Thursday, 1 November 2012

SVP 2012 - The Ceratopsians


 
So this time I will start off with ceratopsian news and highlight current research, some old and some new, which continues to make the study of these popular dinosaurs so fascinating.
Because morphological and ontogenetic variation within genera, as well as individual taxa, can be extreme in ceratopsians, more and more research is focussing on the processes of stratigraphy, paleogeography and taphonomy. Deanna Brandau and Randall Irmis, of the Natural history Museum of Utah, have been comparing monospecific bone beds of both centrosaurines and chasmosaurines from the middle Campanian Kaiparowits formation in southern Utah.
Chasmosaurine bone beds are uncommon and are often composed of two, maybe three individuals whilst centrosaurine beds can contain many animals – certainly into the hundreds. Typically chasmosaurine bone beds are uncommon and display a low degree of re-working in a low energy depositional scenario while, at the other end of the spectrum, centrosaurine bone beds are more common and comprise of many disarticulated elements that display significant taphonomic modelling – evidence for high energy deposition.
The Kaiparowits bone beds follow the same trend as others in Laramidia, especially those in the north when most of the bones beds are located. Why are chasmosaurines beds less common than their brethren? Because the Kaiparowits has a higher ratio of low energy, fine grained flood plain deposits than the north then you would reason that there should be a greater amount of chasmosaurine bone beds exposed – but there are not.
Because all ceratopsid bone beds are broadly deposited under the same conditions, Brandau and Irmis interpret this as good indication of behavioural and/or paleoecological differences between chasmosaurines and centrosaurines in as much that chasmosaurs may have been solitary animals or only lived in smaller groups as opposed to centrosaurs that, at least part of the time, gathered in large numbers.
Psittacosaurus is a genus of small ceratopsian known from countless specimens that have been recovered from Asia over many years and there are several taxa represented and one of these, Psittacosaurus sinensis, may have been a dwarf. Peter Makovicky et al have examined over a dozen specimens recovered from Shandong Province in China and conducted intensive histological analysis.
This analysis reveals that P. sinensis reached adulthood at small body size – especially when compared with both P. mongoliensis and P. lujiatunensis. Indeed, the authors interpret the adult size of P. sinensis to be at the same size level as a four year old of the other two taxa. Since P. sinensis is closely related to P. mongoliensis the author interpret this as the first histological evidence for dwarfism within ceratopsidae.
Despite currently having no sympatric psittacosaur taxa, the case for P. sinenesis to be a dwarf is reinforced since it is likely that the Shandong Peninsula, where this taxa is from, formed part of an island in the Early Cretaceous and, as we are all aware with other finds worldwide,, may be indicative of island dwarfism. Another taxa, P. gobiensis, is also considered a dwarf but does have a large sympatric taxa living along side and this, in comparison, may suggest niche partitioning - and niche partitioning was a recurring theme in many talks and posters this year of which there will be more later.
The same histological research and analysis used for Psittacosaurus is increasingly used in vertebrate palaeontology – so much so now that is almost becoming the norm to utilise it and so far reaching is it in our science that there is a second international paleohistological symposium next year at the Museum of the Rockies (MOR) in Bozeman, Montana. Julie Reizner, of the very same institution, has also been looking at bone histology – this time of the ceratopsian Einiosaurus procurvicornis.
Tibiae recovered and then sectioned, from a newly discovered monodominant bone bed in the Upper Cretaceous Two Medicine Formation of north western Montana, reveal that, like so many dinosaurian taxa, initial growth rates are fast. They tend to slow up after 3 to 5 years but still maintain a steady rate of growth. 
Since all the specimens tested follow the same trend, this suggests that the bone bed is made up of sub-adults and juveniles. Further research will continue with this bone bed and should reveal further insights into ontogeny and morphology in conjunction with phylogenetic and behavioural implications.
The MOR crew have been working in the Hell Creek Formation (HCF) for many years now and have built up an unparalled collection of fossils and data. Their best known work in recent times involves that well known taxon Triceratops and John Scanella et al reveal yet more data highlighting what a complex stratigraphical and diverse formation Hell Creek is.
There are currently two recognised species of TriceratopsT. prorsus and T. horridus. These taxa have clear morphological differences and are found in different stratigraphical levels. But a new collection of skulls and other elements from exposures located midway between where the two recognised taxa are found suggests the existence of what could be described as a transitional Triceratops morph since the fossils display characters of both.
MOR crews made the decision to concentrate on collecting fossils from this upper middle section of the HCF and have put together an ontogenetic sequence of this transitional animal. The fact these specimens lack clear autapomorphic distinctions lead the authors to suggest that Triceratops clearly demonstrates anagenesis although they also point out that juvenile specimens of the transitional form demonstrate what would be considered mature traits found in in T. horridus which is found immediately below.
This highlights the continuous need to cross reference all necessary data when discussing the taxanomic affinities of all non-avian dinosaurs and reinforces the fact that both stratigraphy and ontogeny are crucial in our overall understanding of evolutionary change – especially when heterochrony is involved as may be the case in Triceratops.   
 
 
No SVP meeting has been complete over the last few years without more discussion on the issue of synonymising Triceratops and Torosaurus – and this year was no exception. This time Torosaurus utahensis was the focus of a poster presented by Jelle Wiersma et al, from the Natural History Museum of Utah. As I approached the poster Jack Horner was finishing up a conversation with two of the authors and I could sense an element of defiance in them as Jack walked away – not unpleasant you understand but a sense of belief in themselves and that they believe in their research. This is the meat and drink of science – two competing hypotheses that continually test the others opposing premise and this is exactly as it should be.
The Upper Cretaceous (Maastrichtian) North Horn Formation of central Utah have revealed five chasmosaurine specimens which have been diagnosed as T. utahensis based on a combination of two autapomorphies and a unique combination of character states. Other specimens can only be identified at the generic level so remain as Torosaurus sp. The thing to bear in mind here is that Triceratops is not known from the North Horn – not that I am aware of.
To remind you, John Scanella and Jack Horner in 2010 suggested that Torosaurus was the adult morph of Triceratops and that Triceratops underwent dramatic morphological change throughout ontogeny. Torosaurus is rare in the HCF – indeed only one in ten specimens can be attributed to the taxon. In assemblages that are not so heavily sampled only Triceratops is ever recovered. However, since only Torosaurus is recovered from the North Horn the authors suggest that this is indicative that T. utahensis is indeed a valid taxon.
Otherwise synonymy of Torosaurus and Triceratops suggests that only adult animals are preserved in the North Horn – but this would seem extremely unlikely. However, sampling remains the biggest issue when discussing Torosaurus and the North Horn. Compared to the intensive collecting and sampling of the Hell Creek Formation (as mentioned earlier) then it can be deemed difficult to compare the two localities. It is also worth pointing out that the two formations are geographically separate by quite a distance.
But the authors point out that statistically the chances of only finding Torosaurus in the coeval North Horn are very slim indeed – especially if the two taxa are one and the same and they take this as powerful evidence that T. utahensis is a valid taxon and, by implication, so is T. latus.
Excellent work and keeps the debate alive and I have to say that I, for one, do not find this particular issue tiresome but, on the other hand, find it extremely stimulating and look forward to the next chapter with some relish.

References

Brandau, D.L. and Irmis, R.B. 2012 Comparative taphonomy of ceratopsid bone beds: implications of new data from Southern Laramidia. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 67-68.
Makovicky, P.J., Erickson, G.M., Gao, K. and Zhou, C. 2012 Ceratopsians didn’t just get bigger: evidence for dwarfism in Psittacosaurus.   Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp. 133-134.
Reizner, J.A. 2012 An ontogenetic study and population histology of the ceratopsid dinosaur Einiosaurus procurvicornis. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp. 160-161.
Scanella, J.B. and Horner, J.R. 2010 Torosaurus Marsh, 1891 is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae) Synonymy through ontogeny. Journal of Vertebrate Paleontology, 30 (4), 1157 – 1168.
Scanella, J.B., Fowler, D.W., Goodwin, M.B. and Horner, J.R. 2012 Transitional Triceratops: details of an ontogenetic sequence from the upper middle unit of the Hell Creek Formation, Montana.    Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp. 166.
Wiersma, J.P., Loewen, M.A. and Irmis, R.B. 2012 A re-evaluation of Torosaurus utahensis: implications for Maastrichtian ceratopsian diversity in western North America. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp. 193.

4 comments:

Andrea Cau said...

"Otherwise synonymy of Torosaurus and Triceratops suggests that only adult animals are preserved in the North Horn – but this would seem extremely unlikely."
Since there is evidence of age segregation among dinosaur species, the absence of juveniles (Triceratops morph following the hypothesis of synonymy) in the North Horn Formation could not be unrealistic.
Disclaimer: I'm totally neutral on these ceratopsid taxonomic discussions.

Mark Wildman said...

Hi Andrea. I agree and this is why sampling is a particular issue with the North Horn. I also accede to your disclaimer!

Ian Cost said...

Not to refute the claim that there is evidence of age segregation, but has that been positively identified within Triceratops genera? Can we still make that assumption of segregation if we stick to the idea of these authors that Trikes are Trikes and Toros are Toros, or does that assumption automatically mean that these are age segregated individuals of the same genus? I could see one argument of the North Horn having only "mature Triceratops" skeletons being found so far as it being some sort of breeding ground or elephant graveyard where the juveniles would not venture to, but I do think we need more North Horn exploration in order to fully answer the questions presented by the current conundrum of Ceratopsians. I hope that makes sense.

Mark Wildman said...

Triceratops bone beds are conspicuous by their absence. Indeed, I can think of only the one (see Mathews et al 2009) but this contained the remains of three individuals - all juveniles!

I agree that the concept of both synonymy and age segregation can only be resolved with yet more sampling. And even if that was achieved I am convinced that you would always need more.

At the moment I happy that there is reasonable evidence for age segregation within a variety of non avian dinosaurs since there appears to be so many bone beds that are made up of multiple specimens that are at a similar ontogenetic stage. But I remain open minded as should we all.

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