Guest Post - Roland Sookias Discusses His New Study Examining How Dinosaurs Came to Fill Most Ecological Niches During the Mesozoic

Why were dinosaurs, and other archosauromorphs (the group of animals including crocodiles, dinosaurs, pterosaurs and several other extinct groups), so big? Was natural selection for increasing size responsible for archosauromorphs’ dramatic rise to larger sizes and did selection for decreasing size drive therapsids’ (‘mammal-like reptiles’, which were the dominant land vertebrates before archosauromorphs) reduction in size during the Triassic (see picture)? These are the questions which my, Richard Butler’s and Roger Benson’s recent publication in Proceedings of the Royal Society B – “Rise of dinosaurs reveals major body size transitions are driven by passive processes of trait evolution” – attempts to answer.
Most of the work in the paper was done as part of my MSc thesis project, which Richard, Roger and Andrew Smith supervised. To carry out the project I spent a good deal of last summer collecting femur and skull length measurements (which we used as proxies for body mass) in the Natural History Museum Library, London. Though barely seeing daylight for a month or two, I managed to collect measurements for ~200 species, which, in combination with data from Benson et al. 2011 got us to >400 species in total. To answer the questions above we focused on getting data for archosauromorphs and therapsids from the Late Permian to Middle Jurassic. This allowed comparison between the two groups, and the interval brackets the rise of archosauromorphs to become the dominant terrestrial vertebrates, replacing therapsids. Thus it allowed us to look at body size evolutionary dynamics during a major faunal transition.

Archosauromorphs (dinosaurs, crocodiles, pterosaurs and their relatives) increased greatly in maximum size (black line, black triangles) from the Permian to Jurassic, and therapsids (‘mammal-like reptiles’) decreased. However in both this was due to expansion in the size variance (i.e. both size increase and decrease), but with subsequent extinction of larger therapsid species. Figure from Sookias et al. 2012, Proc Roy Soc B.

Once we’d got the data together we analysed them using maximum likelihood model fitting approaches. We tried both phylogenetic – i.e. incorporating evolutionary relationships – and time series (ignoring within-group evolutionary relationships and simply averaging size within time ‘bins’) models. Time series models confirmed that on average archosauromorphs tended to increase across the time interval, and that therapsids got smaller. However when we included phylogeny (evolutionary relationships) we found that there was no directional trend in either group along individual lineages. Thus the apparent trends through time were in fact due to ‘passive expansion’ in size, but as the original size was nearer the bottom than the top of the eventual size range the average size tended to increase (see picture). We thus can say that the long-repeated idea of “Cope’s rule” – that taxa in a clade tend to get larger over time due to within-lineage natural selection for larger body sizes – is not found in either archosauromorphs or therapsids during this time interval.

Our work excludes larger size in archosauromorphs as an explanation for their success, as if larger size was especially beneficial one would expect a directional evolutionary trend towards larger sizes. Instead, archosauromorphs probably replaced therapsids opportunistically, as many have hypothesized before. However, the exceptionally high growth, and thus reproductive, rates of archosauromorphs may have allowed them to re-fill empty ecological niches especially easily and rapidly after they went empty due to extinction of therapsids. Thus, while size and growth rate probably did not allow archosauromorphs to outcompete therapsids, it did allow them to fill up free niches quickly.

We also found that archosauromorph predators exceeded the size of the largest herbivores – anomodont therapsids – during the Middle-early Late Triassic. This finding – that the largest carnivores are larger than herbivores - is extremely rare in ecosystems throughout time. It demonstrates that extinct archosauromorphs really were exceptionally large, and that they were able to grow to larger sizes than therapsids given the same resources.

Well, there’s not much more to say about that paper except hope you enjoy it! However, we should be publishing some more work based on my MSc thesis in the near future, so stay tuned, and I’ve just started a PhD with Richard Butler on the early archosauromorph radiation, so hopefully I’ll be involved in answering a few more interesting questions in the coming years. Finally, a very big thank you to Bill Parker for giving us a guest slot here on the esteemed Chinleana.

The paper’s full citation is:


Sookias, R. B., Butler, R. J., Benson, R. B. J. (2012). Rise of dinosaurs reveals major body size transitions are driven by passive processes of trait evolution. Proceedings of the Royal Society B.
doi: 10.1098/rspb.2011.2441 



Abstract- A major macroevolutionary question concerns how long-term patterns of body-size evolution are underpinned by smaller scale processes along lineages. One outstanding long-term transition is the replacement of basal therapsids (stem-group mammals) by archosauromorphs, including dinosaurs, as the dominant large-bodied terrestrial fauna during the Triassic (approx. 252–201 million years ago). This landmark event preceded more than 150 million years of archosauromorph dominance. We analyse a new body-size dataset of more than 400 therapsid and archosauromorph species spanning the Late Permian–Middle Jurassic. Maximum-likelihood analyses indicate that Cope’s rule (an active within-lineage trend of body-size increase) is extremely rare, despite conspicuous patterns of body-size turnover, and contrary to proposals that Cope’s rule is central to vertebrate evolution. Instead, passive processes predominate in taxonomically and ecomorphologically more inclusive clades, with stasis common in less inclusive clades. Body-size limits are clade-dependent, suggesting intrinsic, biological factors are more important than the external environment. This clade-dependence is exemplified by maximum size of Middle–early Late Triassic archosauromorph predators exceeding that of contemporary herbivores, breaking a widely accepted ‘rule’ that herbivore maximum size greatly exceeds carnivore maximum size. Archosauromorph and dinosaur dominance occurred via opportunistic replacement of therapsids following extinction, but were facilitated by higher archosauromorph growth rates.



Popular press coverage:

http://news.discovery.com/animals/how-dinosaurs-got-so-big-120131.html

http://news.sciencemag.org/sciencenow/2012/01/the-secret-of-dinos-success.html?ref=hp