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Genus of Late Permian and Early Triassic dicynodont therapsids

Lystrosaurus (,/l//ɪ//s//t//r//oʊ /'/s//ɔːr//ə//s/; 'shovel lizard'; proper Greek is λίστρον 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 250 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. 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. 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, accounting for as many as 95% of the total individuals in some fossil beds. Researchers have offered various hypotheses for why Lystrosaurus survived the extinction event and thrived in the early Triassic.

Dr. Elias Root Beadle, a Philadelphia missionary and avid fossil collector, discovered the first Lystrosaurus skull. Beadle wrote to the eminent paleontologist Othniel Charles Marsh, but received no reply. Marsh's rival, Edward Drinker Cope, was very interested in seeing the find, and described and named 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 belatedly purchased the skull in May 1871, although his interest in an already described specimen was unclear; he may have wanted to carefully scrutinize Cope's description and illustration.

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).

Most Lystrosaurus fossils have been found in the Balfour and Katberg Formations of the Karoo basin in South Africa; these specimens offer the best prospects of identifying species because they are the most numerous and have been studied for the longest time. As so often with fossils, there is debate in the paleontological community as to exactly how many species have been found in the Karoo basin. However, by the 1980s and 1990s, only six 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, 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. 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.

Lystrosaurus was a dicynodont therapsid, between long with an average of about depending upon the species. 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 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.