therapsids

Therapsids are a clade of synapsids that includes the mammal-like amniotes and, ultimately, mammals. They first appeared in the Permian period, about 270 million years ago, and became the dominant terrestrial vertebrates for much of that era. The group survived the end-Permian mass extinction and gave rise to several lineages that led to mammals, most notably the cynodonts.

Therapsids are divided into several major groups, including biarmosuchians (early therapsids), dinocephalians, anomodonts (primarily herbivores such as dicynodonts), and theriodonts (a diverse, often more predatory assemblage). Within theriodonts, cynodonts are the closest relatives of mammals and contain the line that eventually produced true mammals.

Anatomically, therapsids show several evolutionary trends toward mammal-like traits. They generally had more upright limbs, a more powerful and complex jaw musculature, and differentiated teeth (incisors, canines, and postcanine teeth). Some possessed a secondary palate and a larger skull opening, enabling stronger chewing. In the cynodont lineage, the jaw joint shifted toward a dentary-squamosal articulation, while the articular and quadrate bones became the malleus and incus of the mammalian middle ear. Many therapsids also display evidence of increased brain size and metabolic efficiency, though the degree of endothermy varies among groups.

Fossils of therapsids are found worldwide, with rich Permian deposits in South Africa’s Karoo, Russia, China, and other regions. They occupied diverse ecological roles, from large herbivores to agile carnivores. The mammal lineage ultimately arose from cynodonts during the Mesozoic, with true mammals appearing in the Late Triassic. Therapsids thus represent a pivotal stage in the evolution of mammals.