Wednesday, 9 March 2011

More on Ceratopsians


I thought it was time for some more selected snippets from last years SVP meeting, this time focussing on ceratopsians. I’ve been doing a few of these retrospective posts on last years meeting simply because not everyone has access to these details and new information is always appreciated about work going on in the world of vertebrate palaeontology.

To the detail then and yet more data that may throw more light on the Triceratops/Torosaurus dichotomy. Penkalski and Skulan displayed an interesting poster about remains from two partial chasmosaurs from a quarry in Carter County, Montana (Hell Creek Formation). This is interesting since the authors interpret the remains as showing characteristics from both Triceratops and Torosaurus.

Examination of various elements such as the parietal, squamosal, dorsal vertebrae and humerus all display morphological differences, some subtle, others more so, that appear to indicate the presence of both taxa in the quarry. The authors suggest that the remains may indicate the presence of a new taxon (unlikely in my opinion), or that these are sexual diamorphs of Triceratops or that these are, indeed, specimens of both Triceratops and Torosaurus that were simply fossilised together after deposition.

Bearing in mind the continual discussion regarding the synonymy of both taxa, I do tend to favour that these specimens are both Triceratops but it will be interesting to see if there will be a detailed taphonomic study of this quarry.

This brings me nicely to Clayton et al’s poster demonstrating a revised look at the epiparietal homology of chasmosaurs. The “et al” in this instance is the not to be sniffed at trio of Mark Loewen, Andy Farke and Scott Sampson, so the data here is clearly not to be ignored.

They have come up with a new method of classifying chasmosaurine epiossifications which, in conjunction with both newly discovered centrosaurs and chasmosaurs from the Kaiparowits Formation, may help in the continual discussions regarding what are and what are not valid taxa. They reach a number of conclusions but, of course, the most interesting outcome is that Torosaurus is indeed recovered as a valid taxon, distinct from Triceratops. The authors also list other reasons for this such as the existence of morphs of both taxa that are of similar size, the apparent existence of sub-adult torosaurs, and the fact there are formations where fossils of Torosaurus are found but not Triceratops. Thus this debate looks likely to rumble on for some time.

Denver Fowler’s presentation, on the other hand, demonstrates how a combination of stratigraphy and ontogeny influence morphology in chasmosaurines. Using this framework reveals that southern chasmosaurs are likely to be earlier Maastrichtian variants of a Triceratops clade. Adult morphological traits of these earlier forms occur earlier in ontogeny than they do in Triceratops thus supporting earlier trends already highlighted within the clade.

Additionally, it appears that these same trends occur in the late Campanian and two new chasmosaurs from the Kirtland Formation in New Mexico come out as intermediate taxa between Pentaceratops and Anchiceratops, thus consolidating earlier work showing Pentaceratops and Anchiceratops as sister taxa.. Because of this combination of morphological traits that are clearly separated stratigraphically, the author interprets this data as good evidence of anagenesis within Chasmosaurinae. The paper, when published, which demonstrates the new framework and the methodology used, will be compelling reading. Incidentally, Fowler was also co-author of a poster with Liz Freedman, utilising similar techniques describing anagenesis in Hadrosaurs.

Finally, Pachyrhinosaurus lakustai is a ceratopsian known from a monodominant bone bed in Alberta, Canada. The various sized recovered elements were classified as adult, sub-adult and juvenile and, to demonstrate whether this hypothesised ontogenetic growth stage actually had a factual base, Joseph Fredrickson from the University of Wisconsin, ran a detailed cladistic analysis using 67 hypothetical growth characters from 42 specimens.

Fascinatingly, instead of the expected three stage growth series, the analysis reveals a continual ontogenetic growth which appears to split at maturity and may be indicative of sexual dimorphism. Although both morphs have seven identical traits at maturity, one clearly displays another seven defining characteristics including a rostral comb and a tall nasal boss. The other morph is also characterised by six unique traits including a long nasal boss (as opposed to tall) and straight caudal parietal horns. Thus, this analysis has revealed that original diagnostic characters for the taxon are, in fact, only present in one of the adult skulls (probably male) and not in the other skulls – so these are almost certainly female



References

Clayton, K, M. Loewen, A. Farke, and S.D. Sampson 2010 A re-evaluation of epiparietal homology within chasmosaurine ceratopsids (ornithischian) based on newly discovered taxa. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 73A.

Fowler, D.,2010 Anagenesis and long term morphologic trends in Chasmosaurinae (Dinosauria: Ceratopsidae) revealed by a new high resolution chronostratigraphic framework, ontogenetic analysis and description of two new taxa. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 91A.

Frederickson, J., 2010 Craniofacial ontogeny in Pachyrhinosaurus lakustai: evidence for sexual dimorphism in an ornithischian dinosaur. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 92A.

Freedman, E and D. Fowler.2010 Stratigraphic correlation of Judith River Formation (Campanian, Upper Cretaceous) exposures in Kennedy Coulee (north central Montana) to the Foremost Formation (Alberta): implications for anagenesis in hadrosaurid dinosaurs. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 92A.

Penkalski, P and J. Skulan.2010 An unusual ceratopsid quarry from the Hell Creek Formation of Montana. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 145A.

2 comments:

Traumador said...

nice pics of the tyrrell's old ceratopsian display. old skool! they bring back memories. mind you the new ceratopsian display is a lot more comphrensive and impressive.

saurian said...

Good spot Traumador! I was looking through some old photos I took in the Tyrrell back in 1999 and thought they were nice to share.

Thankfully, although museums and science move on, the skulls and bones remain eternal.

Post a Comment