Reconstructing the Skull of Stenocybus

Stenocybus accidentusis (Cheng and Li 1997) is known from a skull and a set of skull parts. Earlier we looked at comparisons to anteosaurs as reported by Kammerer (2011).

The holotype specimen, IGCAGS V 361, is complete, but skewed parasagittally. Photographs of the specimen (Kammerer 2011), my only data source were taken from less than ideal angles using a fairly wide angle lens (which does not flatten perspective). Unfortunately Kammerer (2011) did not delineate the bones, so I attempted to differentiate sutures from cracks. So, based on observation and comparison with potential sister candidates, here they are (having never seen the actual fossil). If I made any mistakes, please bring them to my attention. I certainly made mistakes earlier. This is a learning process. The parietal portion was particularly troublesome as the posterior frontals have a medial hole into which the preparietal snakes. Moreover the parietal was unexpectedly narrow. Nevertheless, it all seems to fit together and no “rules” were broken.

Stenocybus in situ

Figure 1. Stenocybus in situ with bones identified. Colors match those in figure 2. A longer lens (not such a wide angle) and a more direct lateral and dorsal view would have made reconstruction much easier to do. From Kammerer 2011.

 

Figure 2. Stenocybus reconstructed based on tracings in figure 1. Note the lacrimal AND septomaxilla in close proximity to each other.

Figure 2. Stenocybus reconstructed based on tracings in figure 1. Note the lacrimal AND septomaxilla in close proximity to each other.

Overall
The skull has the appearance of a small Haptodus retaining traces of its ophiacodontid ancestry. There is a median ridge on the skull.

Premaxilla
The ascending process remains long, as in ophiacodontids and most therapsids, but unlike sphenacodontids.

Lacrimal and Septomaxilla
Both appear side by side here, the latter external and the former internal, their plesiomorphic positions. One traditional trait separating pelycosaurs from therapsids is the septomaxilla becomes a surface (dermal) bone in therapsids. This can be seen to be happening in Stenocybus with the lacrimal retreating.

Preparietal
This new bone makes an early appearance in Stenocybus. Not sure why, but it appears to occupy a circular foramen in the posterior frontal, just anterior to the parietal foramen, which is relatively small here, as is the parietal itself.

Maxilla
This upper tooth bearing bone contacts the nasal and prefrontal. A tiny sliver of no consequence extends beneath the orbit.

Quadratojugal and Squamosal
Ophiacodonts have a horizontal quadratojugal. Sphenacodonts hide their’s beneath a squamosal. Therapsids have a vertical quadratojugal, sometimes visible, often not. Stenocybus shows the transitional stage, somewhat of a diagonal orientation, somewhat hidden by the squamosal. The squamosal is also overtaking the posterior jugal.

Supratemporal
Pelycosaurs have them. Therapsids don’t. Here they are found as flaked off loose elements near the parietal foramen. So they were of little consequence and probably disappeared rather than fusing to the squamosal.

Teeth
The anterior teeth of Stenocybus are rake-like. Does this indicate the start of an herbivorous diet? If so, perhaps the earliest stages of one.

Angular
In therapsids the posterior angular separates itself from the body of the mandible and develops a retroarticular process. We don’t see much separation here in Stenocybus.

Palate
The choanae are elongated here, shifting the pterygoid and palatine bones largely behind the tooth row, which presages the condition in anomodonts. However the ventral view of the palate remains quite narrow, as in ophiacodonts. The pterygoid transverse processes are vestiges. Palatal teeth were relatively larger. The epipterygoid was reduced to a slim buffer between the jawline and palate.

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)

wiki/Stenocybus

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 )

Google photo

You are commenting using your Google 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 )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.