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.
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
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.
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.