In lateral view Stenocybus and Haptodus are close matches. Even so, the two are distinct taxa separated by others according to the large reptile tree now up to 315 taxa (not counting the therapsid or pterosaur trees). Stenocybus was twice as large, but this specimen of Haptodus was one of the smallest ones known.
The maxilla was shorter than the lacrimal in Haptodus. The teeth were smaller. The quadratojugal was not visible. The ascending process of the premaxilla was shorter.
The skull was narrower in ventral view and the palate was smaller in Stenocybus. The palatal elements (other than the vomers) were largely behind all the teeth in Stenocybus. Haptodus represents the plesiomorphic condition. The pterygoid was smaller in Stenocybus as were the quadrates. The frontal and parietal was smaller producing a more visible lateral termporal fenestra in dorsal view. The canine tooth was more prominent and the maxilla housing its root was much deeper, overlapping the shrinking lacrimal. The septomaxilla shifted more toward the surface and the prefrontals formed larger ‘eyebrows.’
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Credner H 1888. Die Stegocephalen un d’Saurier aus dem Rothliegnden des Plauen’schen Grudes bei Dresden: Zeitschrift der Deutschen Geologischen Gesellschaft, v. 40, p. 490-558.
Currie PJ 1977. A new Haptodontine Sphenacodont (Reptilia: Pelycosauria) from the Late Pennsylvanian of North America: Journal of Paleontology, v. 51, n. 5, p. 927-942.
Huene F von 1925. Ein neuer Pelycosaurier aus der unteren Permformaiton Sachens: Geologische und Palaeontologische Abhandlungen, Jena., v. 18, NF 14, p. 215-264.
Kammerer CF 2011. Systematics of the Anteosauria (Therapsida: Dinocephalia), Journal of Systematic Palaeontology, 9: 2, 261 — 304, First published on: 13 December 2010 (iFirst)
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.