Yesterday I posted about a new paper in Nature which, through phylogenetic study, suggests that modern mammal lineages diversified earlier than once thought and the extinction of non-avian dinosaurs did not increase the lineages diversity. PZ, Larry Moran, and Mike Dunford have all covered this story as well, but there is one important facet of the study that I think is missing from many of the discussions about it popping up around the blogosphere. If the paper does accurately reflect what occurred to lineages that would lead to modern monotreme, marsupial, and placental mammal groups, this does not mean that there was no diversification of mammals after non-avian dinosaurs went extinct. Indeed, the paper itself states the following;
The supertree therefore contains no evidence that the diversification rate of the extant mammalian lineages increased soon after non-avian dinosaurs went extinct. Although there is strong palaeontological evidence that mammalian diversity, driven by a massively elevated rate of speciation, generally rose rapidly immediately after the K/T boundary, there is in fact no conflict between the palaeontological and neontological interpretations of the known facts. Most diversifications immediately after the K/T boundary were in groups such as multituberculates, plesiadapiforms and ‘archaic’ ungulates, as plots of the numbers of genera known in each sub-epoch indicate. These groups declined or went extinct early in the Cenozoic era and so are barely, if at all, represented in the phylogeny of living species. The continuing low rates of extant mammalian diversification through this period imply that the dearth of Palaeocene crown-group fossils is a real reflection of the low diversity of those clades. The low rates are also consistent with (but not direct evidence for) the hypothesis that extant lineages were inhibited in some manner by the diversity of the predominantly Palaeocene groups, and only started to diversify with the decline of the latter. However, like most other proposed competitive exclusion scenarios (for example, see refs 24, 25), this conjecture is based purely on the negative correlation of taxon diversities rather than direct evidence of exclusion.
While this may be a “minor” theme of the paper, I find this aspect of it particularly exciting. If the study is correct, then extant mammal lineages diversified once before the K/T extinction, but did not do so again until later when many of the other “archaic” mammal groups became extinct. Indeed, it almost seems like modern mammal groups could not catch a break, and if this holds up it would be extremely interesting to find out why now-extinct mammal groups were so successful while extant groups were not, and what caused such a reversal in fortune. Even forgetting about the K/T extinction for a moment, if mammals diversified long before the extinction, what led to this diversification? These are questions that certainly beg answers.
While the idea that extant mammals like opossums crawled out of the ash a little after the K/T extinction and led an unstoppable march of progress towards you and I is preposterous, I can’t see how the extinction of many groups of animals would not “set the stage” for further diversity and evolution. The process was likely more chaotic than previously thought, but it seems apparent that some (now extinct) mammal groups did diversify in the wake of the K/T extinction, although the reasons for this will require further investigation. I’m sure that there will be some debate over this paper for a long time to come, but hopefully paleontologists, paleoecologists, and those undertaking phylogenetic studies will be able to communicate each other to make sense of the fossil and genetic data. Either way, the extinction of the dinosaurs did mark a major landmark for mammals, and how all members of that group responded to the changes around them is something that will require much more study to ascertain.
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.