Lystrosaurus (; 'shovel lizard'; from Ancient Greek lístron 'tool for leveling or smoothing, shovel, spade, hoe') is an extinct genus of herbivorous dicynodont therapsids from the late Permian and Early Triassic epochs (around 248 million years ago). It had a widespread distribution across Pangaea, with fossils having been found in what is now Antarctica, India, China, Mongolia, European Russia, South Africa, and possibly Australia and Mozambique. Four to six species are currently recognized, but from the 1930s to 1970s the number of species was thought to be much higher.
As 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. It was a heavily built, herbivorous animal. The structure of its shoulders and hip joints suggests that it moved with a semi-sprawling gait. Its forelimbs were even more robust than its hindlimbs, and it is thought to have been a powerful digger that nested in burrows.
Lystrosaurus survived the Permian-Triassic extinction event (Earth's most severe) 252 million years ago and thrived in the immediate aftermath; its fossils are, by far, the most common terrestrial vertebrate fossils in Early Triassic beds around the world, such as in the Lystrosaurus Assemblage Zone in South Africa. Researchers have offered various hypotheses for why it survived the extinction event and thrived in the early Triassic.
History of discovery
thumb|350px|Map of [[Pangea showing locations of Lystrosaurus remains as yellow disks. Distorted boundaries of modern continents shown as grey lines. (Distributions for lystrosaurs and three other Permian and Triassic fossil groups used as biogeographic evidence for continental drift and certain land bridges.)]]
Dr. Elias Root Beadle, a Philadelphia missionary and avid fossil collector, discovered the first Lystrosaurus skull. He wrote about it to the eminent paleontologist Othniel Charles Marsh, but received no reply. Marsh's rival, Edward Drinker Cope, described and named it Lystrosaurus in the Proceedings of the American Philosophical Society in 1870. Its name is derived from the Ancient Greek words listron 'shovel' and sauros 'lizard'. Marsh purchased the skull in May 1871, although his interest in an already-described specimen was unclear; he may have wanted to scrutinize Cope's description and illustration.
Distribution and species
Lystrosaurus fossils have been found in many Late Permian and Early Triassic terrestrial bone beds, most abundantly in Africa, and to a lesser extent in parts of what are now India, China, Mongolia, European Russia, and Antarctica (which was not over the South Pole at the time). Possible remains have also been reported from Australia and Mozambique. However, by the 1980s and 1990s, only 6 species were recognized in the Karoo: L. curvatus, L. platyceps, L. oviceps, L. maccaigi, L. murrayi, and L. declivis. A study in 2011 reduced that number to four, treating the fossils previously labeled as L. platyceps and L. oviceps as members of L. curvatus.
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, it is very similar to L. curvatus. Some paleontologists have therefore proposed that it was possibly an ancestor of, or closely related to, the ancestors of L. curvatus, while L. maccaigi arose from a different lineage. Seven Lystrosaurus species have been described from the Early Triassic Jiucaiyuan, Guodikeng and Wutonggou formations of the Bogda Mountains 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.
Description
thumb|Size of Lystrosaurus murrayi compared to a human. Note that some other species, such as L. maccaigi, are known to have reached somewhat larger maximum sizes.
Lystrosaurus reached a total length of around . The largest known individuals of the genus, belonging to the species L. maccaigi, reached a total skull length of around .
Unlike other therapsids, dicynodonts had very short snouts and no teeth except for the tusk-like upper canines. Dicynodonts are generally thought to have had keratinous 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 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. 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.
Paleobiology
Reproduction and growth
[[File:Lystrosaurus_embryo.png|thumb|Preserved embryo of Lystrosaurus from the Early Triassic of South Africa, with life restoration by Sophie Vrard showing embryo curled up in inferred (but unpreserved) egg. a, photograph of the specimen; b, 3D digital reconstruction of the segmented bones Colour code for b: vertebral elements in shades of green, ribs in blue, forelimb elements in red, femur in yellow, pelvic girdle elements in grey, skull in light red, mandible in light orange.
An aggregation of nine young juveniles together suggests the clutch of Lystrosaurus may have been relatively small-medium sized, compared to clutch size of 38 known from the Triassic cynodont genus Kayentatherium. which were perhaps elastic and slowed in the face of adverse conditions. At least Early Triassic Lystrosaurus likely had an early onset of sexual maturity before they had reached maximum body size (somatic maturity), Specimens of Lystrosaurus individuals have been found preserved in burrows, suggesting that they were habitually fossorial, and were capable of constructing burrows relatively early in life. It has been suggested that individuals of Lystrosaurus from the Early Triassic of Antarctica went into a state of torpor or hibernation based on stress growth marks preserved in their tusks (which are absent from individuals from the more northern South Africa) to cope with the polar seasonality and limited light levels for significant parts of the year. 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 therocephalians 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 and Middle Triassic.
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:
thumb|right|Lystrosaurus murrayi
- Lystrosaurus has been suggested as a highly adaptable generalist with a flexible physiology able to tolerate a wide range of environmental conditions.
- One of the more recent theories is that the 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.