Adding Ophiacodon to the Origin of the Therapsida
Therapsids were derived from pelycosaurs but the key taxon has not been identified yet. The sphenacodonts, Haptodus and Dimetrodon have been traditional sisters, but both have a relatively shorter, taller skull, a short premaxillary ascending process, a kink at the premaxilla/maxilla jawline, a shorter, taller rostrum and a deeply concave posterior jawline.
The resemblance of Eotitanosuchus to Dimetrodon (convex rorstral margin, loss of pre-canine maxillary teeth) are identified here as homologous traits. Eotitanosuchus nested between Biarmosuchus and higher therapsids.
Here Biarmosuchus nests closer to Ophiacodon. Haptodus and Dimetrodon were likewise derived from a sister to Ophiacodon. The base of the Therapsida lies somewhere inbetween close to the tiny pelycosaur/therapsid Nikkasaurus tatarinvi.
Several Biarmosuchus traits shared with Ophiacodon are not found in Haptodus, Sphenacodon and Dimetrodon: 1) Premaxilla longer than naris; 2) Rostrum twice as long as tall; 3) Quadratojugal not reduced to anearly invisible nub; 4) Premaxilla rises anteriorly; 5) Transition from premaxilla and maxilla without a kink.
Fewer Biarmosuchus traits shared with Haptodus are not found in Ophiacodon: 1) Reflected lamina present. 2) Anterior dentary deep and ventral margin sharply angled. These traits would be expected to appear in the last common ancestor of the Therapsida originating between Ophiacodon and Haptodus.
In therapsids the nasal is relatively narrow, but in sphenacodonts it is broader. The purported septomaxilla in therapsids appears to be the anterior lacrimal beneath the ascending process of the maxilla, perhaps laminated over it.