A paper in this week’s Science uses a lot of data to construct the most complete phylogeny yet of mammalian families. Meredith et al. used 26 genes to not only construct the tree, but estimate divergence times. Their sample comprises 97%-99% of the roughly 150 described mammalian families. Here’s the tree they get (click to enlarge; lots of detail can be seen by zooming as well):
All the nodes except for the ones denoted by solid blue circles are strongly supported. The background shows the transition from light gray (Mesozoic) to white (Cenozoic).
A few highlights for me (and I’m not a mammalogist):
- As Christofer M. Helgen points out in a Perspectives piece in the same issue, families prove to be good monophyletic groups: clades like bears, cats, and dogs all indeed fall into families determined earlier on morphological grounds.
- No surprises for major groupings: monotremes (platypus and echidna) are the most distant relatives of living mammals, equally distantly related to the marsupials (as expected, a monophyletic clade) and the placentals.
- The red panda (Ailuridae) is more closely related to procyonids and mustelids (e.g., skunks and raccoons) than to the giant panda, which is in the family Ursidae (bears; the panda’s status as a bear has been known for forty years). This is probably not a surprise to anyone who knows mammals, but I haven’t kept up with mammalian systematics.
- There are tons of bat families, and they’re well diverged from everything else. Their closest relatives appear to be the families that include whales, rhinos, hippos, giraffes, cats, and pangolins, and not the rodents, which are far more distantly related.
- All marine mammals are a monophyletic group, meaning that they shared a single common ancestor, and the closest relative of those whales, dolphins, and porpoises are the hippos.
- The closest relatives of monkeys and apes (and us) comprise the other primates: the lemurs. No surprise there, but the next most closely related group comprises the Dermoptera (the “flying lemurs” which, as noted below, ain’t lemurs) and, after that, the rodents.
- Sloths, armadillos and anteaters group together in a clade that is distantly related to most other mammals, but a bit more closely related to aardvarks, tenrecs, dugongs, and elephants. And all of these groups diverged from other mammalian families a long time ago—on the order of 100 million years.
- Finally, for connoisseurs of paleobiology, the the origin of new orders (a taxonomic level higher than families; orders comprise what most of us think of as “types” of mammals: carnivores, bats, primates, rodents, and so on) occurred after the “KPg” (Cretaceous-Paleogene; formerly called the Cretaceous-Tertiary) extinction event about 65 million years ago. This extinction wiped out the dinosaurs, of course, but also many marine invertebrates and terrestrial plants. The authors posit that this extinction event played an important role “in the early diversification and adaptive radiation of mammals,” perhaps by opening up the “ecospace available for mammals.” That’s not a new theory: we’ve all heard, for example, that the extinction of the dinosaurs allowed mammals to diversify, but it’s good to see the dates of diversification confirmed in this way.
Meredith, R. W. et al. 2011. Impacts of the Cretaceious terrestrial revolution and KPg extinction on mammal diversification. Science 334:521-524