Thursday, 13 October 2011

Alamosaurus and the Naashoibito

In the latest edition of the Journal of Vertebrate Paleontology, there was an interesting paper by Michael D’emic et al describing a titanosaurid pes from the Naashoibito member of the Kirtland Formation in New Mexico. The specimen (NMMNH P-49967) displays synapomorphies referable to a titanosauriform neosauropod and is almost certainly referable to Alamosaurus sanjuanensis, although additional remains would be needed to confirm validity.

The specimen, however, is from a very large sauropod indeed and, when limb proportions are compared and scaled up, is comparable to both Brachiosaurus altithorax and Paralititan stromeri. Alamosaurus is the only recognised Late Cretaceous titanosaur from North America but its taxanomic status has always been subject to review (eg Sullivan & Lucas 2000) but this paper reaffirms its belief that the Alamosaurus holotype is diagnostic and should be considered a valid genus.

The interesting thing for me in the paper is the referral, yet again, to the geological position of the Naashoibito Member and whether it belongs to the Ojo Alamo Formation or not. There is a consensus these days, however, regarding the age of the Naashoibito and it is now generally accepted to be Maastrichtian. The type specimen of Alamosaurus was recovered from the Ojo Alamo Sandstone which, during 1980, had been considered part of the Naashoibito Member of the Kirtland Shale (Kues et al 1980).

However, the Ojo Alamo is amongst the stratigraphically highest horizons for dinosaur remains, so high in fact that there has been occasion to suspect that a few dinosaurs survived into the Palaeocene but this has been hotly contested (Fasset et al 2002, Lucas & Sullivan 2000) and is generally not considered these days but, as these things do, rumours persist. Today, the Ojo Alamo appears to be distinct from the Naashoibito and is a particularly interesting formation in its own right because it does indeed straddle the Maastrichtian/Danian contact.

From Williamson & Weil 2008
  • The general consensus (it appears to me) is that the Naashoibito is part of the underlying Kirtland Formation demonstrating a Lancian fauna or, as is generally known, the "Alamosaurus fauna". It has been recognised for some time now that there have been distinct Maastrichtian faunas and this provincialism is perhaps better recognised from earlier Campanian times but it is a fact that faunal provincialism continued right through to the end of the Cretaceous.
The paleoenvironment of the Alamosaurus fauna was characterised by valleys that were situated between mountain ranges and were frequented, not only by titanosaurs, but by hadrosaurs, ceratopsians and theropods. The Hell Creek Formation, by contrast, represents a coastal plain environment that was warm, humid and wet and was dominated by ceratopsians, hadrosaurs and tyrannosaurids – but no sauropods. The Scollard and Willow Creek formations demonstrate a third environment described as a semiarid, alluvial plain and contains remains of ceratopsians, hadrosaurs, ankylosaurs and tyrannosaurs but, again, no sauropods.

It is this provincialism that has kept apart Alamosaurus and Tyrannosaurus, thus preventing a possible (and very popular) clash of these titanic animals – but maybe not for much longer. Tyrannosaurus appears to have been able to exist in a range of paleoenvironments and a specimen recovered from the North Horn Formation of Utah (Sampson 2005) was the first example of the giant carnivore apparently co-existing with Alamosaurus.

Just when Alamosaurus appeared in the Late Cretaceous is still subject to debate but it seems that titanosaurs appeared during the Campanian around 73 million years ago although their exact point of origin remains undetermined. Suggestions include a possible migration from South America to the north due to the appearance of a land bridge between the continents. This may have been due to climate change since the Western Interior Seaway was also receding at this time and this may have created the ideal conditions for the migration and establishment of a relatively localised population of sauropods. Another possible titanosauriform doorway into North America may have been via Asia but this appears to lack any substantial fossil evidence at this moment in time.

It is obvious that there are vast gaps in our knowledge regarding this fascinating stage in dinosaurian history including unanswered questions that include, amongst others, sauropod phylogeny, paleoecological implications and, of course, the geology and stratigraphy of the San Juan Basin as a whole. It is apparent that the best is still to be revealed by both the Ojo Alamo and Naashoibito and one can’t help but be intrigued by these fascinating formations

Footnote

There will be yet more detail revealed regarding the taxanomic status and paleobiogeography of Alamosaurus at this years SVP conference in November and I will report on this after the conference has finished and embargo lifted.

From Russel 1992
The above image is a poor quality scan but demonstrates the San Juan Basin quite well. This is Ojo Alamo Spring revealing the yellowish sandstonesof the Naashoibito. The Ojo Alamo Formation is exposed on the horizon in this shot.

References

D’emic, M.D., Jeffrey A. Wilson & Thomas E. Williamson (2011): A sauropod dinosaur pes from the latest Cretaceous of North America and the validity of Alamosaurus sanjuanensis (Sauropoda, Titanosauria), Journal of Vertebrate Paleontology, 31:5, 1072-1079

Fassett, J. E., R. A. Zielinski, and J. R. Budahn. 2002. Dinosaurs that did not die: evidence for Palaeocene dinosaurs in the Ojo Alamo Sandstone, San Juan Basin, New Mexico; pp. 307–336 in C. Koeberl and K. G. MacLeod (eds), Catastrophic Events and Mass Extinctions. Geological Society of America Special Paper 356.

Kues, B. S., T. Lehman, and J. K. Rigby Jr. 1980. The teeth of Alamosaurus sanjuanensis, a Late Cretaceous sauropod. Journal of Paleontology 54:864–868.

Lucas, S. G., and R. M. Sullivan. 2000. The sauropod dinosaur Alamosaurus from the Upper Cretaceous of the San Juan Basin, New Mexico. New Mexico Museum of Natural History and Science Bulletin 17:147–156.

Russel, D.A. 1992. The “Modern” Cretaceous in An Odyssey in Time: The Dinosaurs of North America. University of Toronto Press in association with the National Museum of Natural Sciences. ISBN 0-08020-7718-8.

Sampson, S. D., and M. A. Loewen. 2005. Tyrannosaurus rex from the Upper Cretaceous (Maastrichtian) North Horn Formation of Utah: biogeographic and paleoecologic implications. Journal of Vertebrate Paleontology 25:469–472.

Sullivan, R. M., and S. G. Lucas. 2000. Alamosaurus (Dinosauria: Sauropoda) from the late Campanian of New Mexico and its significance. Journal of Vertebrate Paleontology 20:400–403.

4 comments:

Thomas Holtz said...

One solution that might serve us in the vert paleo community (although not sure how the stratigraphers would like it): raise the Naashoibito to Formation status, so people a) don't care if it belongs to the Kirtland or the Ojo Alamo and b) don't get hung up on confusing lithostratigraphic boundaries with isochronous ones...

Denver Fowler said...

A lot of this is incorrect.

Formations are mappable units. The Naashoibito is distinct from the underlying De-Na-Zin Mbr of the KIRTland Fm, but less distinguishable from the overlying Kimbeto Mbr of the Ojo Alamo, hence the confusion regarding Paleocene dinosaurs (etc). It is for this reason some of us keep it in the Ojo Alamo Fm.

The figure you use from Williamson and Weil misrepresented the stratigraphic views of Lucas and Sullivan (check their 2006 paper on the "Kirtlandian"; or read the erratum in the following JVP).

Tyrannosaur material was known from Alamosaurus-bearing units long before Sampson and Loewen's paper: Lawson (1976) for example (the source of most of the problems correlating Maastrichtian units of the SW).

The photo caption is also incorrect: there is no Fruitland Fm exposed at Ojo Alamo spring. The Fruitland Fm is up to 594m (max combined thickness of the De-na-zin, Farmington, & Hunter Wash Mbrs of the Kirtland Fm) below the Naashoibito.
The most up-to-date summary of Alamosaurus stratigraphy can be found in our upcoming APP paper (Fowler & Sullivan, in press).

Mark Wildman said...

Hi Denver and thanks for the comments - I figured you might have an opinion or two. Firstly I've sorted out the "Kirtland" typo - should have spotted that in the first place.

The object of this post was really to highlight what I believe are a couple of really interesting formations and perhaps introduce them to those not so familiar with them. The fact that Alamosaurus is involved made the piece more interesting.

I appreciate the information regarding the figure from Williamson & Weil (20008) and will add the appropriate amendment in the upcoming post after SVP. Again, I used the figure to give people an idea of the geological units involved and their relationship with one another.

I also knew about the various tyrannosaurid elements in the Alamosaurus beds before (from you in fact!) but for simplicity I highlighted the Sampson (2005) example as it was more substantial and made the relevent point.

I used the scanned image of Ojo Alamo Spring since that was all I had really and, again, thanks for pointing out the error - I've edited accordingly!

Finally, I look forward to your new APP paper with great interest and am grateful for your input and interest in the blog.Good luck!

Denver Fowler said...

Whoops, I take it back. The figure was the regional strat figure, not the publications comparison one. Sorry.

Back in 2006, we found a nice shed Tyrannosaurid tooth (SMP-VP-2352) with a couple of damaged Alamosaurus cervicals. Tyrannosaur material is still pretty rare in the Naashoibito, but we do find some. Steve Jasinski et al recently figured most of the Naashoibito material collected by the SMP fieldwork, including a large tyrannosaurid scapula-coracoid that we found in ?2004? (Jasinski, et al., 2011). As big as a rex. You can find that paper on Bob Sullivan's website:

http://www.robertmsullivanphd.com/Publications.html

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