page_title: Lystrosaurus
section_title: Dominance of the Early Triassic
hierarchical_section_title: Lystrosaurus / Paleoecology / Dominance of the Early Triassic
caption_attribution_description: Lystrosaurus murrayi, a dicynodont from the Early Triassic of South Africa, pencil drawing, digital coloring
context_page_description: Lystrosaurus was a herbivorous genus of dicynodont therapsids from the late Permian and Early Triassic epochs. 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 2.5 meters long.
  Being a dicynodont, Lystrosaurus had only two teeth, 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 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.
context_section_description: 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 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 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:
  Growth marks in fossilized tusks suggest that Lystrosaurus could enter a prolonged state of torpor analogous to hibernation.
  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. 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. However, 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 counterparts, 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 temnospondyls become more abundant in the Karoo's Triassic sediments, they were much less numerous than 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.
  Only the 1.5 m (5 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.
  According to Benton, "Perhaps the survival of Lystrosaurus was simply a matter of luck".
caption: Lystrosaurus murrayi
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