More and more biologists are beginning to realize that the history of life is not as determined by natural selection as they once thought. They are beginning to take to heart the idea that if you rewind and rerun the tape of life it will not turn out the same. A lot of the history is due to chance, luck, and accident.
This week's issue of
Science contains a paper by Brusatt et al. (2008) discussing the evolution of dinosaurs. The dinosaurs (Dinosaura) rose to prominence around 200 million years ago in what is often called an "adaptive radiation." The idea was that dinosaurs outcompeted all other land animals because they were better able to adapt to new environments.
At the beginning of this period, the main competitors were the crurotarsans, animals that resembled dinosaurs in many ways but which aren't classified as dinosaurs. Crocodiles are crurotarsans but all other families have gone extinct.
Recent work has revealed that the crurotarsans were as diverse and abundant as the dinosaurs 200 million years ago. The authors explain the problem ...
The critical interval to consider is the Late Triassic, especially the Norian and Rhaetian (Fig. 1), a 28-million-year span between the CNEE [Carnian-Norian extinction event] and TJEE [Triassic-Jurassic extinction event]. The key "competitors" of the early dinosaurs were the crurotarsans, the "crocodile-line" archosaurs, which show a range of morphologies and adaptations during this time: long-snouted fish- and flesh-eating phytosaurs, armored herbivorous aetosaurs, and large to giant carnivorous "rauisuchians." The crurotarsans even replicated many dinosaurian body plans (large terrestrial predators; small swift predators; mid- to large-bodied low-browsing herbivores; agile bipedal herbivores). Several new discoveries show striking convergences between crurotarsans and dinosaurs (10), and many Triassic crurotarsans were previously erroneously identified as dinosaur ancestors (11) or even as true dinosaurs (12). Such morphological convergence suggests that dinosaurs and crurotarsans were exploiting similar resources in the Late Triassic. In some Norian faunas, crurotarsans were numerically more abundant than dinosaurs (3) and seem to have exploited a wider range of body plans. However, by the end of the Triassic all crurotarsans were extinct, save a few lineages of crocodylomorphs.
The key question is why the major dinosaur lineages survived the TJEE, ushering in the 135-million-year "age of dinosaurs," while most crurotarsan groups went extinct.
Brusatte et al. measured evolution rates, speciations, and morphological disparity for the two groups (crurotarsans and dinosaurs) before the Triassic-Jurassic extinction event. They found no evidence that the dinosaurs were evolving more quickly or were becoming better adapted than the crurotarsans.
So, if crurotarsons were so successful why did they die out and why did the dinosaurs survive? Maybe the dinosaurs weren't better adapted, maybe they were just lucky.
The lead author of the study, Steve Brusatt, an M.Sc. student, puts it very well in the press release [
Good Luck, Not Superiority, Gave Dinosaurs Their Edge, Study Of Crocodile Cousins Reveals].
"If we were standing in the Late Triassic, 210 million years ago or so, and had to bet on which group would eventually dominate ecosystems, all reasonable gamblers would go with the crurotarsans. There was no sign that dinosaurs were eventually going to succeed so why did they? The answer is two mass extinction events: the dinosaurs not only got lucky, but they got lucky twice.
"They first weathered the storm during the Carnian-Norian event 228 million years ago, but so did the crurotarsans. In contrast, many other potential competitor groups went extinct. Then dinosaurs weathered a second, much bigger, storm 200 million years ago. This was the end Triassic extinction event, which was a sudden and catastrophic extinction caused by rapid climate change, possibly facilitated by an asteroid impact. Strangely, and suddenly, all crurotarsans except for a few lineages of crocodiles went extinct. On the other hand, the dinosaurs did not. They survived and then radiated in the Early Jurassic, and very quickly established themselves as the dominant vertebrate group on land across the world.
"Why did crurotarsans go extinct and not dinosaurs? We don't know the answer to that, but we suspect that it was nothing more than luck, plain and simple.
This paper is relevant for a number of reasons unrelated to the history of dinosaurs.
- It shows a trend away from pure adaptationist thinking toward consideration of other explanations (e.g., accident).
- It emphasizes the importance of understanding mass extinctions and incorporating them into macrevolutionary studies. The conclusions echo those of David Raup in his book Extinction: Bad Genes or Bad Luck [see Good Science Writers: David Raup].
- It is relevant to the discussion about convergent evolution—a feature of the history of life used by theistic evolution proponents to indicate that there's a plan to evolution [Convergence]. Note that there are many examples of convergence in the crurotarsan and dinosaur lineages. Similar examples with marsupials and placental mammals are used as evidence that evolution may have had a purpose in mind. But if we apply the same reasoning to crurotarsans and dinosaurs, the purpose becomes less obvious, since both groups eventually become extinct.
[Image Credit: The drawing of a crurotarsan archosaur is from the Palaeos website, specifically Archosauromorpha: Rauisuchiformes]
Brusatte, S.L., Benton, M.J., Ruta, M. and Lloyd, G.T. (2008) Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs. Science 321:1485-1488. [DOI: 10.1126/science.1161833]
S. L. Brusatte, M. J. Benton, M. Ruta, G. T. Lloyd (2008). Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs Science, 321 (5895), 1485-1488 DOI: 10.1126/science.1161833
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The 37th edition of 
