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Lystrosaurus (; 'shovel lizard'; proper Ancient Greek is lístron 'tool for leveling or smoothing, shovel, spade, hoe') is an extinct genus of herbivore dicynodont from the Lopingian and Early Triassic epochs (around 248 million years ago). It lived in what is now Antarctica, India, China, Mongolia, European Russia and South Africa. Four to six species are currently recognized, although from the 1930s to 1970s the number of species was thought to be much higher. They ranged in size from that of a small dog to 8 feet (2.5 meters) long. (1978). 9780908407583, The South African Museum. ISBN 9780908407583
As a dicynodont, Lystrosaurus had only two teeth (a pair of tusk-like Canine tooth), 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. The structure of its shoulders and hip joints suggests 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 survived the Permian-Triassic extinction, 252 million years ago. In the Early Triassic, they were by far the most common terrestrial vertebrates, present in fossil beds around the world and accounting for as many as 95% of the total tetrapods in the Lystrosaurus Assemblage Zone in South Africa. Researchers have offered various hypotheses for why Lystrosaurus survived the extinction event and thrived in the early Triassic.
L. maccaigi is 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. Some paleontologists have therefore proposed that K. shakai was possibly 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 fossil record 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, a re-examination of Permian specimens in the Karoo has identified some as L. curvatus, and there is no need to assume immigration.
L. murrayi and L. declivis are found only in Triassic sediments.
L. murrayi, in addition to two undescribed species presently assigned to L. curvatus and L. declivis, is known from the Early Triassic Panchet Formation of the Damodar River and the Kamthi Formation of the Pranhita-Godavari Basin in India. Seven Lystrosaurus species have been described from the Early Triassic Jiucaiyuan, Guodikeng and Wutonggou formations of the Bogda Shan in Xinjiang, China, although it is possible that only two ( L. youngi and L. hedini) are valid; unusually, no Chinese Lystrosaurus specimens are known below the Permian-Triassic boundary in this region. L. curvatus, L. murrayi, and L. maccaigi are known from the Fremouw Formation in the Transantarctic Mountains of Antarctica.
Unlike other therapsids, dicynodonts had very short snouts and no teeth except for the tusk-like upper canine teeth. Dicynodonts are generally thought to have had horny like those of turtles, for shearing off pieces of vegetation, which were then ground on a horny secondary palate when the mouth was shut. The jaw joint was weak and moved backwards and forwards with a shearing action, instead of the more common sideways or up and down movements. 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.
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 the sideways flexing of the body. The five 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. Therapsida with fewer than five sacral vertebrae are thought to have had sprawling limbs, like those of modern lizards. In and , which have erect limbs, the sacral vertebrae are fused to each other and to the pelvis. 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 of Lystrosaurus were massive, and Lystrosaurus is thought to have been a powerful burrower.
Mummified specimens recovered from the Karoo Basin and described in 2022 revealed that Lystrosaurus had dimpled, leathery, and hairless skin.
Several attempts have been made to explain why Lystrosaurus survived the Permian–Triassic extinction event, the "mother of all mass extinctions", and why it dominated Early Triassic fauna to such an unprecedented extent:
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.
Description
Lystrosaurus was a dicynodont therapsid, between long with an average of about depending upon the species.
Paleoecology
Dominance of the Early Triassic
Lystrosaurus is notable for dominating southern Pangaea for millions of years during the Early Triassic. At least one unidentified species of this genus survived the end-Permian mass extinction and, in the absence of predators and herbivorous competitors, went on to thrive and re-radiate into a number of species within the genus, becoming the most common group of terrestrial vertebrates during the Early Triassic; for a while, 95% of land vertebrates in the Lystrosaurus Assemblage Zone in South Africa were Lystrosaurus. This is the only time that a single species or genus of land animal dominated the Earth to such a degree. A few other Permian therapsid genera also survived the mass extinction and appear in Triassic rocks—the Tetracynodon, Moschorhinus, and Promoschorhynchus (the latter based on specimens originally attributed to Ictidosuchoides)—but do not appear to have been abundant in the Triassic; complete ecological recovery took 30 million years, spanning the Early Triassic and Middle Triassic.
See also
External links
Categories: Dicynodontia, Triassic Synapsids, Permian–triassic Extinction Event, Prehistoric Synapsids Of Asia, Lopingian Synapsids Of Africa, Early Triassic Synapsids, Prehistoric Vertebrates Of Antarctica, Triassic Antarctica, Taxa Named By Edward Drinker Cope, Fossil Taxa Described In 1870, Lopingian Genus First Appearances, Changhsingian Genera, Induan Genera, Early Triassic Genus Extinctions, Anomodont Genera
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