Sorry to be away for awhile.
I was updating the basal synapsid portion of the large reptile tree at ReptileEvolution.com. Still working on the website as of this writing, but the tree is more robust with a few added taxa. Notably Nikkasaurus and Niaftasuchus have been removed from the Synapsida. The former is now a basal prodiapsid nesting with Mycterosaurus. The latter now nests as another prodiapsid with Mesenosaurus.
Now that all the hard work is done,
let’s take a fresh look at the basal therapsid, Burnetia (Fig. 1), the most derived member of the Burnetiidae. Sutures delineate bones and in order to correctly score the bones you have to see the sutures. And they have to closely resemble those of clade members (Fig. 2). See what you think of these. And note that those who had the fossil in their hands and presumably under the microscope were not able to provide the sutures shown here, gleaned from published photographs.
(Broom 1923, Rubidge and Sidor 2002; BMNH R5397; Late Permian) had a flat, wide skull with exceptional skull ornamentation. The squamosal cheeks flared widely. The teeth are very small. Derived from a sister to Proburnetia.
Derived from a sister to Hipposaurus,
the Burnetidae were basal therapsids from the Middle to Late Permian that evolved bizarre skull ornamentation. Rubidge and Sidor (2002) report, “The systematic position of the Burnetiidae has been unsure largely because of a poor understanding of the cranial morphology of these two enigmatic skulls. In the past they have been considered gorgonopsians (Boonstra, 1934; Haughton and Brink, 1955; Sigogneau, 1970), dinocephalians (von Huene, 1956), and more recently, biarmosuchians (Hopson and Barghusen, 1986; Sigogneau-Russell, 1989). Like Gorgonopsids, this clade has anterior facing nares and a proparietal by convergence.”
We’ll take a look at the other members of this clade later.
But for now here’s the data for the taxa (Fig. 2). Lemurosaurus and Proburnetia appear to have antorbital fenestrae/foraminae and the lacrimal overlaps the jugal. Note the gradual reduction of the teeth in this clade and the gradual widening of the back of the skull. The supratemporals are supposed to be missing from al therapsids, but I found they are missing from all therapsids more derived than this clade.
Broom 1923. On the structure of the skull in the carnivorous dinocephalian reptiles. Proceedings of the Zoological Society of London 2:661–684.
Rubidge BS and Sidor CA 2002. On the crnial morphology of the basal therapsids Burnetia and Proburnetia (Therapsida: Burnetiidae). Journal of Vertebrate Paleontology 22(2):257–267.