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.