A 120-million-year graveyard
The Karoo Supergroup is a vast expanse of sedimentary rock covering nearly two-thirds of South Africa. This semi-desert region holds the world's most abundant fossil beds of therapsids, the creatures that mark the evolutionary transition from reptiles to mammals. The rock layers provide an almost continuous 120-million-year record of terrestrial life, from the Late Carboniferous (about 300 million years ago) to the Early Jurassic (180 million years ago). The sheer density of fossils is astonishing; thousands have been discovered since the 1830s, with many more eroding out of the hillsides.
The primary fossil-bearing layer is the Beaufort Group, a sequence of mudstones and sandstones deposited by ancient river systems on the supercontinent of Gondwana. These rocks record two of the most significant extinction events in Earth's history. One, a mid-Permian extinction around 260 million years ago, wiped out a group of large, dominant therapsids called dinocephalians. The second, and more catastrophic, was the end-Permian extinction, or "The Great Dying," around 252 million years ago. This event eliminated about 70% of terrestrial vertebrate species and reshaped life on the planet. The Karoo's rocks document this turnover with unmatched clarity.
The creatures of the Karoo
The therapsids found here represent a diverse fauna. Among the most fearsome were the gorgonopsians, saber-toothed predators that were the apex carnivores of the late Permian. These creatures, which could reach the size of a lion, had long skulls and canine teeth up to 4 inches long. For more than 150 years of fossil hunting, complete gorgonopsian skeletons remained elusive until a major discovery in 1998.
The most common herbivore was Lystrosaurus, a pig-sized animal with a beaked snout and two tusks. After the end-Permian extinction, Lystrosaurus became extraordinarily dominant, with its fossils accounting for up to 95% of the terrestrial vertebrates in some early Triassic layers. Mummified specimens found in the Karoo reveal the animal had leathery, hairless skin. Other common therapsids include the small, burrowing Diictodon. Thousands of their skeletons have been found, many curled up inside their fossilized burrows.
The first therapsid fossils were brought to scientific attention in the 1830s by Andrew Geddes Bain, a road engineer and geologist. His work was followed by legendary paleontologists like Robert Broom and James Kitching, who uncovered thousands of specimens. The fossils provided important early evidence for the theory of continental drift, as identical species like Lystrosaurus were found in South Africa, India, and Antarctica.