The great dying
Around 252 million years ago, life on Earth faced its most severe challenge: the Permian-Triassic extinction event. Massive volcanic eruptions in the Siberian Traps released up to 43,000 gigatons of carbon into the atmosphere, triggering catastrophic climate change. This event, often called "The Great Dying," eliminated an estimated 96% of all marine species and 70% of terrestrial vertebrate species. The planet's ecosystems collapsed, leaving a largely empty world for the survivors. In the aftermath of this global catastrophe, one unlikely creature rose to dominance. Its name was Lystrosaurus.
The fossil beds of South Africa's Karoo region provide one of the world's clearest records of this extinction and the strange world that followed. The boundary between the Permian and Triassic periods is starkly visible in the rock layers. Below it, a diverse community of animals exists in the Dicynodon Assemblage Zone. Above it, the area changes, and the fossil record becomes eerily uniform. This new geological layer is named the Lystrosaurus Assemblage Zone, for good reason.
A world of Lystrosaurus
Lystrosaurus was a therapsid, a group of reptiles with some mammal-like characteristics, but it was not a direct ancestor of mammals. It was a stout, pig-sized herbivore, typically about one meter long, with a turtle-like beak and a pair of canine tusks used for digging up roots and vegetation. It had a barrel chest, powerful forelimbs, and a semi-sprawling gait.
Before the extinction, Lystrosaurus was a relatively uncommon animal. After the extinction, it became the most common terrestrial vertebrate on the planet. In the Early Triassic rocks of the Karoo Basin, Lystrosaurus fossils can constitute up to 95% of all vertebrate remains found. This is a level of dominance by a single genus unparalleled in Earth's history.
Scientists have proposed several reasons for its remarkable success. Its powerful front legs and skull shape suggest it was a proficient burrower, an ability that could have helped it survive the harsh surface conditions during the extinction event. Its large lungs, housed in a barrel chest, may have helped it cope with the low-oxygen atmosphere that followed the volcanic eruptions. As a generalist herbivore, it could feed on the hardy, low-lying plants like ferns that were among the first to recover. Post-extinction, its lifespan shortened dramatically from about 13 years to just 2-3 years, a change that was accompanied by breeding at a much younger age, allowing it to rapidly populate the vacant area.