Therapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, including an erect posture and lactation. The earliest fossil attributed to Therapsida is Tetraceratops insignis (Lower Permian). Therapsids evolved from pelycosaurs (specifically sphenacodonts) 275 million years ago. They replaced the pelycosaurs as the dominant large land animals in the Middle Permian. They remained the dominant fauna until replaced by archosaurs and rhynchosaurs in the Middle Triassic although some therapsids, the kannemeyeriiforms for example, remained diverse in the Late Triassic.
Contributions by NatureA16, Peter M. Brown, and Dinoguy2.