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extinction and radiation

21 july 2015

If there's one thing that everybody knows about the extinction of the dinosaurs, it's that a huge asteroid hit the Earth 66 million years ago, and asteroid winter ensued, freezing the terrible lizards out.

Well, we think we know this, and we think that scientists are sure of it, because a train of inferences and assumptions becomes for us a fact of prehistory. The asteroid itself, which left the great Chicxulub crater and the iridium layer that separates Cretaceous rocks from Tertiary ones, is a fairly huge fact, though unsuspected till about 35 years ago. The extinction of the dinosaurs is not quite so factual in the details, though. I mean, they're not around anymore, but that they vanished overnight in the wake of the asteroid is far from settled fact.

J. David Archibald, who freely describes himself as one of the leading skeptics of the catastrophic-extinction scenario, is one of the reasons it's not. Archibald's major research interest is early mammals. Mammals have coexisted with dinosaurs for most of their mutual run on the planet. (The dinosaurs' isn't over yet, because birds are dinosaurs; hence Archibald usually calls the big reptiles "nonavian dinosaurs.")

Archibald's chapters on late-Cretaceous mammals are fascinating, even if they have a bit of "and then I discovered" about them. And even if the mammals themselves are hard to distinguish, even given Archibald's scrupulous descriptions and entertaining drawings. Most mammals, 70 million years ago or so, looked basically like rats; the more outré examples looked somewhat like raccoons. But these little guys were survivors, and their very homogeneity set the stage for the "radiation" part of Archibald's title Extinction and Radiation. After the "K/T" mass extinction, the rat-like critters took off.

Archibald's discussion of the causes of that mass extinction, at the end of the Cretaceous, is a model of scientific logic. We know that a large object hit Earth 66 million years ago. We know that a lot of species went extinct at about that time, including all nonavian dinosaurs. Coincidence??

Well, as Archibald shows, it actually could be coincidence. Large things land on Earth all the time, and mass extinctions happen when they don't, or don't happen when they do. Or everything could have been going swimmingly, or trompingly, and then boom! an entire fauna wiped out. But of course entire faunas weren't wiped out: crocodiles, and frogs, and mammals themselves seemed barely to notice the big boom.

Or it could be that a number of other trends, like extreme volcanism and recession of the inland seas that characterized Cretaceous continents, contributed, and the big impact was just another contributor, or the straw that broke the dinosaur's back. If volcanism and receding shorelines were cramping the style of nonavian dinosaurs to begin with (and the best evidence, Archibald shows, suggests they were), then the asteroid was "a literally Earth-shattering event [that] magnified the differences between the 'have' and 'have not' species" (61-62). A bottleneck that might have opened into a new era of dinosaurs was stoppered all at once by the impact.

The beneficiaries, Archibald notes, were mammals. The small scurrying mammals of the Cretaceous did not out-compete the majestic dinosaurs by eating their eggs or something; if you learned that in school, as I did, you must unlearn it. No, the volcanoes and asteroids finished off the big guys, and left an evolutionary blank slate for the little guys.

In his final chapter, Archibald deals with the confounding contradictions between fossil evidence, which places most of the evolutionary radiation of mammals after the K/T extinctions, and molecular-clock evidence, which suggests that mammals diverged into their present lineages well before the K/T boundary 66 million years ago, in fact long before some of the famous dinosaur species like T. rex even evolved. What gives? Archibald can't be sure, but he suspects that the very littleness of early mammals holds the key. Early-Tertiary mammals reproduced quickly and colonized their newly-empty world very rapidly. After they radiated into marsupials and primates and glires and ungulates, their generations became much longer, and their clocks slower. (And there may have been special circs associated with the K/T catastrophe that disrupted the clock.) His argument seems cogent to me; it depends on noticing that the clock is consistently off from the fossil record, across nearly a score of diverse evolutionary groups.

I'm always amazed and delighted by the theories of paleontologists, who boldly deduce vast prehistorical stories from a bunch of old bones. We cannot suspend the telling of such stories, even though we realize that the next bunch of bones will prove them false. J. David Archibald does an exceptional job in Extinction and Radiation with the vast but fragmentary evidence at hand.

Archibald, J. David. Extinction and Radiation: How the fall of dinosaurs led to the rise of mammals. Baltimore: Johns Hopkins University Press, 2011. QE 861.6 .E95A73