More upright limb posture. Started when synapsids became more mammal-like in body plan, especially with a more upright posture and more differentiated teeth. Britannica also notes their single temporal opening, mammal-like incisors, canines, cheek teeth, and modified limbs.
The clean marker here is Tritylodontidae. They were cynodont therapsids close to mammals but outside Mammaliaformes, and they survived into the Early Cretaceous. Britannica specifically notes that the last known tritylodontids died out about 125 MYA.
Last Non-Mammaliaform Therapsids Read More »
Raranimus is a good first-known marker for Therapsida. It is usually described as the most basal known therapsid from the Middle Permian. Britannica gives therapsids as a synapsid subgroup that first appears in the Permian and later gives rise to mammals.
