Biarmosuchus tener (Tchudinov 1960) Late Permian ~260 mya is the most primitive therapsid. Derived from a sister to Ophiacodon and Haptodus, Biarmosuchus phylogenetically preceded Aelurognathus and the rest of the Therapsida, including mammals and man.
Distinct from Haptodus garnettensis, the skull of Biarmosuchus was elongated with an elongated premaxilla dorsal process (both as in Ophiacodon). Huge canine teeth dominated the jawline, which otherwise had fewer teeth. The maxilla was much deeper to house the canine root. The lacrimal did not contact the naris, obstructed by the rising maxilla. The quadratojugal did not descend but the quadrate did. Rather than a premaxillary notch, the premaxill rose anteriorly. The pineal opening (third eye) was raised like a little volcano. The anterior dentary developed a strong chin. The reflected lamina became a thin sheet of bone reinforced by a cross of thicker bone.
The neck was slightly longer and included more vertebrae. The posterior dorsal ribs were shorter.
The scapula was longer, less bulky and rode more loosely on the ribcage. The lateral fingers and toes were shorter, brought about by the reductions of m3.2, m4.2 and m4.3 (see below). This permitted the hands and feet to swing more directly beneath each limb, rather than out to the sides (as in Gephyrostegus), during the recovery stroke of each stride. This change helped reduce the amount of torso undulation and thus enabled Biarmosuchus to breathe more easily while walking.
The pelvis was relatively much larger. The femur was longer and elevated the torso higher off the ground while walking.
If you want to read the book, "From the Beginning - The Story of Human Evolution" by David Peters (Little, Brown 1991), which is where the above images were first published, click here for the PDF.
Tetraceratops (Matthew 1908) was considered the most primitive therapsid by Laurin and Reisz (1996), but cladistic analysis indicates it shares more characters with Tseajaia, a limnoscelid, which has never been included in previous cladistic analyses of Tetraceratops.