Is Stenocybus an Anomodont Ancestor?

Earlier we looked at the unlikely connection between Stenocybus and the anteosaurs Sinophoneus. We also looked at a skull reconstruction. Today we’ll take a look at another candidate for a closest kin to Stenocybus at the base of the Therapsida.

The dicynodonts were plant-eating therapsids with bizarre skulls distinctly different from those that retained a carnivorous diet and eventually spun off the mammals and a number of other plant-eating clades along the way. The origin of the dicynodonts has been pegged to the basal anomodonts Patranomodon (Fig. 1) Anomocephalus and their kin.

In our search for the closest kin to Stenocybus its worthwhile to consider a comparison to Patranomodon, with which it appears to share more traits.

 

Figure 1. The skulls of Stenocybus and Patranomodon, side by side and to scale (the smaller Patranomodon illustrations are to scale with Stenocybus). If larger eyes and a shorter snout are indeed signs of neotony in this lineage then we may be seeing the evolution of one form into another here. While the skull of Patranomodon appears wider, that is about the only dimension that has remained the same in the otherwise overall reduction of this taxon from Stenocybus. Note, in particular, the similar palates as Stenocybus transits from the pelycosaurian-grade palate to that approaching the dicynodont grade. This illustration improves on an earlier attempt.

Figure 1. The skulls of Stenocybus and Patranomodon, side by side and to scale (the smaller Patranomodon illustrations are to scale with Stenocybus). If larger eyes and a shorter snout are indeed signs of neotony in this lineage then we may be seeing the evolution of one form into another here. While the skull of Patranomodon appears wider, that is about the only dimension that has remained the same in the otherwise overall reduction of this taxon from Stenocybus. Note, in particular, the similar palates as Stenocybus transits from the pelycosaurian-grade palate to that approaching the dicynodont grade. This illustration improves on an earlier attempt.

Neotony at Work
Currently the therapsid branch of the large reptile tree nests Patranomodon close to Stenocybus at the base of the Therapsida. There’s good reason for this as they share more traits than any other current candidates. Patranomodon is quite a bit smaller (judging by the only data: skulls) than Stenocybus. Patranomodon has a relatively shorter rostrum and larger eyes, both of which make it a candidate for neotony in this relationship. Patranomodon also had smaller teeth, but the breadth of its occiput changed the least.

Premaxilla Ascending Process
Shorter in Patranomodon, along with the shorter rostrum. Also it is nearly vertical, as in dicynodonts. The premaxilla likely included rake-like teeth, considering the ascent of the anterior premaxilla, though partly missing.

Disappearance and Verticalization of the Quadratojugal
In Stenocybus we see the quadratojugal rotate and reduce from chiefly horizontal, as in ophiacodonts, to chiefly vertical and associated only with the quadrate, as in more derived therapsids. We also see the squamosal overtake the quadratojugal.

Supratemporals
Pelycosaurs have them. Therapsids fuse them or lose them. In Stenocybus there are two loose supratemporal-like bones scattered randomly on the parietal, disarticulated from their original positions. So therapsids probably lost them.

Lacrimal
In primitive synapsids the lacrimal contacts the naris. In derived synapsids and all therapsids, the maxilla expands dorsally covering the lacrimal. Earlier I wondered if the lacrimal was the external septomaxilla. The septomaxilla is traditionally a bone inside the naris. In Stenocybus the anterior tip of the lacrimal and the septomaxilla are both present. Part of the broken maxilla exposes the underlying lacrimal. In Patranomodon there is no lacrimal connection, but strangely, neither is the septomaxilla delineated (Fig.1). Probably just an oversight in the original illustration (Fig. 1).

Growth of the Preparietal
Stenocybus has a small new medial bone anterior to the parietal foramen, the preparietal. It is much larger in Patranomodon.

Expansion of the Temporal Fenestrae and Adductor Chamber
In Patranomodon the adductor chamber (area devoted to jaw muscles) is relatively larger despite the smaller teeth. This probably marks the transition to herbivory or insectivory or both, considering the overall size reduction.

Shifting of the Teeth vs. Palate
In Ophiacodon, Haptodus and most therapsids, the palate bones lie between the teeth. In Stenocybus most of the palate is posterior to most of the teeth. In Patranomodon all of the palate, other than the vomers, is posterior to all of the teeth. The palatal elements in both taxa have similar shapes and proportions.

Postcrania
Little to nothing is known of the post-crania in these two taxa. However, we can make certain assumptions based on phylogenetic bracketing more primitive and more derived taxa, Haptodus and Suminia. Torso: long and low. Tail: long and meaty. Legs: splayed with a tendency toward longer toes. The tail shortens quite a bit when these basal forms evolve into dicynodonts. (Or does the body just get bigger and the tail remains the same length?)

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again. Crushing and cracking make both skulls (Fig. 1) difficult to restore the sutures. If I have made any mistakes, please bring them to my attention.

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)

wiki/Stenocybus

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