The mammalian tree is rooted deeply and branched early!
That’s the message of a new paper in Nature that compiled sequence data from 4,510 mammalian species (out of 4,554) to assembly that lovely diagram above. Challenging the ‘conventional wisdom’ that mammalian diversity is the product of an opportunistic radiation of species after the dinosaurs were wiped out at the end of the Cretaceous 65 million years ago, the authors instead identified two broad periods of evolutionary expansion among the mammals: an early event 100-85 million years ago when the extant orders first appeared, and a radiation of modern families in the late Eocene/Miocene. A key point is that there is no change in rates of taxon formation across the Cretaceous/Tertiary (K/T) boundary—mammalian diversity was rich before the dinosaurs disappeared.
In addition to the pretty tree, the authors plotted lineages through time of these mammalian groups—the plots pass throught the K/T boundary (the red line) without even a stutter. The second graph below shows rates of diversification, and the peaks occur well before and well after the K/T boundary.
Now the question is … why the delay? Why weren’t modern mammals expanding into new niches once the dinosaurs disappeared? One reason might be that the early exploiters were oddball (from our perspective, at least) groups like the archaic ungulate and multituberculates (ratty creatures, now extinct) that would have filled those places first.
Another is a matter of perspective. Big animals are the most obvious creatures in a biome to our eyes, but they are not the most important or diverse—we have a perceptual bias for the charismatic megafauna. The KT extinction was devastating for just those animals we are attuned to recognize most readily, but may not have been quite so troubling to others; the bacteria didn’t even notice, insects would have gone on buzzing (except those with dependent parasitic relationships), plants may have seen one minor class of leaf-nibblers disappear, the rat-like and lizard-like creatures of the undergrowth might well have found their environments relatively unchanged. That transient period of intense trauma might not have been as disturbing to the overall life of the planet as we big apes might like to imagine.
One other important lesson along the same lines is that mammalian diversity arose quietly and gradually among small creatures; it is again a consequence of our bias that we imagine the triumphs of mammalian evolution were the appearances of elephants and whales and tigers and of course, large apes. The important business of evolution was going on in creatures not even big enough to reach our knees, small scurriers who were founding independent dynasties 100 million years ago. We just didn’t pay much attention to them until some subset of their descendants grew large enough to trample us or eat us.
Bininda-Emonds ORP, Cardillo M, Jones KE, MacPhee RDE, Beck RMD, Grenyer R, Price SA, Vos RA, Gittleman JL, Purvis A (2007) The delayed rise of present-day mammals. Nature 446:507-511.