Basal Dinocephalian – svp abstracts 2013

From the abstract
Jansen et al. 2013 report, “The transition from pelycosaur-grade synapsids to therapsids documents a major macroevolutionary transition with early therapsids providing important information on the evolutionary history of the mammalian body plan. Dinocephalians were an early therapsid group that occupied a wide range of ecological niches and dominated terrestrial ecosystems during the middle Permian. The Mezen fauna (Roadian/Wordian in age) of northern European Russia yields one of the most basal known therapsid faunas, including the enigmatic and poorly known genera Alrausuchus, Niaftasuchus, Nikkasaurus, and Reiszia.

“For the present study, we investigated a new skull, representing an additional basal therapsid from Mezen, via three-dimensional computed tomography (micro-CT) and a detailed retro-deformed reconstruction. Additional isolated cranial and postcranial material can also be assigned to the new taxon. The new taxon has a complex heterodont dentition, with intermeshing, heeled incisors, small precanines, a very small canine, and nine postcanines.

“Inclusion of the new taxon in a phylogenetic analysis of early therapsids recovers it as a basal dinocephalian, although its precise position is variable (as the most basal dinocephalian or the most basal member of either of the major dinocephalian subclades, Tapinocephalia and Anteosauria). This taxon exhibits a mosaic of tapinocephalian and anteosaur characters, including roughly denticulated, leaf-shaped postcanine teeth, a temporal fenestra that undercuts the orbit, and restriction of the palatine dentition to a small, reniform boss. Remarkably, this taxon bears a distinct preparietal bone at the anterior edge of the pineal foramen, a feature widely distributed in basal therapsids but previously unknown in dinocephalians. The new taxon provides novel insight into the early evolution of dinocephalians specifically and therapsids in general.”

I haven’t seen this taxon but we can follow the Jansen et al. data. The large reptile tree also nests anteosaurs and tapinocephalids together with dinocephalians (Fig. 1).

Figure 1. Click to enlarge. Basal Therapsida. Note superficial similarities between Haptodus and Stenocybus. Note superficial similarities between Dimetrodon and Eotitanosuchus.

Figure 1. Click to enlarge. Basal Therapsida. The new Jansen et al. specimen nests either at the base of Anteosauria and Tapinocephalia, or at the base of the Dinocephalia.

Preparietal present
As Jansen et al. reported, typically the preparietal found in the new specimen is also found in gorgonopsids and dicynodonts, two unrelated therapids. I don’t see it in Eotitanosuchus (Fig. 1) or biarmosuchids. So this may be a third convergent appearance.

Small canine
The very small canine is also at odds with most candidate sister taxa, but may, perhaps, also be found in Sinophoneus (Fig. 1), also at the base of the Dinocephalia. Not sure if Sinophoneus had large canines. Teeth are unknown.

Undercut orbit
The orbital bones are undercut by the lateral temporal fenestra, as in Sinophoneus. So, sight unseen following the clues, the new specimen seems to fit with Sinophoneus.

Looking forward to seeing the details.

Jansen M, Reisz R, Kammerer C and Fröbisch J 2013.
A new basal dinocephalian from the middle Permian Mezen fauna (Russia). Journal of Vertebrate Paleontology abstracts 2013.

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