Turn the clock back 230 million years, and the land was covered with big, toothy reptiles.*** But as many a nine-year-old can tell you, not all of them were dinosaurs. Some were "crurotarsans," a lineage that all but died out just as the dinosaurs were acheiving global domination. Today, the only crurotarsans are the crocodiles. But alas! It all could have been so different, according to research published in Science today by Stephen Brusatte, of Columbia University, and colleagues.
The Age of Dinosaurs may have been a matter of luck, they say: just a matter of which group was hit harder by a mass extinction 200 million years ago. Before then, for nearly 30 million years, dinosaurs and crurotarsans had vied for superiority in a classic Darwinian struggle.
And the crurotarsans should have won, the scientists argue. After analyzing the fossils of 64 species, they found the beasts had a greater variety of body plans - and evolved new species at about the same rate - as dinosaurs. They take this as evidence that dinosaurs weren't innately superior creatures (otherwise, the reasoning goes, more dinosaur species would have evolved as they usurped the crurotarsans). In the race for supremacy, it wasn't that the dinosaurs outpaced the crurotarsans - it's more like the crurotarsans were felled in the home stretch by a calamity.
But hang on a second. I'm all for exciting new theories that offer explanations no one's thought of before (i.e., prairie-stalking pterosaurs). But this logic sounds wonky in a few places. Does a lack of species divergence have to mean an ecological stalemate was going on? Or could it mean that the species in existence at that time were doing phenomenally well on their own? For that matter, might the rapid appearance of new species signal a sputtering lineage, dying out in a flash of ill-fated new forms?
More problematically, how does a mass extinction kill nearly all the members of one group (crurotarsans) without destroying a similar number of the other (dinosaurs)? That doesn't sound like the luck of the draw; it sounds like one of those groups had a competitive advantage - what the regular person might call "superiority."
Full disclosure: I'm not a paleontologist. Perhaps these are well-thought-through ideas that the authors lacked the room to explain in their paper. (If so, I'd love it if a real paleontologist would write in and educate me.) Maybe the authors imagine that a different kind of mass extinction (meteoric fireball vs. global warming, for instance) could easily have switched the tables and led to an Age of Crurotarsans.
But then, the crocodiles did survive, apparently content to hide out in the swamps for 200 million years while the dinosaurs enjoyed their 135 million years of fame - and then died out. Come to think of it, maybe the crurotarsans are superior after all.
***To be fair, there were also plenty of small and medium sized reptiles, some with rather ordinary teeth.