Is Stenocybus a Juvenile Sinophoneus?

Christian Kammerer (2011) reports that Stenocybus acidentatus (IGCAGS V 361, Middle Permian, Cheng and Li 1997) is a juvenile Sinophoneus. I don’t see it. Just too many differences, even with allometric growth. You be the judge. Here they are side by side (Fig.1).

Figure 1. Stenocybus (left) is considered a juvenile Sinophoneus (right) by Kammerer (2011). I think the changes are too great. You be the judge. Also note the differences in the new Stenocybus reconstruction, including the palate. Crushing and cracking make both skulls difficult to restore the sutures. If I have made any mistakes, please bring them to my attention.

Figure 1. Stenocybus (left) is considered a juvenile Sinophoneus (right) by Kammerer (2011). I think the changes are too great. You be the judge. Also note the differences in the new Stenocybus reconstruction, including the palate. Crushing and cracking make both skulls difficult to restore the sutures. If I have made any mistakes, please bring them to my attention.

According to Kammerer (2011), Cheng & Li (1997) “distinguished Stenocybidae from Anteosauridae on the basis of the lightly structured skull, large orbits, and small temporal fenestrae in the former. Additionally, they argued that the triangular maxilla, restriction of the septomaxilla within the naris, and anteriorly stretched lacrimal of Stenocybus are unique in therapsids and more closely resemble the condition in ‘pelycosaur’-grade synapsids.”

Kammerer continued, “Examination of the type skull reveals that some of the features used to diagnose Stenocybus are artefactual results of crushing. Examination of the type skull reveals that some of the features used to diagnose Stenocybus are artefactual results of crushing.” In contrast, I found the palate could be restored by skewing the ventral photograph in Photoshop (Fig. 1). The disarticulated palatal elements could be restored to their original positions.

Kammerer (2011) reported that a “small and light-structured skull” and “large-sized orbit and small triangular-shaped temporal fenestra are commonly observed in juvenile therapsids.” He also mentioned that juvenile anteosaurs lose their palatal teeth during maturation and that both share a “dorsal median ridge on the snout that is diagnostic for Sinophoneus,” among other traits.

I have not seen the other juvenile therapsids Kammerer is referring to. It would have been great to see these illustrated in his paper. In personal communication, Kammerer said he would try to find some images to send. When I get those we can reexamine this question with that data.

We’ll look at another pairing of Stenocybus tomorrow, another taxon that shares more traits in common. We’ll also look at the process of reconstructing the skull of Stenocybus the day after.

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)

 

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