Stenocybus and Daemonosaurus Convergence

Figure 1. Daemonosaurus a basal phytodinosaur, has several features convergent with those of Stenocybus, a basal anomodont therapsid.

Figure 1. Daemonosaurus a basal phytodinosaur, has several features convergent with those of Stenocybus, a basal anomodont therapsid.

Updated February 28, 2015 with a new skull for Daemonosaurus.

Dinosaurs produced several herbivorous clades, but the big one is the Phytodinosauria, which includes sauropodomorphs, poposaurids and the ornithischians. At its base is Daemonosaurus (Fig. 1).

Therapsids produced several herbivorous clades, but one of the first was the Anomodontia (dicynodonts and dromosaurs). At its base is Stenocybus (Fig. 1).

Despite their phylogenetic differences, Damonosaurus and Stenocybus (known only from skulls so far) share several character traits (Fig. 1).

Among these are: 1. short, convex rostrum; 2. large round orbit about a quarter the length of the skull; 3. Elongated, rake-like teeth; 4. Ventrally convex maxilla; 5. Canine tooth; 6. Reduced mandibular fenestra; 7. Reduced quadratojugal; 8. Small coronoid process; 9. Elongated anterior dentary teeth; 10. Little to no retroarticular process. 11. Jaw joint descends below tooth row. Perhaps you’ll see others.

These traits can be labeled superficial or convergent due to their phylogenetic differences. The relatives of Daemonosaurus were dinosaur-ish bipeds. The relatives of Stenocybus were pelycosaur-ish quadrupeds. Even so, at these taxa we see two transitions from carnivory to herbivory.

So What Do We See and What Does It Mean?
Taken alone, neither Daemonosaurus nor Stenocybus would strike anyone as a plant-eater, and perhaps they weren’t — but their descendants were.

The rake-like teeth would have been suitable for pulling leaves off of stems. Together with this, as in all therapsids and sphenacodonts the jaw joint was lowered in Stenocybus. Similarly, compared to Pampadromaeus the jaw joint was lower in Daemonosaurus. This arrangement helps retain jaw joint articulation with muscles pulling the jaw back in opposition to forces pulling the jaw out while feeding.

Compared to ancestors, Pampadromaeus and Ophiacodon, both Daemonosaurus and Stenocybus, respectively, had a shorter rostrum. A longer rostrum makes it a little easier to grab prey. Plants don’t run and fight.

The mandibular fenestra was smaller than in ancestors. Not sure what this means other than reinforcing the mandible structure. I have no idea what the palate of Daemonosaurus looks like, so no comparisons can be made there.

Anyway, It thought the similarities were curious.

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.

References
Cheng Z and Li J 1997. A new genus of primitive dinocephalian – the third report on Late Permian Dashankou lower tetrapod fauna. Vertebrata PalAsiatica 35 (1): 35-43. [in Chinese with English summary]
Kammerer CF 2011. Systematics of the Anteosauria (Therapsida: Dinocephalia), Journal of Systematic Palaeontology, 9: 2, 261 — 304, First published on: 13 December 2010 (iFirst)
Sues H-D, Nesbitt SJ, Berman DS and Henrici AC 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society Bpublished online 

wiki/Daemonosaurus
wiki/Stenocybus

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s