There’s been quite a few news reports over the past 24 hours involving a new paper in the upcoming issue of nature entitled “The delayed rise of present day mammals.” The conventional wisdom is this; modern mammal groups were generally prevented from evolving and diversifying into the forms we would see post K/P extinction by the presence of dinosaurs. Once the dinosaurs were out of the picture, the mammals had lots of space and resources to evolve into a variety of forms. The new paper, however, suggests that modern mammal groups were already relatively diverse during the time of the dinosaurs (the paper states that this diversification occurred about 93 million years ago), but then placental mammal group diversity fell off, not to rise again until well after the K/P extinction.
While there is some amount of controversy surrounding the paper (being based on phylogenetic supertrees and some palentologists pulling the emergence dates for the mammal groups into question), the popular articles covering this story have certainly made things confusing. While many of the articles do mention that some mammal groups did diversify (only to lead to dead ends), the articles sport titles like “Dinosaur demise didn’t lead to new species” and don’t give readers the requisite background information as to what mammal groups existed during the time to get a good picture of what this study says occurred. This study also brings to mind something my paleontology professor discussed last semester (I wish I knew what paper/study he was referring to), in which the diversity of North American fauna dropped dramatically across the K/P boundary, warm-blooded and large animals (even mammals) faring worst of all. If I remember correctly, the professor stated there was only one surviving mammal group in North America, although until I find the study he was citing for myself I won’t put this forward as absolute fact. Even so, it does make sense; if North America was flash-fried by the impact of a meteor in the Yucatan, very little would survive. As I said, I am not an expert in these matters (so please correct me if I’m wrong), but in order to fully understand what happened to evolution through the K/P extinction we need not only to know what existed, but where and when (i.e. biogeography) and always be comparing the fossil evidence with the phylogenetic evidence.
I was also a bit puzzled by this inclusion into the ScienceDaily article on the topic;
The tree of life shows that after the MEE, certain mammals did experience a rapid period of diversification and evolution. However, most of these groups have since either died out completely, such as Andrewsarchus (an aggressive wolf-like cow), or declined in diversity, such as the group containing sloths and armadillos.
Andrewsarchus has long been a media-darling, despite the fact that the actual fossil material we have is exceedingly limited (the primary part being a skull, on display at the American Museum of Natural History). This beast (almost always striped like a tiger in life reconstructions) belonged to the Order Mesonychia (or Mesonychids, once thought to be the ancestors of whales), and was an angulate (hoofed-mammal), although there has been much debate over how closely related ungulate groups really are. While the Order Andrewsarchus belonged to became entirely extinct, the “agressive wolf-like cow” description does nothing but confuse. Andrewsarchus was not a cow, nor a wolf, but a variety of large carnivore that cannot be shoehorned into modern placental mammal groups, and referring to it as some sort of meat-eating cow certainly doesn’t do Andrewsarchus justice.
Like I said, while the study is interesting, I believe the treatment that it has gotten from many media outlets has been a little confusing (such is what you have [I'm assuming] non-scientists trying to cover such a story). Rather than a big paradigm shift, this study poses some new questions; why did placental mammal groups diversify, decline, and then diversify once more long after the dinosaurs became extinct? What sort of competition was there? Were modern mammal groups “held down” by other groups in the same way they weren’t allowed to diversify under the dinosaurs? If so, what caused this change that allowed the rise of modern mammal groups? Hopefully as more fossil material is collected and fine-tuning of phylogenetic study occurs, more on the oscillation of modern-mammal diversity will be illuminated, but until then it still is clear that the extinction of non-avian dinosaurs did leave the door open for mammal diversity to rise, even if it wasn’t among the lineages that would lead to extant taxa.