A Slight Conundrum

For the most part the remains of Wannanosaurus have been fairly well recognized as those of a basal pachycephalosaur. This turns up pictures we expect to see of typical dome headed dinosaurs wandering a Cretaceous landscape most of the time. Additionally, it occasionally shows us results like the page from a book over here (anyone that can identify that book gets the satisfaction of being helpful) which looks a lot less like a pachycephalosaur and more like a hypsilphodontid, though early pachcephalosaurs were more than likely fairly bland like this on account of the fact that the skull adaptations were not quite as ostentatious as they would be later in the familial lines. Regardless, these are what we expect to see when we here pachycephalosaur or basal pachycephalosaur.

©Hou 1977

The material on which Wannanosaurus is based, in part, is a non-articulated mandible. The evidence in the mandible, the teeth, which was used to aid in the classification of Wannanosaurus as a pachycephalosaur indicate what could possibly be an omnivorous diet, as many members of the family are thought to have also been. Wannanosaurus had a number of leaf shaped teeth, some of which overlapped, and a few which may very well have been chipped canines. This evidence was very important in Hou's initial description and assigning of Wannanosaurus to the pachycephalosauridae with the addition of the upper cranial elements (part of the skull roof) also discovered. There is a slight problem with this evidence however, in that it is so little in number that it can also be assigned elsewhere in the dinosauria if not carefully examined. The skull roof, however, is quite clearly that of a pachycephalosaur when compared to other members of the extended family.

Regardless of careless indexing, see above, Wannanosaurus is widely agreed to be an adult pachycephalosaur of a small basal species. Very few sources, Gregory Paul's Princeton Field Guide as an example, consider this an immature animal. Paul, though, despite being on the minority side of this consensus as to the maturity of the individual does not, at least, assign it to another genus or species; he recognizes Wannanosaurus as a separate and distinct species despite considering it an immature specimen. The edition of The Dinosauria (Weishampel, Dodson, Osmolska 1990) that I have is very old, but in its pachycephalosauria section (Maryanska 1990) it mentions that "it probably represents an adult individual" based on the roof of the skull.

References
Hou, Lianhai, "A primitive pachycephalosaurid from the Cretaceous of Anhui, China, Wannanosaurus yansiensis gen. et sp. nov," Vertebrata PalAsiatica, Volume 15, Number 3, July, 1977 198-202

Maryanska, T., "Pachycephalosauria," The Dinosauria, Ed. Weishampel, D.B, Dodson, P., Osmolska, H, Los Angeles: University of California Press, 1990

Paul, G.S., The Princeton Field Guide to Dinosaurs, Princeton, NJ.: Princeton University Press, 2010