Of Mammals and Mass Extinctions
9 May, 2007 01:10 pm
It's a story that everyone knows. Some 65 million years ago a giant asteroid slams into the Earth. The resulting catastrophe spreads around the world, snuffing out the majority of animals, including the dinosaurs, which had to that point been the dominant land fauna for the preceding 160 million years. In the new landscape, new opportunities arise, particularly for those small furry animals that had largely escaped notice living in the shadow of the dinosaurs for at least 135 million years. So began the Tertiary - the Age of Mammals - and a crucial event in our own history: an open-and-shut case for the textbooks.
Although the Tertiary is indeed the Age of Mammals (if not of birds as well), early on it's not that of the mammals we commonly think of, namely the ones that we see around us today. This is one of the key results of a new study published in Nature by a worldwide team of researchers who examined the evolutionary dynamics of the mammal radiation on the basis of a supertree containing 99% of all 4554 species of living mammal. The unprecedented completeness of the tree means that they could, for the first time, statistically identify those times in the history of mammals when the group was either undergoing a dramatic increase or decrease at the rate at which new species and lineages were evolving, evolutionary hotspots and notspots as it were.
Two time periods were found to be especially important for the extant lineages. The largest peak in diversification rate (i.e., net rate of increase in the number of lineages) occurred between 100 and 85 million years ago coincident with the origins and first divergences within the placental superorders and orders, the major groups of mammals that we recognize today (e.g., primates, rodents, carnivores, or bats). A second peak took place starting some 50 million years ago, representing the real flourishing within the orders to generate the lineages that are present today.
But what was happening between these times, especially at the Cretaceous-Tertiary boundary when the dinosaurs went extinct? The surprising answer was "not a lot", or at least nothing out of the ordinary. Although the extant lineages were around at the time and diversifying steadily, diverse statistical tests showed that the mass extinction event had no real effect on their net diversification rate whatsoever, for good or for bad: rates before the boundary were statistically identical to those after the boundary. But, it was the Age of Mammals, so why weren't the mammals taking advantage of the opportunity handed to them?
Again, the answer is that they did indeed flourish after the extinction of the dinosaurs, and the fossil record shows this quite clearly, but it's just that other, now wholly extinct mammalian groups such as multituberculates (rodent-like forms), plesiadapiforms (primate-like forms) and "archaic" ungulates were quicker off the mark. The present-day mammals, instead, had to bide their time for another 15 million years until they got their chance at the Early Eocene faunal turnover, another event clearly written in the fossil record. This apparently delayed rise in present-day mammals that can be gleaned from their evolutionary tree explains the known gap that exists for these groups in the fossil record before about 55 million years ago. The very lack of information defining the gap meant that paleontologists previously could not explain it fully: had the groups simply not arisen yet or, if they had, had we simply not found the fossils yet, possibly because the groups were in relatively low abundance so as to not leave many in the first place? Taken together, the historical information in both the supertree and the fossil record point to the latter solution: the lineages of present-day mammals were indeed around, as they had been in the Age of Dinosaurs, but in such low numbers that they too went largely unnoticed. This answer is not uncontroversial, however, with many paleontologists strongly questioning the Cretaceous antiquity of the present-day groups in the absence of any fossil evidence for it whatsoever.
Hiding behind this controversy, however, is the supertree itself. For the first time ever, mammalian biologists now have a virtually complete evolutionary tree of the group at their disposal. We can now attempt to answer questions at the scale of all mammals that have long intrigued us. For instance, why are there nearly 2000 species of rodent, especially given that there are only about 80 species of rabbits and pikas, the sister-group of rodents that arose at the same time and from the same common ancestor? More importantly, can we use this kind of information to inform our conservation decisions? In other words, can we associate differences in group sizes with differences in their extinction risk, or use the evolutionary information more generally to identify those endangered species containing the most unique evolutionary history in their genes? We are standing on the edge of yet another mass-extinction event, but this time mammals will probably not be the ones to benefit from it.
Bininda-Emonds, O.R.P. et al., 446 507 - 512 Nature , March 29, 2007