Therapsida

Therapsida is a major group of eupelycosaurian synapsids. It includes mammals, as well as their ancestors and close relatives. Many defining traits of modern mammals had their origins within early therapsids. Changes in skeletal structure brought their limbs underneath their body mass which resulted in a more "standing" quadrupedal posture, as opposed to the lower sprawling posture of many reptiles and amphibians. The teeth of therapsids are differentiated into incisors, canines, and postcanines (the latter becoming differentiated into premolars and molars in mammals). Their necks also gained more flexibility allowing therapsids greater range of motion in moving their heads. Therapsids split from their closest relatives within the "pelycosaurs", the carnivorous Sphenacodontidae (including the famous Dimetrodon) during the Late Carboniferous or earliest Permian. Their early evolutionary history during the Cisuralian (Early Permian), is obscure, and they lack confirmed fossil records during this time period leaving a substantial ghost lineage. They replaced the pelycosaurs as the dominant large land animals in the Guadalupian (Middle Permian) through to the Early Triassic. In the aftermath of the Permian–Triassic extinction event, therapsids declined in relative importance to the rapidly diversifying archosaurian sauropsids (pseudosuchians, dinosaurs and pterosaurs, etc.) during the Middle Triassic. The therapsids include the cynodonts, the group that gave rise to mammals (Mammaliaformes) in the Late Triassic around 225 million years ago, the only therapsid clade that survived beyond the end of the Triassic. The only other group of therapsids to have survived into the Late Triassic, the dicynodonts, became extinct towards the end of the period. The last surviving group of non-mammaliaform cynodonts were the Tritylodontidae, which became extinct during the Early Cretaceous.