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The validity of the Late Cretaceous pachycephalosaurid Stegoceras novomexicanum (Dinosauria: Pachycephalosauridae).

December 20 , 2016:

In light of a recent paper, we review, and reassess, the validity of the pachycephalosaurid dinosaur Stegoceras novomexicanum (Dinosauria: Ornithischia: Pachycephalosauridae). Specimens that are referred to Stegoceras novomexicanum are all late Campanian (early Kirtlandian) in age and are not only from a restricted stratigraphic horizon at the Fruitland/Kirtland transition, but also from a small geographic area of the San Juan Basin, New Mexico. While some of the characters initially used in diagnosing this taxon may be representative of an earlier (sub-adult) ontogenetic stage, such as the reduction of the size of the posteromedial extension of the parietal and the size of the supratemporal fenestrae, several other characters confirm its validity. These include the shape of the posteromedial extension of the parietal, the relative position of the supratemporal fenestrae, the shape and degree of inflation of the nasal boss, morphology of the prefrontal-frontal suture, curvature of the frontal-parietal suture, and its relative overall size and gracile form relative to the type (lectotype) of Stegoceras validum from the Judithian of Alberta, Canada. Although the holotype of Stegoceras novomexicanum may represent a sub-adult individual, it also possesses some diagnostic features that are indicative of an adult. This combination of features may indicate heterochrony for Stegoceras novomexicanum within the Pachycephalosauridae. Recently recovered small-bodied, high-domed pachycephalosaurid specimens from the Fruitland-Kirtland transition further support our interpretation that this taxon represents a distinct, small-bodied adult pachycephalosaurid in New Mexico. The previously described paratypes, and newly collected specimens, are conservatively assigned to cf. Stegoceras novomexicanum, as all this material comes from a very restricted stratigraphic interval and geographic area.

Steven E. Jasinski and Robert M. Sullivan (2016)
The validity of the Late Cretaceous pachycephalosaurid Stegoceras novomexicanum (Dinosauria: Pachycephalosauridae).
In Sullivan, R.M. and Lucas, S.G., eds., 2016, Fossil Record 5. New Mexico Museum of Natural History and Science Bulletin 74: 107-11


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