The Origin of the Dicynodonts

Updated Nov. 16, 2013
Dicynodonts were herbivorous therapsids that lost most of their teeth, save the canine fangs. They originated during the Middle Permian (265 mya) and continued at least until the Late Triassic (215 mya), but a specimen from Australia may extend that to the Early Cretaceous (105 mya). Fossils are found world wide.

Anomodonts (including dicynodonts) had a distinct front-back sliding movement of the mandible that affected the development of jaw muscles and the bones that anchored them.

Traditional Dicynodont Origins
Dr. Kenneth D. Angielczyk reported online* that dicynodonts descended from Cynodonts + Therocephalians and the following non-dicynodont anomodonts: Biseridens, Anomocephalus, Venjukovioidea, Patranomodon and Galeops, in order of increasing similarity to dicynodonts. Eodicynodon is considered the most basal dicynodont.

Problem?
It seems difficult to derive anomodonts from the hypercarnivorous therocephalians. Moreover, several basal taxa were not included in the Angielczyk tree.

(*Sorry this was unavailable when I composed this earlier. I’ll keep looking for it.) There’s also the Rubidge Sidor tree here.

Figure 1. Click to enlarge. Basal therapsids in phylogenetic order as shown by their skulls.

Figure 1. Click to enlarge. Basal therapsids in phylogenetic order as shown by their skulls.

Heretical Dicynodont Origins
The large phylogenetic analysis included a larger number of basal therapsids and recovered anomodonts (including dicynodonts) nesting separate from therocephalians and biarmosuchians, at the base of the therapsida. Here (Fig. 1) the basal dicynodont, Eodicynodon, descended from sisters to , Venjukovia, Otsheria, Microurania, IVPP V18120, Stenocybus and Ophiacodon in order of increasing distance. None of these were hyper-carnivores. The non-dicynodont anomodonts, Anomocephalus, Suminia, Venjukovia [now Ulemica], Galechirus and Patranomodon, also descended from a sister to Stenocybus (Fig. 1).

The New Dicynodont Ancestors and the Reappearance of Canine Fangs
While Archaeothyris, one of the oldest synapsids yet discovered, had moderate canine fangs. Ophiacodon had relatively smaller fangs and Nikkasaurus had no trace of fangs. In Microurania (Ivakhnenko 2003) small canine fangs reappeared. Non-dicynodont anomodonts, such as Anomocephalus and Suminia had no fangs. The tusks of Tiarajudens appear to be new structures, further back from the typical canine position. Similarly, the large canine teeth of dicynodonts appear after a reduction of the canines in ancestral taxa.

References
Ivakhnenko MF 2003. Eotherapsids from the East European Placket (Late Permian). Paleontological Journal, 37, Suppl. 4: S339-S464.
Kammerer CF and Angielczyk KD 2009. A proposed higher taxonomy of anomodont therapsids. Zootaxa 2018:1–24.

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