A major new paper is out today on the vexed subject of dinosaur diversity and it is one that is of special importance for vertebrate palaeontology because I am one of the authors. It is also important for a bunch of other reasons but I won’t trouble you with them here. Oh, OK, if you insist…
The project was a major collaboration between various researchers at the University of Bristol and a couple of recent former members of the palaeo lab, namely Davide Pisani and myself. The intention was to look rather more closely at the issues of dinosaur diversity – could we detect when they were diversifying and expanding, and did the so-called Cretaceous Terrestrial Revolution (KTR) affect them?*
*The KTR is the massive change in terrestrial faunas that supposedly came about as a result of the evolution and subsequent radiation of angiosperms and includes major radiations by insects and mammals amongst others.
Despite my opening, anyone who reads the paper (or even just the full reference) will realise that I am a pretty minor player in all this, and my primary job was collecting and analysing various sauropod trees. At the time this work was started properly (we already had some vague discussions about it while I was based in Bristol more than three years ago now – some things take a long time to get to fruition) I was in Munich working on sauropod phylogenies, so it was an obvious task for me to get stuck into. Full credit must go to Graeme Lloyd who assembled (and a far harder task) marshalled his team to get the work done, it is no mean feat organising such a group of people to do such complex and time consuming tasks and have everyone getting the necessary data or analysis to the right people at the right time. Authorship lists do not always (if ever) reflect exactly who did how much of what work and from where I was sitting Graeme certainly deserves to be at least first and second author.
While the crux of this paper is about diversity, you need something to found your analysis on. What constitutes an increase in diversity? How do you measure that? The simple answer is with a phylogeny, which then immediately begs the question, which phylogeny do you use? How do you pick between the various competing hypotheses and trees of the evolution of various dinosaur groups? And how do you allow for the fact that most analyses focus on relationships within clades and not between them? How can you do this? Which trees do you use?
Well, we used all of them – some 155 cladistic trees by my count.*
* The number we actually checked is quite a lot higher – 240 – as we exclude or combine non-independent trees e.g. where someone has taken an existing dataset and simply added one new taxon and then republished it, plus those trees for which were not actually based on a cladistic analysis.
Subsampling of the data suggests that many if not all of these might simply be a result of collection bias (numbers of exposed areas, amount of time dedicated to digging, size of fossilerous beds etc.) and the curves are significantly flattened. There are many, large fossil beds known for the Late Cretaceous and they have been heavily studied, so it is perhaps no surprise they yield the most species.
You can actually put trees together using a series of methods to build what are called supertrees. Supertrees are not without their problems (pseudoreplication being the most obvious – some trees will feature the same underlying data many times over) but many of them can be minimised or circumvented. If you are careful and clever in what you do, you can assemble a supertree that gives you the best consensus of how science perceives dinosaur species to be related to each other. Some of you may know that this was tried before (waaay back in 2002 by another Bristol team then led by Davide and featuring Adam Yates) which featured some 277 genera, from 130 trees.
This time it has been ramped up significantly to some 420 species and actually the result is surprisingly well resolved (some 99.2% in fact). The early supertree suffered in its resolution as there were still some major contradictions and controversies at the time, notably in the relationships of the maniraptorans (where did alveresaurids go for example) and the sauropodomorphs. With subsequent analyses coming closer and closer to a consensus (and there being more and more of them), naturally the new supertree reflects this – more congruence means greater resolution, in addition to a fair few methodological tweaks and of course new taxa and analyses being included.
(Clicking on this will bring up the whole tree – warning, it’s big).
So now we have a tree and of course we have temporal data from the literature (when each species and thus each clade first appeared, and when they were around) we can turn our attention to diversity. Several analyses were done to account for different issues (e.g. ghost ranges) the primary one being a comparison of the tree shape (and of course branches) and an evolutionary model of diversification to see how they differed (i.e. where high rates of diversification was occurring). Plotted out this shows several large peaks where there was an apparently significant increase in diversity. The results show that actually the majority of diversity increases occurred in the Late Triassic and Early Jurassic and that these tended to coincide with the origins of major clades and not the Cretaceous when the KTR was taking place.
One might expect this, after all an initial radiation can potentially be explosive as a new clade expands to fill various ecological niches, but things do not always occur in this manner and all kinds of patterns are known (continual expansions, an initial radiation, then stasis, low levels of expansion followed by a late radiation etc.). Certainly there is an early increase for dinosaurs, but diversification continues to increase afterwards throughout their history, if less dramatically. Major new radiations (such as that of the derived maniraptoran theropods) do not have the impact one might expect when set against the background increase in diversity.
So did the dinosaurs take part in the KTR? While there clearly is a serious boost in diversity of dinosaurs in the Late Cretaceous, much of this can be attributed to sampling bias. More recent rocks preserve more fossils and are more easily accessed and their presence in large numbers in continental North America makes them a prime candidate for study. While the ceratopsians and hadrosaurs underwent significant increases in diversity in this period, there was no overall significant increase in diversity that could correspond to the KTR.
In short, the KTR was certainly going on, and dinosaurs diversified many times over the course of their history including during the Late Cretaceous, but the two were not linked – dinosaurs were not part of the terrestrial revolution. While there is evidence from stomach contents, coprolites and some anatomical shifts that at least some dinosaurs fed on angiosperms, this does not seem to have been a significant factor in their evolution and diversification. Hopefully, this study can also put to bed the idea that dinosaurs were already on the way out before the KT extinction, despite the sampling bias, the clade was in rude health and highly diverse when the curtain fell on their stage.
You can find more info here on the tree and the results and download the paper and copies of the tree too. We really are very good you know.
Lloyd, G. T., Davis, K. E., Pisani, D., Tarver, J. E., Ruta, M., Sakamoto, M., Hone, D. W. E., Jennings, R., and Benton, M. J. 2008. Dinosaurs and the Cretaceous Terrestrial Revolution. Proceedings of the Royal Society, Series B, in press. doi:10.1098/rspb.2008.0715