Smithsonian.com

A family tree of early mammals showing diversity through time. The red line marks the end-Cretaceous mass extinction; notice that some groups go extinct or dwindle after this line. Modified from Luo, 2007 in Nature.

Had the non-avian dinosaurs not been wiped out 65 million years ago, our species would probably never existed. The mass extinction that struck at the end of the Cretaceous was one of the major events in earth’s history that greatly affected evolution by pruning back the tree of life, and it was in the wake of the extinction that mammals became the dominant vertebrates on land. What scientists have been trying to figure out, however, is why mammals survived while the dinosaurs perished.

According to Penn State researcher Russ Graham, the lifestyles of mammals gave them an advantage when the asteroid struck the area that is today’s Yucatan peninsula about 65 million years ago. In response to a “probing question” published on the university’s website, Graham opined that mammals that used burrows or lived in aquatic environments would have been shielded from the intense heat that briefly followed the impact. Once the heat was off, mammals could come back out and make the most of the remaining food resources. There may not have been enough food for dinosaurs, but the more generalized tastes of mammals allowed them to hang on.

Yet the Cretaceous mass extinction is not quite so clear cut. Much of what we know about the last days of the dinosaurs has come from North America, close to the site of impact, so how the pattern of extinction emerged elsewhere in the world is still poorly understood. And, contrary to popular perceptions, mammals did not escape the extinction event unscathed. Several groups of mammals most people have never heard of (like the triconodontids, spalacotheroids, dryolestids and multituberculates) perished right at or not long after the extinction event. Some groups of mammals did survive, but others were either wiped out or so reduced in diversity that, like the dinosaurs, they fell into extinction.

Mass extinctions are the greatest murder mysteries ever known. Figuring out why some forms went extinct and others survived is no easy task, and I doubt that even the mythical deductive powers of Sherlock Holmes could have resolved the puzzles facing paleontologists. The survival of some mammals is itself just one mystery embedded in a more perplexing question, and scientists are still busily collecting evidence from the scene of the crime.

[Image modified from Luo, Zhe-Xi. (2007) "Transformation and diversification in early mammal evolution." Nature, 450 , pp. 1011-1019]