Stegoceras novomexicanum, a tiny southern dome-headed pachycephalosaur

Illustration of Pachycephalosaurus wyomingensis

Stegoceras novomexicanum is a new (ish) pachycephalosaur from the late Cretaceous of northwest New Mexico (Jasinski and Sullivan, 2011). I was lucky enough to name and describe this taxon recently with Bob Sullivan, a paleontologist who has spent a lot of time working on this enigmatic group of ornithischian dinosaurs. Pachycephalosaurids are essentially characterized by a dome on top of their skulls of thickened bone. They do have other synapomorphies that make them a monophyletic group, but the thickened skull rook is the most recognizable characteristic. Pachycephalosaurs are only known from the Late Cretaceous, and are best known from North America and Asia. Recently, Watabe et al. (2011) reported on what may be the earliest definite pachycephalosaur known, Amtocephale gobiensis from the Cenomanian-late Santonian of the Gobi Desert in Mongolia. I was lucky enough to help out with that study a bit, although I was not an author on the paper. Although Amtocephale appears to currently be the oldest known pachycephalosaurid known, it is not the most basal.

There are many problems surrounding pachycephalosaurs. These include questions about taxonomy, paleobiology, systematics, origin, and many other things. While these are all great topics for discussion, we will not be getting into them at the moment, although perhaps at a later time we will.

Skull of Pachycephalosaurus wyomingensis with lower jaws

In North America, pachycephaolsaurs are most well known from Canada and the northern United States. South of Wyoming there is almost nothing known of the pachycephalosaurs that roamed the land n the late Cretaceous. Longrich et al. (2010) recently reported on one such southern pachycephalosaur from Texas. Texacephale langstoni is based on a highly weathered frontoparietal dome The specimen itself is not highly diagnostic in general. In my newly published paper (Jasinski and Sullivan, 2011), we discuss its validity and make a case for why the holotype and referred specimens are not actually diagnostic and do not have any autapomorphies. We conclude that Texacephale langstoni is, therefore, a nomen dubuim. The specimens themselves, however, are still quite important. They mark some of the southern-most pachycephalosaur specimens known from North America and I hope that further material is found. Lehman (2010) actually reported on other pacycephalosaurid specimens from Texas that same year, although he felt that the material wasn't complete enough and only identified it to an indeterminate pachycepalosaurid. Hopefully more specimens are found that provide actual diagnostic features and it can be shown that it really is a valid taxon. I hope that it is, or will be, but only time will tell.

In 2002, Williamson and Carr reported on a juvenile pachycephalosaur from the Fossil Forest Member of the Fruitland Formation and a part of the Hunter Wash local fauna. Following this, Sullivan and Lucas (2006) reported that the specimen represented a juvenile Stegoceras validum based on, among other things, a squamosal shelf at the posterior of the frontoparietal dome. This would have been a vast geographic expansion of the taxon southward.

Figure of the skull and frontoparietal dome of a pachycephalosaur, Stegoceras validum

Stygimoloch spinifer

In the summer of 2008, I went out to the San Juan Basin of New Mexico with Dr. Bob Sullivan for field work. I had never done field work in Mesozoic strata before, so it was all new to me at that time. Oh the things I had to learn, so anything I found that I thought was interesting of weird or whatever the case may be, I showed to Bob in the hopes that I had struck "gold". While exploring a new area, I came across a weird piece of bone with no idea of what it was. When I showed Bob, he immediately knew it was a partial frontoparietal dome. I went back and found a few more fragments, but nothing substantial. The following summer we were at a relatively close site about a mile away, and I found a small bone sticking out of the side of a small hill. While Bob and I couldn't tell what it was, I made sure to get it out quickly before we left for the day. In the field it was identified as part of a croc skull, but we later saw it was a pachycephalosaur squamosal.


We were eventually able to identify both as from the same type of animal as the juvenile pachycephalosaur reported on by both Williamson and Carr (2002) and Sullivan and Lucas (2006). The paper was originally going to report on further Stegoceras validum specimens based on the earlier reports. But something didn't seem quite right. Thanks to the one specimen I had found the first summer (2008), we could tell that it was from an animal nearly full-grown and essentially the same age range as the type of Stegoceras validum. The specimens were all smaller and more gracile though. There were several other differences that I could find. I talked to Bob and told him that I felt it was a taxon distinct from Stegoceras validum. After discussing it for awhile, Bob agreed.

Stegoceras

Stegoceras novomexicanum is distinguished from Stegoceras validum based on possessing the following features: posteromedial extensionof the parietal reduced and sub-rectangular; squamosal sutural surface contacts of the posteromedial extension of the parietal roughlyparallel; supratemporal fenestrae more medial and enlarged; gracile and small adult size. While the small adult size can possibly be a contentious one and not the lone feature to distinguish two taxa, it can, nonetheless, be an added characteristic. The other features though, were confirmed by both Bob and myself through the direct inspection and comparison of many, and perhaps all, of the Stegoceras specimens known from Canada and northern United States.

Size comparison between Stegoceras validum and Stegoceras novomexicanum

Stegoceras novomexicanum is a smaller version of Stegoceras validum, and contains a more pronounced squamosal shelf, giving the back of its skull a somewhat more flattened appearance. Other than Texacephale langstoni (considered a nomen dubium), Stegoceras novomexicanum is the southernmost named member of the Pachycephalosauridae in North America. This small, taxon provides further evidence to the distinct nature of the animals from this time (late Campanian) from New Mexico and furthers the difference between the animals from the North (Canada, Montana, Dakotas, Wyoming, etc.) and those further south (New Mexico, Texas, etc). Here's hoping that more information is found on Stegoceras novomexicanum and other pachycephalosaurs in the not-too-distant future and that people realize that this will help to show more about this strange group of dinosaurs than previously known.

For another view of Stegoceras novomexicanum and a look at Amtocephale gobiensis, take a look at Jamie Headden's blog (The Bite Stuff - New domes-New Interpretations)

Literature Cited

Jasinski, S.E. and Sullivan, R.M., 2011, Re-evaluation of pachycephalosaurids from the Fruitland-Kirtland transition (Kirtlandian, late Campanian), San Juan Basin, New Mexico, with a description of a new species of
Stegoceras and a reassessment of Texacephale langstoni. New Mexico Museum of Natural History and Science, Bulletin 53, p. 202-215.

Lehman, T.M., 2010, Pachycephalosauridae from the San Carlos and Aguja formations (Upper Cretaceous) of West Texas, and observations of the frontoparietal dome: Journal of Vertebrate Paleontology, v. 30, p. 786-798.

Longrich, N.R., Sankey, J. and Tanke, D., 2010, Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA: Cretaceous Research, v. 31, p. 274-284.

Sullivan, R.M. and Lucas, S.G., 2006b, The pachycephalosaurid dinosaur Stegoceras validum from the Upper Cretaceous Fruitland Formation, San Juan Basin, New Mexico: New Mexico Museum of Natural History and Science, Bulletin 35, p. 329-330.

Williamson, T.E. and Carr, T.D., 2002a, A juvenile pachycephalosaur (Dinosauria: Pachycephalosauridae) from the Fruitland Formation, New Mexico: New Mexico Geology, v. 24, p. 67-68.