A recent paper by Liu (2013) proposed that three small skull specimens (Fig. 1), including one formerly considered the holotype of Stenocybus, are really just juvenile versions of the much larger anteosaurian dinocephalian, Sinophoneus. This idea was put forth earlier by Kammerer (2011). To support this hypothesis Liu reported the following synapomorphies he said were shared by these four specimens.
From the abstract
“Sinophoneus yumenensis and Stenocybus acidentatus are the only dinocephalians from China, and the latter taxon has been proposed to be a junior synonym of the former. Here I confirm this synonymy on the grounds that the differences between the two putative taxa are due to ontogenetic variation. Sinophoneus yumenensis differs from all other anteosaurs in having premaxillary dorsal processes that are separated by relatively long nasal anteromedial processes, and vomers without raised, elongated edges; from all other anteosaurs except Archaeosyodon praeventor in having distinct frontal posterolateral processes, and a wide intertemporal region formed partly by long posterior processes of the postfrontals that approach the posterior edge of the skull roof; and from Archaeosyodon praeventor in having a well-developed midline ridge on skull roof.”
A phylogenetic analysis by Liu (2013) nested Sinophoneus as the most basal anteosaurid. The purported juveniles were not analyzed separately.
Well, you know what that means… Somebody has to do it and here it is (Fig. 2).
Three specimens, including the holotype of Stenocybus (the IGCAGS specimen) nested several nodes away from Sinophoneus. The Stenocybus holotype still nests at the base of the Anomodontia (Figs. 2, 3), as we noted earlier. Specimens V18117 and V18120 nest at the bases of the two major subclasses within the Anomodontia. One leads to dicynodonts and their ancestors (Fig. 3). The other leads to dromasaurs and their kin.
So it looks like China is where anomodont radiation had its genesis. Or at least this was a refuge for this taxa.
Then the fourth small specimen:
IVPP V18119 nests between Sinophoneus and Deuterosaurus. So, yes, it could be a juvenile Sinophoneus! Then again, it could be a transitional taxon. Only one step is added when the two Sinophoneus nest together as a clade.
Why small specimens considered juveniles?
It’s probably just human nature that juvenile-looking taxa are considered juveniles and therefore are not include in phylogenetic studies. This has to stop. The proof of relationships comes from phylogenetic analysis, not from a cold reading.
We’ve seen this before
Taxon exclusion by assumption is a problem we found earlier in pterosaurs. Virtually all analyses of the Pterosauria exclude the tiny pterosaurs under the presumption that they, too, were juveniles. That hypothesis has been falsified in phylogenetic analysis here that includes tiny pterosaurs, and, like here, they turned out to be key taxa in evolutionary sequences.
Now, about those synapomorphies…
Liu’s (2013) analysis compared the large Sinophoneus only to other anteosaurs, but a larger gamut analysis (Fig. 2) nests three of them far outside the anteosaurs, at the base of the Anomodontia. I can’t remark on the presence or absence of a nasal separating the premaxillary ascending processes, nor the presence or absence of a midline ridge on sister taxa as most of my data is in lateral view and based on drawings.
Sure three of the four new small skulls look similar to Sinophoneus, but they look more like and nest closer to basal therapsids in the anomodont clade. Unfortunately, neither Kammerer (2011) nor Liu (2013) considered these possibilities by including the small skulls in larger gamut studies.
Kammerer CF 2011. ‘Systematics of the Anteosauria (Therapsida: Dinocephalia)’, Journal of Systematic Palaeontology, 9: 2, 261—304, First published on: 13 December 2010 (iFirst) To link to this Article: DOI: 10.1080/14772019.2010.492645\
Liu J 2013. Osteology, ontogeny, and phylogenetic position of Sinophoneus yumenensis (Therapsida, Dinocephalia) from the Middle Permian Dashankou Fauna of China, Journal of Vertebrate Paleontology, 33:6, 1394-1407, DOI:10.1080/02724634.2013.781505