Alamosaurus and the North American sauropod hiatus

Hello all, I know I've really dropped the ball on this one. It's been far too long. But I'm finally back from SVP in Las Vegas and have a bit of free time to try to catch up and write some things. The meetings went rather well and, as is always the case with these things, I gained tons of information and ideas, and I can't begin to actual use all of them. That leads me in to today's post though. Jeff Wilson and Mike D'Emic, two excellent sauropod paleontologists gave two very good talks on titanosaurs, Alamosaurus, and North America at the meeting, and I will discuss them briefly here, along with some of my questions and ideas.

Image of Alamosaurus from SV-POW blog: http://svpow.wordpress.com/2009/09/02/how-big-was-alamosaurus/

Sphaerotholus (also sometimes called Prenocephale)

First, D'Emic's talk (D'Emic, 2011) focused on much of his Ph.D. research on titanosaurs, and this led into the "27 million year sauropod hiatus' in North America toward the end of the Cretaceous. This was ended, for anyone who isn't aware, by the appearance of Alamosaurus in the Late Cretaceous. D'Emic presented a very interesting phylogeny for the Titanosauriformes. Alamosaurus came out on the end of the cladogram as one of, if not the most, dervied member of the Titanosauria. D'Emic hypothesized that the sauropod hiatus was a result of regional extinction. This regional extinction may have been the result of the infusion of other taxa (such as hadrosauroids), or could have been brought about by other factors, of which we are not certain or aware of at this time. Still, many authors have hypothesized that sauropods, namely Alamosaurus, migrated to North America from South America (where titanosaurs are relatively abundant) or from Asia, where a large number of other taxa are thought to have come from, such as anykylosaurids (Nodocephalosaurus) and pachycephalosaurids (Sphaerotholus, Prenocephale). While we aren't sure yet where they came from, we do know they were here.

The presentation by Wilson (actually presented by D'Emic at the meeting) was mainly discussing whether Alamosaurus is a valid taxon (Wilson and D'Emic, 2011). As reported by Jasinski et al. (2011) Alamosaurus was named based on a nearly complete scapula as the holotype and a nearly complete ischium as the paratype, The material was collected by Reeside in 1921 in the Naashoibito Member of the Ojo Alamo Formation in the San Juan Basin of New Mexico, and named and described by Gilmore (1922).

Fig. 12 from Jasinski et al. (2011), showing A-B, USNM10486 (holotype), left scapula and C-D, USNM 10487 (paratype), right ischium, bars scales  = 10 cm.

These specimens, and this taxon have been the topic of a lot of discussion. Part of the reason for this is the fact that they are somewhat scrappy, and this means that the taxon itself is based on less then ideal material. This also makes it quite difficult to confidently refer other material to Alamosaurus. Nevertheless, the prevailing thought has been that there is a single taxon of sauropod present in North America during the Late Cretaceous. Due to the normalized acceptance of this idea, essentially all sauropod material from North America during this time has been referred to Alamosaurus. Some have conservatively identified some material to Sauropoda indet. or to Titanosauria indet., but this is a small minority. Wilson and D'Emic (2011), however, completed a thorough revision of the type material for Alamosaurus and took this re-investigation further. As was commonly the practive in the early twentieth century, Gilmore (1922) did little to offer some kind of diagnosis for a newly named taxon. Wilson and D'Emic (2011) found that the holotype (and other type) material did contain several autapomorphies, signifying that Alamosaurus is, certainly, a valid taxon.

Herd of Alamosaurus sanjuanensis

Now this was an important step to take. Many authors had used Alamosaurus in phylogenetic analyses and comparisons without being certain of its validity. Some authors, however, have other questions regarding Alamosaurus. Jasinski et al. (2011) did not question its validity, but they did, however, question whether it was being used as a garbage (or waste-basket) taxon. A garbage taxon is when where many taxa or specimens are commonly lumped together without real distinct reasons or definitions. Their thought was that all sauropod specimens were being thrown together even though they couldn't be confidently referred to Alamosaurus. Regardless of whether it was true or not, Alamosaurus was being made the only possible sauropod in North America at this time.

Who's to say there wasn't more than one sauropod taxa in North America in the Late Cretaceous?

Wilson and D'Emic (2011) attempted to address this issue as well. Due to their re-evaluation of the type material, and the material that could then be referred to the taxon through their determined autapomorphies, they were able to come up with a far more thorough diagnosis. Several of the newly referred specimens were far more complete, and this allowed for several confident referrals.This does clear up some of the confusion and some of the problems. It does not, however, clear up all of them. While Wilson and D'Emic attempted to use this as a way of saying that Alamosaurus was, indeed, not a waste-basket taxon, I must thoughtfully disagree.

Although they were able to refer more specimens to Alamosaurus, they were not able to confidently refer all Late Cretaceous sauropod material from North America to Alamosaurus. Their is still plenty of material out their that has not, or can not, be referred. I have seen several specimens of the same element that appear to be significantly different in various aspects, including morphologically. This means that Alamosaurus is still being used, in many ways, as a waste-basket taxon.

I find it somewhat unlikely that only a single sauropod taxon occupied North America from the time of a "sauropod migration" back into North America until sauropods died at at the end of the Cretaceous or just before. It seems far more likely for there to have been more taxa and, assuming they were rare (or more rare) we have not found their remains or have just not been able to positively identify them as such. Wilson and D'Emic (2011) took a very positive first step in this, and their study is very important. I hope to see a complete paper on this in the not too distant future. One thing to remember is that even some of what they do is referral by provenance, so they do have some problems with their study. Still, this may be a good first step in, not only re-evaluating Alamosaurus, but re-evaluating numerous other taxa that face similar or other key problems.

Alamosaurus defending themselves, special thanks to artist for this great artwork

Let me know if you have any thoughts on this though. I would love to know if I am vastly in the minority on this subject, or if other people share my reservations and thoughts.



REFERENCES

D'Emic, M. 2011. Early evolution of titanosuriform sauropod dinosaurs: taxonomic revision, phylogeny, and paleobiogeography. Journal of Vertebrate Paleontology 31(Supplement): 95A. (abstract) 

Gilmore, C. W. 1922. A new sauropod from the Ojo Alamo Formation of New Mexico. Smithsonian Miscellaneous Collections 72: 1-9.


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.

Wilson, J. and M. D'Emic. 2011. The validity and paleobiogeographic history of the titanosaur sauropod Alamosaurus sanjuanensis from the latest Cretaceous of North America. Journal of Vertebrate Paleontology 31(Supplement): 215A. (abstract)