Therapsiden

Therapsida, or therapsids, is a major clade of synapsids that includes mammals and their extinct relatives. They first appear in the Middle Permian and become the dominant terrestrial vertebrates in the Late Permian, diversifying into carnivores and herbivores before the end-Permian mass extinction. Some therapsid lineages survived into the Triassic, with cynodonts leading to mammals.

Therapsids exhibit a suite of derived features relative to their pelycosid ancestors. They generally show differentiated teeth (incisors, canines, postcanines) and more robust jaw musculature. The skull and jaw joints evolved toward greater efficiency, and a more upright limb posture is common in many groups. Many therapsids display adaptations for higher activity and sensitivity, and debates about their exact metabolic rate continue; most paleontologists view them as more thermally capable than earlier synapsids.

The group includes several subgroups, such as Biarmosuchia, Dinocephalia, and Theriodontia, with cynodonts occupying the lineage closest to mammals. Dicynodonts were a successful herbivore group, while predatory forms included gorgonopsians and therocephalians. Among cynodonts, taxa such as early Triassic forms illustrate the transition toward true mammals.

Impact and legacy: Therapsids shaped Permian ecosystems and provided the anatomical and developmental framework for mammals. After the Permian extinction, surviving therapsids gave rise to more advanced cynodonts, culminating in crown-group mammals during the Mesozoic.