Cretaceous Turtles of New Mexico Part 1: Here comes the meteor?

Since some of you may be aware that one of my major interests revolve around turtles, both alive and dead, its surprising that its taken me this long to post anything on this amazing group or reptiles. While there are an incredible number of things that can be posted, this will be more of a simple post based around one of my recent papers (Jasinski et al., 2011). From what is known about the San Juan Basin, specifically in New Mexico, these are the last turtles known before the K-T extinction event and the moment in time when the bolide hit the Yucatan.

Glyptodontopelta mimus from Naashoibito Member (Ojo Alamo Formation)

Ojoceratops fowleri

The study itself was on all the fossil vertebrates recovered from the Naashoibito Member of the Ojo Alamo Formation (Jasinski et al., 2011). The fossil vertebrates from this stratigraphic unit have been grouped together in what is called the Alamo Wash local fauna and were originally studied by Lehman (1981), but were in thorough need of revision. There are several key dinosaur taxa within this fauna, including the recently named Ojoraptorsaurus boerei. Other endemic taxa to this fauna include the ankylosaur Glyptodontopelta mimus and the ceratopsid Ojoceratops fowleri. It is also contains the types of Alamosaurus sanjuanensis, the youngest sauropod from North America. Alamosaurus has a number of potential problems though, and I will be touching on those in later posts.

So, the focus of this post is on those lovely creatures how keep their houses on their backs. The turtles from the two underlying formations are under study by Dr. Robert Sullivan and others and the study is currently waiting to come out in an upcoming volume on fossil turtles dedicated to Dr. Eugene Gaffney. So we will hope it comes out sooner rather than later. The turtles from the Naashoibito Member are vastly understudied and little attention has been paid to them. Much of this is because most material is very fragmentary, and there has been little there that would even qualify as a potential holotype for a turtle species.

reconstructed Basilemys, similar to Basilemys nobilis

Even so, two turtle species have been named from this unit. The first, called Basilemys nobilis, was named by Hay (1911) based on several carapace and plastron fragments. Basilemys is a member of the Nanhsiungchelyidae, and represents the only definitive terrestrial turtle from this turtle fauna. The forthcoming study by Sullivan et al. came to the conclusion that B. nobilis was a nomen dubium. While a recently recovered specimen may yet prove that a legitimate species of Basilemys is present in the Naashoibito Member, the holotype of B. nobilis will still probably be taxonomically undiagnostic.

reconstructed shell of Adocus

The second holotype, named Adocus vigoratus by Hay (1911), is based on several carapace fragments. Adocus is currently part of the family Adocidae, although at one point it was considered in the same family as Basilemys. Being an aquatic species, it probably inhabited a familiar niche throughout the late Cretaceous and into the Paleocene. Jasinski et al. (2011) felt that the type specimen of A. vigoratus was undiagnostic though, and considered it a nomen dubium, although it does represent an adocid in the fauna.

Kinosternon subrubrum, close living relative of Hoplochelys

Recent studies on Compsemys (Knauss et al., 2011) and Hoplochelys (Lyson and Joyce, 2011), have revealed many synonymies for these two previously ill-studied turtles. While the specimens of the ?pleurosternid Compsemys and the kinosternoid Hoplochelys are generally quite fragmentary in this fauna, both have been referred to specific species. The ?pleurosternid is now called Compsemys victa and all North American Compsemys are thought to represent one species. The kinosternoid Hoplochelys has now been referred to the species H. clark, as has all Cretaceous specimens of Hoplochelys. Both turtles would have represented small aquatic species. Compsemys was a small gracile turtle with a number of distinct places on the shell with small and distinct bumps and small ridges.

Neurankylus (a baenid) showing color spots found on specimen from Paleocene in San Juan Basin

Boremys (Baenidae)

The Baenidae are a group of aquatic turtles well-represented throughout the Cretaceous and Paleocene. They come in a variety of sizes, but are commonly quite robust for water turtles. Considering the overall number of turtle fossils recovered from the Naashoibito Member, baenids are some of the most common. Even with the generally large number of turtle fossils, none have been identified to a given genus yet. This is because most are very fragmentary or undiagnostic.

Plastomenus, similar to a turtle found in Naashoibito Member

The Trionychidae are a group of aquatic turtles today commonly called soft-shell turtles. These turtles have been around since the Cretaceous and continue today around the world. Surface texture makes shell fragments easy to identify to the species, and one or more new species may yet be named from the trionychid material already collected from the Naashoibito Member. The only turtle skull material from this member is also from a trionychid, identified by Jasinski et al. (2011) as the right parietal of a large trionychid called Plastomenus.

A modern trionychid, Apalone spinifera, the spiny softshell turtle

Various other fragments of turtles have been recovered as well, but these are too fragmentary to assign to any specific family of turtles. I realize that this was a relatively small post, but I wanted to put this out there for the future. This is a small look at the last turtles known from the Cretaceous in this region of the world. While the age of this member has been debated for years, the two generalized camps range from either an early Maastrichtian age (~68 Mya) or later Maastrichtian (~66 Mya). I am currently working on a study into how the turtle fauna of the San Juan Basin across the K-T Boundary, which is leading to some interesting conclusions. I won't go into to much detail, but if you are going to SVP this year in Las Vegas, hopefully you will seek me out and ask me about them.

REFERENCES CITED

Hay, O. P. 1911. Descriptions of eight new species of fossil turtles from the west of the one hundredth meridian. Proceedings of the United States National Museum 38:307-325.

Jasinski, S. E., R. M. Sullivan and S. G. Lucas. 2011. Taxonomic composition of the Alamo Wash local fauna from the upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico. New Mexico Museum of Natural History and Science Bulletin 53:216-271.

Knauss, G. E., W. G. Joyce, T. R. Lyson, D. Pearson. 2011. A new kinosternoid from the Late Cretaceous Hell Creek Formation of North Dakota and Montana and the origin of Dermatemys mawii lineage. Paläontologische Zeitschrift 85:125-142.

Lehman, T. M. 1981. The Alamo Wash local fauna: A new look at the old Ojo Alamo fauna; pp. 189-221 in Lucas, S. G. (ed.), Advances in San Juan Basin paleontology. University of New Mexico Press, Albuquerque. 

Lyson, T. R., and W. G. Joyce. 2011. Cranial anatomy and phylogenetic placement of the enigmatic turtle Compsemys victa Leidy, 1856. Journal of Paleontology 85:789-801.

Sullivan, R. M., S. E. Jasinski, and S. G. Lucas. in press. Re-assessment of Late Campanian (Kirtlandian) turtles from the Upper Cretaceous Fruitland and Kirtland formations, San Juan Basin, New Mexico; in Brinkman, D., J. Gardner, and P. A. Holroyd. (eds.). Morphology and evolution of turtles. Springer Press, Dordrecht.

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.