Therapsida

Therapsids are a major group of animals that include all living mammals and their ancestors. They belong to a larger group called synapsids. These creatures were once the most successful large animals on land. They first appeared more than 279.5 million years ago. They evolved from earlier synapsids known as pelycosaurs, specifically from a group called the Sphenacodontia.

By studying these fossils, scientists can trace the long history of how mammals eventually appeared.

One of the most important changes in therapsids was their physical structure. Many reptiles and pelycosaurs have a sprawling posture with legs sticking out to the sides. Therapsids developed limbs that were oriented more vertically underneath their bodies. This created a more "standing" quadrupedal posture. Their feet also became more symmetrical. The first and last toes were short, while the middle toes were long. This change allowed for a more mammal-like gait rather than a lizard-like crawl.

Their feeding tools also became much more advanced. The jaws of many therapsids grew more complex and powerful. Their teeth became specialized for different tasks, a process called differentiation. They had frontal incisors for nipping food. They had large lateral canines for puncturing and tearing meat. They also had molars for shearing and chopping.

This variety allowed different species to eat many different types of food.

Scientists are still learning about how therapsids managed their body heat. Many early Permian therapsids had a pineal foramen. This is a small hole in the skull for a parietal eye. This eye helped animals regulate temperature and biological rhythms. As therapsids evolved, many groups lost this hole. For example, probainognathian cynodonts lost the hole entirely. Some evidence suggests that endothermy, or the ability to maintain a constant body temperature, evolved in groups like dicynodonts and cynodonts near the end of the Permian.

Therapsids are divided into several major scientific groups or clades. These include the Biarmosuchia, the Dinocephalia, the Anomodontia, the Gorgonopsia, the Therocephalia, and the Cynodontia. The dinocephalians lived during the Middle Permian but died out later. The anomodont dicynodonts were very successful herbivores. The gorgonopsians were famous saber-toothed predators. The cynodonts are especially important because they gave rise to the Mammaliaformes. This happened in the Late Triassic around 225 million years ago.

History shows that therapsids faced many massive challenges. They were the dominant land animals from the Guadalupian through the Early Triassic. However, they were hit hard by the Permian–Triassic extinction event. Many successful groups, like the gorgonopsians, disappeared completely. During the Middle Triassic, they lost their dominance to the archosaurian sauropsids. These included dinosaurs and pterosaurs. Only the cynodont lineage truly survived the end of the Triassic. The Tritylodontidae, a family of very mammal-like cynodonts, survived even longer into the Early Cretaceous.

We can also find clues about what therapsid skin looked like. Most direct evidence of fur comes from docodonts, which are very close relatives of mammals. Some fossils of the dicynodont Lystrosaurus show skin that was leathery and dimpled, similar to an elephant. Other fossils, like the dinocephalian Estemmenosuchus, show skin that was glandular but lacked hair or scales. Some researchers believe hair may have first evolved in probainognathian cynodonts. This idea is supported by the way their skulls changed.

Today, mammals are the only living members of the therapsid lineage.