Saturday, October 11, 2008

Were the dinosaurs "lucky"?

A recent article in Science by Brusatte, Benton, Ruta and Lloyd proposes that dinosaurs may have been "lucky" in gaining dominance over their initial competitors, the crurotarsans, which are currently represented only by crocodilia. The setting is the late Triassic of 228 to 200 million years ago, beginning with the Carnian-Norian boundary and ending with the Triassic-Jurassic mass extinction brought on (according to the most common theory) by a massive outpouring of lava during the breakup of Pangaea.

The crurotarsans went mostly extinct at the Triassic-Jurassic boundary, while many dinosaur groups did not, setting the stage for their diversification and dominance in the "the age of dinosaurs"- the Jurassic. The question is- what caused this differential survival? Were the dinosaurs continuously competing with and defeating the crurotarsans during the late Triassic, or did they just catch a lucky break during the boundary period?

The authors compile morphological series for lineages in each group during the late-Triassic interval, graphing out how they diversified and changed with time. Their conclusion is that "There is no clear evidence for differences in overall evolutionary rates between dinosaurs and crurotarsans during the Triassic as a whole." So, they propose that whatever the ecological competition between late crurotarsans and early dinosaurs, each held its own until the Triassic-Jurassic boundary, favoring the "lucky break" characterization. Indeed, the crurotarsans were more diverse and generally more abundant during this period, so they were not only holding their own, but thriving.

This closely resembles the debate about the later rise of mammals- were the dinosaurs losing diversity well before the end of the Jurassic, or were they fine right up until the asteroid impact at the Cretaceous-Tertiary boundary? Here increased sampling and better statistics have been important, demonstrating that what had previously been thought of as a gradual decline of dinosaurs going into the boundary might better be thought of as a statistical error attributable to having a continuous set of data truncated at zero at one boundary, which can show up in some trend averaging statistics as a downward curve into that boundary.

Be that as it may, the concept of a "lucky break" in evolution is problematic. I'm no expert, being more of a molecular biologist. But having battled and despaired through Steve Gould's magnum opus, I have some interest in how the language of the field works.

Firstly, luck can be a moral concept. Luck is something we didn't merit or work for- something that falls into our lap, good fortune. Bad luck is likewise something we did not deserve, meaning that we took reasonable foresight and lived a blameless existence, and still the storm destroyed our house. Luck comments on how our moral qualities of foresight and adherence to social norms stack up against eventual results, all of which has nothing to do with evolution. By this meaning, luck signifies some extra quality of deservedness by which success or failure are judged. Did the Dodo bird get its just deserts by being so trusting of humans, or was its demise simply bad luck? From an evolutionary perspective, success or failure are not judged by any other criterion- they are neither undeserved nor deserved, but occur as a matter of fact, not value.

Luck could be defined more objectively as beating the odds. If you win the lottery, you are lucky by definition. But that depends on knowing the odds, which is exactly what we do not know about the evolutionary crises of the past. It is inconceivable that the differential survival of dinosaurs versus crurotarsans was due entirely to random chance- where a comet hit, or who fell into a volcanic rift. There must have been relatively long-term climate conditions at play, such as extended extreme temperatures or years without sunlight, which caused the die-off. These are conditions that do not kill randomly, but kill those without the necessary traits (preadaptations) to weather extremely harsh conditions, (thermoregulation, hibernation, etc.). Just because we are ignorant of either the conditions or the adaptations at work does not justify using the term "luck".

The one thing we can say is that the dinosaurs had nothing to do with the occurrence of whatever caused the end-Triassic environmental crisis, and could thus be termed "lucky" in benefiting from this rare event. But the same can really be said of any condition that separates the fit from the unfit- some are lucky to have the traits they need to survive, and others are unlucky to not have them. It is all equally luck in this case, irrespective of how dramatic the events look to us in retrospect.

Switching to a better terminology, one might ask whether natural selection is more relevant to slow episodes of change where one group of organisms gradually encroach on ecological niches occupied by others, rather than to sudden events that indiscriminately cut down many groups, from which few survive. In a word, no. The latter may seem more fortuitous than the former, and would not even involve morphological change, but both are just as much episodes of selection- equally products of variation plus differential survival. No adaptation is aforethought, so whether a differentiating trait is one of tiny scope that allows one more acre to be colonized in a forest, or is a capacity (perhaps like endothermy) that allows survival past a catastrophic epoch, each trait is a product of the past and its survival or loss into the future is best termed "natural selection". It would be anthropomorphization to cheer on one group or another, or to project onto them values like luck, determination, destiny, etc. Only if we knew that trait differences played no role at all in that differential survival would "luck" be operative in a bare statistical sense, and this is unlikely in the extreme.

Surely, evolutionists who are paleontologists are more comfortable with long series of small changes that add up to a recognizable morphological progression- a narrative, if you will, of evolution in action combining presumed cause with observed effect. If local seeds get tougher, and the beaks of birds get bigger, we might say one caused the other, by way of variation and natural selection. Is survival through a catastrophe by virtue of preexisting adaptations any different? Not really, except that our view of it is quite different. The marker is not morphological change through time, but the absence of competing species- differential survival in the starkest possible terms, though usually with little clue as to why that survival occurred.

So the real question this paper gets us only slightly closer to is: what happened to the crurotarsans? If they were abundant right up to the Triassic-Jurassic boundary, then this mass extinction made all the difference. Calling this difference in extinction fates "luck" neither fairly describes the general theory involved, nor tells us what the difference was ... what actually happened. It is a rhetorical marker for ignorance and really should not have been the primary hook for media coverage.

We are responsible for a mass extinction right now. Will our successors 200 million years in the future call the animals that are now being extinguished "unlucky"? Perhaps.

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