WITH MASS EXTINCTIONS, ECOLOGY MATTERS

Bininda-Emonds et al. 2007 established a detailed timescale of the phylogeny of mammals based on a new molecular database. However, their claim that the Cretaceous/Tertiary (K/T) extinction event does “… not seem to have had a substantial direct impact on the evolutionary dynamics of the extant mammalian lineages …” contradicts the extensive fossil record. Without the K/T extinction, dinosaurs not mammals would still be the dominant large animals on Earth. Deciphering the history of mammals from only molecular data is akin to claiming that human history could be understood if only the dates and ancestral tree of all humans were known. However, with this approach historical events such as the Roman Empire, the Chinese dynasties, and people such as Darwin would go unrecognized. The fossil record demonstrates that the ecological importance of species lost during an extinction is at least as important as losses of species diversity.

All large, fully terrestrial animals in the Late Cretaceous were herbivorous or carnivorous dinosaurs. The K/T extinction eliminated them and allowed mammals to evolve into the vacated ecospace. Pre-extinction mammals were small insectivores, seedeaters or omnivores. Within a few million years of the extinction, surviving mammals evolved into herbivorous and carnivorous lifestyles and became the dominant large land animals.

The fact that many modern mammals did not radiate until long after the dinosaurs became extinct is presented as a new idea, but the paleontologic record clearly shows this pattern. The post-K/T radiation of mammals did not give rise to many of the modern mammals, but lineages of modern mammals survived the K/T event and radiated later.

Even though the ancestry of groups such as the artiodactyls, Perissodactyls and carnivores can be traced by molecular studies into the Cretaceous, the actual Cretaceous fossils are difficult to recognize as members of these clades because they had not begun to evolve the lifestyles or morphologic traits of modern members of these lineages. Bininda-Emonds et al. show that most modern clades of herbivorous and carnivorous mammals originated well back in the Cretaceous, while the fossil record reveals when early members of a clade began to look and act like modern members of the clade and when they became ecologic dominants. The lines of evidence are complementary not conflicting.