Although a few early phylogenies in the 1990s did not find that these two formed a natural clade,[2] the relation between these two is now considered to be more stable, and has been found in practically every major analysis of rhynchocephalians since Apesteguía & Novas (2003).

Evans (1988) and Ahmad (1993) have even considered that they may all belong to a single species, although a shortage of good studies and descriptions focusing on this group in particular means that any conclusions on the relationships between different sapheosaurs are uncertain at best.

[1] The upper temporal fenestrae (a pair of large holes on the top of the rear part of the skull) are long but fairly thin, a feature also known in Vadasaurus and Palaeopleurosaurus, potential relatives of the group.

[1] However, it was later shown that Sapheosaurus and Oenosaurus possessed tooth plates on the upper and lower jaws,[8] formed by the fusion of maxillary and dentary teeth.

Sapheosaur vertebrae also had swollen neural arches (the area above the spinal cord) and zygapophyses (connecting joint plates), features also present in Ankylosphenodon.

[1] Some have proposed that sapheosaurs were at least partially aquatic due to some similarities and/or possible close relations to Ankylosphenodon (now believed to be convergently evolved) or Vadasaurus and pleurosaurs.