Friday, 25 November 2011

SVP 2011 - Hadrosaurs & Ceratopsians

Research into hadrosaurs and ceratopsians continues to be popular and vibrant and there was some fascinating data discussed at SVP. Prosaurolophus maximus is an understated hadrosaurine from the Campanian of the Dinosaur Park Formation in Alberta but despite being represented by multiple articulated specimens, the taxon has received little attention in the way of both morphological and phylogenetic analysis.

But Christopher McGarrity of the University of Toronto has looked at the cranial characteristics and ontogenetic features closely and found some interesting detail. P. maximus displays an allometric solid nasal crest that, as appears to be so often the case these days, is probably a deirved feature for sexual selection and recognition.   
P. maximus, as well as a second species,  P.blackfeetensis from the Two Medicine Formation of Montana, were included in a new phylogenetic analysis which recovered the two species  as sister taxa and the author states that the morphological differences between the two are hard to quantify and is indicative that P. blackfeetensis is a junior synonym of  P.maximus.  This suggests that Prosaurolophus has an extended chronological range of 1.6 million years and that is actually a long time when you consider species longevity.
I’ve already mentioned one poster regarding unusual skin preservation in a specimen of Gryposaurus but just why does hadrosaur skin appear to preserve more readily than other dinosaurian taxa? Matt Davis, of Yale University, has provided some detail. Hadrosaurs have been, and continue to be, heavily sampled due to the fact that many Upper Cretaceous Formations are exceptionally well known and it is also the peak period when hadrosaurs were incredibly numerous and diverse. The early 20th century saw an abundance of hadrosaur skin fossils recovered, especially from the extensive fossil fields of the Cretaceous Western Interior.

They are also nearly always found in sandstone exposures which tend to preserve more hadrosaur skin impressions than other facies. And although hadrosaurs were more abundant than other clades in these exposures, there is still an obvious bias towards hadrosaurian skin preservation. And yet despite these quantative statistics there is still no obvious reason to explain this unusual bias.
Just because hadrosaurs were abundant and the formations they are found in are extremely well known there is still no physical evidence to theorise and account for this unusual skin preservation. One can see that it is not easily explained but the one theory that cannot be ruled out is that hadrosaurs did indeed have some form of morphological skin condition or unusual behaviour that aided this unusual preservational bias.
Everybody is aware that the hadrosaur dental battery was a miracle of evolution and produced one of the most efficient plant processing machines that ever evolved. Greg Erickson and Mark Norell have conducted an extensive histological study of hadrosaur cheek teeth and have found that they were truly complex and even more specialised than mammal teeth. One particular tissue identified on the teeth was coronal cementum and this has actually been proposed as evidence for superior advancement in mammalian dentition. The study provides further evidence and enhances the fact that hadrosaurs possessed a truly remarkable and innovational dental arrangement.
Moving on to ceratopsians and the following focusses, again, on the on going research into dinosaurian synonymy. To begin with is a study of two large “Triceratops” skulls at Brigham Young University by Andrew Beach who re-evaluated the specimens after the very high profile synonymizing of Triceratops and Torosaurus last year. The first specimen, BYU 12183, was unfortunately shown to have extensive restoration and this covered much of the original skull morphology. But the second skull, BYU 19974, has suffered no such remodelling and this turned out to be very much more interesting indeed.
The specimen, which essentially comprises the frill, displays all the ontogenetic markers of an adult Triceratops and yet the parietal is actually thinning down where you would expect to find the parietal fenestrae in Torosaurus. This appears to provide yet more powerful evidence that Triceratops and Torosaurus are the same taxa and the author describes BYU 19974 as an ontogentically transitional form.

Triceratops fossils are remarkably abundant in the Hell Creek Formation and this provides ample opportunity to research gross morphology, ontogeny and phylogeny in this taxon taking into account stratigraphic variability. John Scanella et al have performed just such an analysis focussing on cranial variation in multiple specimens of Triceratops and the results are illuminating and indeed reinforce current thinking and thus open new doors which may affect dinosaur taxonomy as a whole.
It is apparent that ontogenetically identical specimens of Triceratops, even from the same stratigraphic level, can demonstrate astonishing variation. As expected, there is always variation in cranial ornamentation and there are significant morphological differences within individual animals. More surprising is that suture fusion, both within the skull and epi-ossifications, is not necessarily a sign of a mature animal and also that some sub-adult animals are actually bigger than adults. This makes such features unreliable as indication of adulthood. The implications of this are obvious when you consider something like Nanotyrannus where the sutres of the type skull (CMNH 7541) are fully fused and was one of the principle reasons why the taxon was raised in the first place instead of being identified as a juvenile Tyrannosaurus – which is the general consensus today.
Furthermore, histological evidence indicates that even the larger, more mature animals were still undergoing morphological change even at an advanced ontogenetic stage. Many other morphologic variations were revealed by the study and suggest that if other dinosaurs were also subject to the same rapid morphological change then a rapid rethink is necessary when quantifying the amount of dinosaurian taxa in any given formation. Incidentally, the authors report no evidence for sexual dimorphism in Triceratops and suggest that gender recognition was a much more subtle affair and may have been indicated by a visual signal, perhaps even coloration.
From the same stable, and featuring three of the same authors, comes yet more data indicating that it is not just Triceratops and Torosaurus that need to be synonymized. Denver Fowler et al introduce us to unified frames of reference which is a combination of methodologies combining, not only morphology and ontogeny, but stratigraphy, phylogeny, geography and tapohistory.
Not only has Triceratops displayed multiple morphologies at different ontogenetic stages, the same can be said for other chasmosaurines and it appears that both Mojoceratops and Kosmoceratops may both be juvenile morphs of Chasmosaurus sp. and Chasmosaurus irvensis respectively. Similarly, but at the other end of the ontogenetic stage, Titanoceratops displays morphologies that you would expect to find in older specimens of Pentaceratops.
A blast from the past
There are further implications from this research that focusses on biogeographical distribution and faunal endemism and that is currently a very heavily researched and debated subject in the palaeoworld, especially where Laramidia is concerned, and this on-going and important work will make a valuable contribution to our understanding of the diversification of dinosaurs and the environments they inhabited.
Beach, A. 2011. Triceratops and Torosaurus synonymy: an evaluation of two large specimens from Brigham Young University. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp68.
Davis, M. 2011. Complete census of published fossil dinosaur integument quantifies taphonomic bias towards prevalence of hadrosaurid skin. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp96.
Erickson, G. and Norell, M. 2011. The histology of hadrosaurid dinosaur teeth – reptiles that exceeded mammals in dental complexity?   Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp105.
Fowler, D., Scanella, J. and Horner, J. 2011. Reassessing ceratopsid diversity using unified frames of reference.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp111.
McGarrity, C. 2011. Cranial morphology and variation in Prosaurolophus maximus with implications for hadrosaurid diversity and evolution.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp155.
Scanella, J., Fowler, D., Trevethan, I., Roberts, D. and Horner, J. 2011. Individual variation in Triceratops from the Hell Creek Formation, Montana: implications for dinosaur taxonomy.    Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp187.


Anonymous said...

I usually say that this past century was mostly building up in our knowledge of what dinosaurs were and now we're entering in an era in which we're actually knowing who dinosaurs were.

I won't be surprised if a lot of supposed different genera were actually the same genus or species. The Nanotyrannus/T. rex juvenile debate might be close to an end (as much as an end this sort of debates can have) now due the Montana’s “Dueling Dinosaurs”.

Mark Wildman said...

Hi Henrique - I completely agree with you. The MOR crew have certainly made everyone think and their arguments are quite convincing.

As for the Nanotyrannus debate - well that still has a long way to go in my opinion. There are a couple of papers in the not too distant future which apparently will "confirm" the validity of the genus - we'll have to wait and see. Until then it's a juvenile Tyrannosaurus until proven otherwise.

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