karoo Basin of South Africa | accessdate=2008-07-02 ]
*In the 1980s and 1990s 6 species were recognized: "Lystrosaurus curvatus", "Lystrosaurus platyceps", "Lystrosaurus oviceps", "Lystrosaurus maccaigi", "Lystrosaurus murrayi" and "Lystrosaurus declivis".
*A study in 2006 reduced that to 4, treating the fossils previously labelled as "Lystrosaurus platyceps" and "Lystrosaurus oviceps" as members of "Lystrosaurus curvatus". [ citation | author=Grine, F.E., Forster, C.A., Cluver, M.A. & Georgi, J.A. | date=2006 | contribution=Cranial variability, ontogeny and taxonomy of "Lystrosaurus" from the Karoo Basin of South Africa | title=Amniote paleobiology. Perspectives on the Evolution of Mammals, Birds, and Reptiles,
pages= 432–503 | punlisher=University of Chicago Press
]

"L. maccaigi" was the largest and apparently most specialized species, while "L. curvatus" was the least specialized. A "Lystrosaurus"-like fossil, "Kwazulusaurus shakai", has also been found in South Africa. Although not assigned to the same genus, "K. shakai" is very similar to "L. curvatus". Hence it is possible that "K. shakai" was an ancestor of or closely related to the ancestors of "L. curvatus", while "L. maccaigi" arose from a different lineage.

"L. maccaigi" is found only in sediments from the Permian period, and apparently did not survive the Permian–Triassic extinction event. Its specialized features and sudden appearance in the fossilrecord without an obvious ancestor may indicate that it immigrated into the Karoo from an area in which Late Permian sediments have not been found.

"L. curvatus" is found in a relatively narrow band of sediments from shortly before and after the extinction, and can be used as an approximate marker for the boundary between the Permian and Triassic periods. A skull identified as "L. curvatus" has been found in late Permian sediments from Zambia. For many years it had been thought that there were no Permian specimens of "L. curvatus" in the Karoo, which led to suggestions that "L. curvatus" immigrated from Zambia into the Karoo. However re-examination of Permian specimens in the Karoo has identified some as "L. curvatus", and there is no need to assume immigration.

"Lystrosaurus murrayi" and "Lystrosaurus declivis" are found only in Permian sediments.

Other species

"Lystrosaurus georgi" fossils have been found in the Earliest Triassic sediments of the Moscow Basin in Russia. It was probably closely related to the African "Lystrosaurus curvatus", which is regarded as one of the least specialized species and has been found in very Late Permian and very Early Triassic sediments.

Dominance of the Early Triassic

"Lystrosaurus" is notable for dominating southern Pangea during the Early Triassic for millions of years. At least one unidentified species of this genus survived the end-Permian mass extinction and, in the absence of predators and of herbivorous competitors, went on to thrive and re-radiate into a number of species within the genus, cite book | author=Michael J. Benton |title=When Life Nearly Died. The Greatest Mass Extinction of All Time |date=2006 | isbn=050028573X ] becoming the most common group of terrestrial vertebrates during the Early Triassic; for a while 95% of land vertebrates were "Lystrosaurus". [http://www.orionmagazine.org/index.php/articles/article/268/ The Consolations of Extinction: includes section on Lystrosaurus and end-Permian extinction] ] This is the only time that a single species or genus of land animal dominated the Earth to such a degree. [BBC: Life Before Dinosaurs] A few other Permian therapsid genera also survived the mass extinction and appear in Triassic rocks — the therocephalians "Tetracynodon", "Moschorhinus" and "Ictidosuchoides" — but do not appear to have been abundant in the Triassic.

Several attempts have been made to explain why "Lystrosaurus" survived the Permian–Triassic extinction event, the "mother of all mass extinctions",cite book|author=Erwin DH|date=1993|title=The great Paleozoic crisis; Life and death in the Permian|publisher=Columbia University Press|isbn=0231074670] and why it dominated Early Triassic fauna to such an unprecedented extent:

* One of the more recent theories is that the Permian–Triassic extinction event reduced the atmosphere's oxygen content and increased its carbon dioxide content, so that many terrestrial species died out because they found breathing too difficult. It has therefore been suggested that "Lystrosaurus" survived and became dominant because its burrowing life-style made it able to cope with an atmosphere of "stale air", and that specific features of its anatomy were part of this adaptation: a barrel chest that accommodated large lungs, short internal nostrils that facilitated rapid breathing, and high neural spines (projections on the dorsal side of the vertebrae) that gave greater leverage to the muscles that expanded and contracted its chest. Unfortunately there are weaknesses in all these points: The chest of "Lystrosaurus" was not significantly larger in proportion to its size than in other dicynodonts that became extinct; although Triassic dicynodonts appear to have had longer neural spines than their Permian counter-parts, this feature may be related to posture, locomotion or even body size rather than respiratory efficiency; "L. murrayi" and "L. declivis" are much more abundant than other Early Triassic burrowers such as "Procolophon" or "Thrinaxodon".
* The suggestion that "Lystrosaurus" was helped to survive and dominate by being semi-aquatic has a similar weakness: although amphibians become more abundant in the Karoo's Triassic sediments, they are not nearly as numerous as "L. murrayi" and "L. declivis".

* The most specialized and the largest animals are at higher risk in mass extinctions; this may explain why the unspecialized "L. curvatus" survived while the larger and more specialized "L. maccaigi" perished along with all the other large Permian herbivores and carnivores. Although "Lystrosaurus" generally looks adapted to feed on plants similar to "Dicroidium", which dominated the Early Triassic, the larger size of "L. maccaigi" may have forced it to rely on the larger members of the "Glossopteris" flora, which did not survive the end-Permian extinction.

* Perhaps the survival of "Lystrosaurus" was simply a matter of luck.

* Only the convert|1.5|m|ft-long therocephalian "Moschorhinus" and the large archosauriform "Proterosuchus" appear large enough to have preyed on the Triassic "Lystrosaurus" species, and this shortage of predators may have been responsible for a "Lystrosaurus" population boom in the Early Triassic.

Plate Tectonics

The discovery of "Lystrosaurus" fossils at Coalsack Bluff in the Transantarctic Mountains by Edwin H. Colbert and his team in 1969-70 helped confirm the theory of plate tectonics and convince the last of the doubters, since "Lystrosaurus" had already been found in the lower Triassic of southern Africa as well as in India and China. [Naomi Lubick, " [http://www.agiweb.org/geotimes/feb05/feature_Antarcticfossils.html Investigating the Antarctic] ", Geotimes, 2005.]

ee also

* Evolution of mammals

Notes

External links

* [http://www.palaeos.com/Vertebrates/Units/400Therapsida/400.725.html#Dicynodontia Palaeos.com: Dicynodontia]
* [http://geowords.com/histbooknetscape/i07.htm Hugh Rance, "The Present is the Key to the Past": "Mammal-like reptiles of Pangea"]
* [http://www.gondwanastudios.com/info/lys.htm Lystrosaurus, Gondwana Studios]


Wikimedia Foundation. 2010.

Игры ⚽ Нужно решить контрольную?

Look at other dictionaries:

  • Lystrosaurus — Lebendrekonstruktion von Lystrosaurus georgi Zeitraum Changhsingium (Oberperm ) bis Olenekium (Untertrias) 253,8 bis 245,9 Mio. Jahre …   Deutsch Wikipedia

  • Lystrosaurus — Lystrosaurus …   Wikipédia en Français

  • Lystrosaurus — Lystrosaurus,   zu den säugetierähnlichen Therapsida gehörende Reptiliengattung aus der untersten Trias, etwa 1,20 m lang; plumpe Pflanzenfresser, mit großen Eckzähnen; weltweit, v. a. in Gondwana, in Marschen und Deltaablagerungen verbreitet …   Universal-Lexikon

  • Lystrosaurus — Lystrosaurus …   Wikipédia en Français

  • Lystrosaurus —   Lystrosaurus Rango temporal: Pérmico Superior Triásico Inferior (hace 250 a 248 ma) …   Wikipedia Español

  • Lystrosaurus Assemblage Zone — The Lystrosaurus Assemblage Zone is a geological stratum and a faunal zone of the Beaufort Group, of the South African Karoo. The name refers to Lystrosaurus, a genus of mammal like reptile, a dominant life form of the Early Triassic Period,… …   Wikipedia

  • Lystrosaurus — …   Википедия

  • Листрозавр — ? † Листрозавры Lystrosaurus murray Научная классификация …   Википедия

  • Листрозавры — ? † Листрозавры Lystrosaurus murray Научная классификация …   Википедия

  • Walking with Monsters — Título Walking with Monsters Género Documental Narrado por Kenneth Branagh País de origen …   Wikipedia Español

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”