Lucky dinosaurs

ScienceDaily has an intriguing article titled Good Luck, Not Superiority, Gave Dinosaurs Their Edge, Study Of Crocodile Cousins Reveals

Back when dinosaurs first started to evolve to the types we have hear about (or have seen in Jurassic Park), there were several competing groups of animals that might evolve to the dominant species. Among those were the ancestors to the modern day crocodiles, the crurotarsan archosaurs, which together with dinosaurs formed the Archosauria group (which now consists of the crocodiles and the decedents of the dinosaurs - the birds).

It has been assumed that dinosaurs had some kind of edge over the other groups, including their cousins, the proto-crocodiles. The research mentioned in the ScienceDaily article, however shows otherwise.

The researchers examined the evolutionary pattern of dinosaurs and crurotarsans in the Late Triassic. Using a very large dataset of anatomical characters – nearly 500 features of the skeleton – and a new family tree of the entire archosaur group, they measured evolutionary rates and morphological disparity (a measurement of the range of different body plans and lifestyles that a group has).

They found no difference in the rates at which dinosaurs and crurotarsans were evolving. This was surprising as, if dinosaurs were truly 'superior' or 'out-competing' crurotarsans in the Triassic, they should be expected to evolve faster. Instead, crurotarsans were keeping pace.

The results for the second measure, morphological disparity, were even more remarkable. Crurotarsans had a much higher disparity than dinosaurs in the Triassic. In other words, crurotarsans were exploring a larger range of body types, diets, and lifestyles. This greatly contrasts with the classic image of dinosaur superiority since their greatest competitors, the crurotarsans, were doing so much more.

To these surprising results can be added two other, previously known, findings: crurotarsans were more abundant (more individuals, more fossils, more species) than dinosaurs in many Triassic ecosystems, and crurotarsans were in some cases more diverse (greater number of species). Putting all this together, it is very difficult to argue that dinosaurs were 'superior' to crurotarsans, or that they were out-competing crurotarsans.

So, it's debatable if dinosaurs actually were the dominant species when looking at the period as a whole. Why then the impression that they were? Well, first of all, it's not easy to tell fossils from the two subgroups apart, so in the past many crurotarsan fossils were considered dinosaur fossils. Second of all, dinosaurs won out in the end, continuing after the crurotarsans died in great numbers. This is also explained in the ScienceDaily article

Steve Brusatte, who conducted the research while an MSc student in Bristol University's Department of Earth Sciences, said: "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."

When we talk about randomness in evolution, this is the sort of things we mean. It's a typical case of a major impact occurrence which for some reason affected the one group of animals more than the other.

The Science article by Brusatte et al is behind a paywall, but can be found here: Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs