Lystrosaurus ( / ˌ l ɪ s t r oʊ ˈ s ɔːr ə s / ; "shovel lizard"; from Ancient Greek λίστρον lístron, meaning "tool for leveling or smoothing, shovel, spade, hoe") is an extinct group of plant-eating dicynodont therapsids that lived during the late Permian and Early Triassic periods (about 248 million years ago). It was found in many places across Pangaea, with fossils discovered in areas that are now Antarctica, India, China, Mongolia, European Russia, South Africa, and possibly Australia and Mozambique. Scientists currently recognize four to six species of Lystrosaurus, though from the 1930s to 1970s, more species were believed to exist.

As a dicynodont, Lystrosaurus had only two teeth (a pair of tusk-like canines) and likely used a horny beak to bite pieces of plants. It was a large, plant-eating animal. The structure of its shoulders and hip joints suggests it moved with a semi-sprawling gait, spreading its legs out to the sides when walking. Its front legs were stronger than its back legs, and it is thought to have been a strong digger that lived in burrows.

Lystrosaurus survived the Permian-Triassic extinction event (Earth's most severe extinction) 252 million years ago and became very common in the time that followed. Its fossils are the most frequently found remains of land-dwelling vertebrates in Early Triassic rock layers worldwide, such as in the Lystrosaurus Assemblage Zone in South Africa. Scientists have proposed several possible reasons for its survival and success during the Early Triassic period.

History of discovery

Dr. Elias Root Beadle, a missionary from Philadelphia and a person who collected fossils, found the first Lystrosaurus skull. He sent a letter about it to Othniel Charles Marsh, a well-known paleontologist, but Marsh did not reply. Marsh’s competitor, Edward Drinker Cope, described and named the skull Lystrosaurus in the Proceedings of the American Philosophical Society in 1870. The name comes from the Ancient Greek words listron, meaning "shovel," and sauros, meaning "lizard." Marsh bought the skull in May 1871, but it was unclear why he was interested in a fossil that had already been described. He may have wanted to check Cope’s description and drawing.

In 1969–1970, Edwin H. Colbert and his team discovered Lystrosaurus fossils at Coalsack Bluff in the Transantarctic Mountains. This helped support the idea of plate tectonics, a theory that explains how Earth’s continents move. Scientists already knew Lystrosaurus fossils existed in southern Africa, India, and China.

Lystrosaurus fossils have been found in many Late Permian and Early Triassic land areas, most often in Africa. They have also been found in parts of what are now India, China, Mongolia, European Russia, and Antarctica (which was not near the South Pole at that time). Possible remains have also been reported in Australia and Mozambique.

Most Lystrosaurus fossils are found in the Balfour and Katberg Formations of the Karoo basin in South Africa. These fossils are the most studied and help scientists identify species. However, scientists often debate how many species exist in the Karoo. Studies from the 1930s to 1970s suggested many species (as many as 23). By the 1980s and 1990s, only six species were recognized: L. curvatus, L. platyceps, L. oviceps, L. maccaigi, L. murrayi, and L. declivis. A study in 2011 reduced this number to four, grouping L. platyceps and L. oviceps with L. curvatus.

L. maccaigi is the largest and most specialized species, while L. curvatus is the least specialized. A fossil similar to Lystrosaurus, called Kwazulusaurus shakai, was found in South Africa. Though not in the same group, it looks very similar to L. curvatus. Some scientists think Kwazulusaurus shakai may be related to the ancestors of L. curvatus, while L. maccaigi came from a different line. L. maccaigi fossils are only found in Late Permian sediments and did not survive the Permian–Triassic extinction event. Its sudden appearance in the fossil record and specialized features suggest it may have moved into the Karoo from a place where Late Permian sediments are not found. L. curvatus is found in sediments just before and after the extinction and is used to mark the boundary between the Permian and Triassic periods. A skull identified as L. curvatus was found in late Permian sediments in Zambia. For many years, scientists thought no Permian L. curvatus fossils existed in the Karoo, leading to ideas that it moved from Zambia to the Karoo. However, a new study found L. curvatus in Permian sediments in the Karoo, so scientists no longer need to assume it moved.

L. murrayi and L. declivis are found only in Triassic sediments.

Fossils of Lystrosaurus georgi have been found in the Earliest Triassic sediments of the Moscow Basin in Russia. It was likely closely related to L. curvatus, a species found in very Late Permian and very Early Triassic sediments in Africa.

L. murrayi, along with two species not yet named (currently grouped with L. curvatus and L. declivis), has been found in the Early Triassic Panchet Formation of the Damodar Valley 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 Mountains in Xinjiang, China. However, only two (L. youngi and L. hedini) may be correct. No Lystrosaurus fossils from below the Permian-Triassic boundary have been found in this region. L. curvatus, L. murrayi, and L. maccaigi have been found in the Fremouw Formation of the Transantarctic Mountains in Antarctica.

Description

Lystrosaurus was about 1 metre (3.3 feet) long. The largest known Lystrosaurus, from the species L. maccaigi, had a skull about 27.8 centimetres (10.9 inches) long. Fossils found in the Northern Hemisphere are often larger than those in the Southern Hemisphere, but this may be because more fossils have been studied in the North. Evidence suggests that individuals from both hemispheres could grow to the same maximum size.

Dicynodonts, a type of therapsid, had short snouts and only had teeth in their upper canines, which looked like tusks. Scientists believe they used a tough, beak-like structure made of keratin (like turtle beaks) to cut plants. When their mouths closed, the plants were ground on a bony, horn-like surface inside the mouth. Their jaw joint moved forward and backward in a shearing motion, unlike the side-to-side or up-and-down movements seen in many animals. Their jaw muscles were attached far forward on the skull, taking up space on the top and back of the skull. This caused their eyes to be positioned high and forward on the skull, and their faces to be short.

The bones of Lystrosaurus show it walked with a semi-sprawling gait, where the legs were partly spread out. The lower part of the shoulder blade was made of strong bone, which helped the front legs move in a longer stride and reduced side-to-side body movement. Five large sacral vertebrae (bones in the lower back) were not fused to each other or the pelvis, making the back more rigid and limiting side-to-side movement. Other therapsids with fewer than five sacral vertebrae likely had sprawling limbs, like lizards. Dinosaurs and mammals, which have upright legs, have fused sacral vertebrae connected to the pelvis. A bony ridge above each hip socket likely helped keep the thigh bone in place during movement. The front legs of Lystrosaurus were very strong.

Fossils found in the Karoo Basin in 2022 showed that Lystrosaurus had smooth, leathery skin with small dimples and no hair.

Paleobiology

A fossil of a curled-up embryo from the Early Triassic of South Africa, likely from the species Lystrosaurus murrayi, shows that dicynodonts may have laid eggs, as is believed for synapsids and amniotes. However, no eggshells were found, suggesting the eggs were soft and leathery, similar to those of modern monotreme mammals, which are thought to be the earliest type for amniotes. The large size of the eggs compared to the adult's body suggests that Lystrosaurus young were able to care for themselves soon after birth, as they had enough yolk in the egg to grow without needing milk. This implies Lystrosaurus and other dicynodonts likely did not produce milk. The well-developed jaws of young Lystrosaurus individuals indicate they could eat tough plant materials immediately after hatching.

Nine juvenile Lystrosaurus fossils found together suggest their nests may have held a small to medium number of eggs, compared to the 38 eggs found in the Triassic genus Kayentatherium. Studies suggest Lystrosaurus grew quickly, but this growth may have slowed during harsh conditions. Early Triassic Lystrosaurus likely reached sexual maturity before reaching full adult size, as many known fossils of large individuals show they were still growing when they died. Juvenile Lystrosaurus may have faced high death rates.

Analysis of Lystrosaurus skulls shows their deep, short skulls and movable skull joints allowed them to deliver quick, snapping bites to break tough plants, possibly including horsetail relatives like Phyllotheca. Fossils found in burrows suggest Lystrosaurus were likely burrowing animals, able to dig tunnels early in life. Some Lystrosaurus fossils from Antarctica show growth marks in their tusks, which may indicate they entered a state of reduced activity, like hibernation, to survive long, dark winters.

Some scientists, starting with Robert Broom in 1902, proposed Lystrosaurus lived partly in water, but this idea is debated. Fossils show Lystrosaurus lived in dry environments, and their anatomy lacks features typical of aquatic animals. While some argue the structure of their limb bones, which contain certain internal features found in aquatic mammals, supports an aquatic lifestyle, others say these features likely helped strengthen bones for digging, not swimming. Most scientists now believe Lystrosaurus was fully land-dwelling.

Paleoecology

Lystrosaurus was a very common animal in southern Pangaea during the Early Triassic period, which was millions of years ago. At least one species of this group survived the end-Permian mass extinction, a time when many animals died out. Without many predators or plant-eating competitors, Lystrosaurus grew in numbers and spread into many different species. During the Early Triassic, Lystrosaurus was the most common type of land animal. In some areas of South Africa, about 95% of land vertebrates were Lystrosaurus. This was the only time in history when one species or group of land animals was so widespread. Other animals from the Permian period, such as Tetracynodon, Moschorhinus, and Promoschorhynchus, also survived the mass extinction and appeared in Triassic rocks, but they were not as common as Lystrosaurus. It took about 30 million years for ecosystems to fully recover after the extinction event, which lasted through the Early and Middle Triassic periods.

Scientists have suggested several reasons for why Lystrosaurus survived the Permian–Triassic extinction event, which was the largest mass extinction in Earth’s history, and why it became so dominant in the Early Triassic:

• Lystrosaurus may have been a generalist, meaning it could live in many different environments and adapt to changes.
• Some scientists think that Lystrosaurus living near the South Pole, such as in Antarctica, could have survived the extinction by entering a state of reduced activity, like hibernation. Antarctica may have been a safe place for animals during this time despite its harsh climate.
• One theory suggests that the extinction event lowered oxygen levels and raised carbon dioxide levels in the atmosphere, making it hard for many animals to breathe. Lystrosaurus may have survived because it lived underground, where air conditions might have been better. Features like a large chest for big lungs, short nostrils for faster breathing, and tall vertebrae that helped move its chest muscles may have helped it adapt. However, some scientists argue that these features were not unique to Lystrosaurus and that other similar animals did not survive.
• Another idea is that Lystrosaurus was partly aquatic, but evidence shows that other animals, like temnospondyls, were more common in Triassic rocks than Lystrosaurus.
• Larger and more specialized animals are often more likely to die out during mass extinctions. This may explain why L. curvatus, a less specialized species, survived, while L. maccaigi, a larger and more specialized species, did not. L. maccaigi may have relied on plants that did not survive the extinction.
• Few animals in the Early Triassic were large enough to hunt Lystrosaurus, which may have allowed its population to grow quickly.
• Some scientists, like Benton, suggest that Lystrosaurus survived simply by chance.