The world's greatest fossil graveyard
South Africa's Karoo Supergroup is a vast geological formation covering almost two-thirds of the country. It represents a nearly continuous 120-million-year record of Earth's history, from the late Carboniferous (around 300 million years ago) to the Early Jurassic (around 180 million years ago). The layers of shale and sandstone, which reach a cumulative thickness of up to 12 kilometers in some places, were deposited in a vast inland sea and the successive terrestrial environments that replaced it. This sequence of rock holds one of the most complete records of terrestrial life during the Permian and Triassic periods, making it the world's most significant collecting ground for fossils of our distant ancestors: the therapsids.
Therapsids are a group of synapsids, often called "mammal-like reptiles," that represent the evolutionary transition between reptiles and true mammals. They flourished during the Permian Period, diversifying into a wide array of forms that filled ecological niches much like modern mammals. Some were large, tusked herbivores like the Dicynodonts, which had tortoise-like beaks. Others were terrifying predators. The Dinocephalians ("terrible heads") were among the first large land animals, reaching 3 meters in length with extraordinarily thick skulls. The Gorgonopsians were tiger-sized apex predators, characterized by enormous saber-like canine teeth. The most mammal-like of all were the Cynodonts ("dog teeth"), the group that gave rise to all mammals around 225 million years ago.
Surviving the Great Dying
The Karoo fossil record records the most severe extinction event in Earth's history: the Permian-Triassic extinction, or "The Great Dying," around 252 million years ago. This event wiped out an estimated 70% of terrestrial vertebrate species. In the Karoo, the rock layers show a dramatic shift. The diverse therapsid communities of the late Permian, filled with Gorgonopsians and large Dicynodonts, disappear. Above this boundary, in the early Triassic layers, the fauna is dominated by a single, hardy, pig-sized herbivore named Lystrosaurus. In some areas of the Karoo's Lystrosaurus Assemblage Zone, its fossils account for 95% of all vertebrates found. The survival and subsequent dominance of Lystrosaurus is a example of a "disaster taxon," an organism that thrives in the aftermath of a mass extinction. The ancestors of mammals, the smaller cynodonts like Thrinaxodon, also survived the extinction. Fossil evidence of cynodonts nesting in burrows, sometimes with their young, suggests complex parental care behaviors that may have helped them endure the harsh post-extinction environment.