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Brusatte et al. 2008



...claims that dinosaurs may have been "lucky" in their successful passage through extinction "events", events which destroyed crurotarsans. The competing hypothesis is that dinosaurs were superior in some way and beat out their competitors because of some crucial adaptation (e.g., they were endothermic). In the paper, the latter model is questioned due to their finding that the two clades "lived side by side for 30 million years, and crurotarsans occupied more morphological space and were often more abundant and diverse than dinosaurs."

The paper brings alot of excellent data to bear...but the conclusion that the "dinosaurs were (likely) the beneficiaries...of some good luck" does not stand up.

Firstly, there is no discussion of the many possible critical adaptations that could have given dinosaurs a real edge--indeed, the idea of any dinosaurian "superiority" is couched in terms of a dusty old notion that has "long pervaded the literature". Further, it is suggested that research into competition hypotheses is not that profitable because they are difficult to test and vague. This is true of course. But it is also true that factors that are difficult to test because they are lost in geological time, might have happened nevertheless. And there are some prime adaptations that potentially gave dinosaurs advantage that are not considered--increased parental investment, for example.

But more than this, I personally find the "lived side by side" argument unconvincing...and if it pleases the group I would like to drop an analogy on you all for your consideration:
I was listening to some big band jazz. I was struck by the limited role of the guitar. It was often restricted to the role of rhythm instrument. There was more diversity in the brass section. Occassionally, the guitarist would step forward and take a solo, but it was surely the brass players that were the stars of the day. So, what happened to allow the guitar to claim the dominant role in popular culture that it has occupied since the fifties? It was not because all the sax, trombone, and trumpet players were hit by a train; it was because a critical adaptation--amplification--changed the game. So, the fallacy behind the "lived side by side argument" is the assumption that organisms don't change, don't acquire game-changing adaptations; or, if they do, that their competitors can always counter-adapt.


Finally, here we go again with the luck of the extinction event. But unlike the end Cretaceous extinctions where there is at least a large body size differential between the winners and the losers, in the extinction events noted in the paper between dinosaurs and crurotarsans, there are no reasons proposed other than chance. Yes, chance plays a role in evolution, but it is increasingly being used to "explain" things as a kind of default, i.e., when we can't test a hypothesis the phenomena must be due to chance. This doesn't follow. And yet the notion is increasingly pervading the literature.