You’ve heard the story of how an asteroid slammed into the Gulf of Mexico about 65 million years ago, igniting fires on the ground and sending solar debris high into the atmosphere. In the millennia that followed, harsh environmental conditions wiped out more than 75 percent of the planet’s species. Most dinosaurs died and mammals rose in their ashes. This dark period of extinction is called the KT mass extinction and marks the boundary between the Cretaceous and Tertiary Periods in the geologic record. But a new study challenges that view by suggesting that mammals were killed at rates similar to the dinosaurs. Mammals simply recovered better than their dinosaur counterparts.
Write in the Journal of Evolutionary Biology, a group of British biologists offers a portrait of the KT mass extinction that deviates from conventional wisdom in a number of ways. First, their reassessment of fossil evidence shows that mammalian species suffered as much as dinosaurs during the asteroid climate disaster. And second, biodiversity returned to the planet faster than previously thought. In some areas, rich ecosystems flourished just 200,000 years after the asteroid impact. Previous studies have estimated that it took at least a million years for several ecosystems to return.
The researchers say our understanding of the catastrophe 65 million years ago has been distorted by both an incomplete fossil record and observational bias. The animals most likely to be wiped out by mass extinctions are those that can only live in a specific, small habitat. These are also the same animals that are least likely to be preserved in the fossil record simply because there were so few of them and they lived in only one place. So we very likely underestimated how many mammal species died out during the KT because we didn’t account for all these rare species that lived in small areas with low population sizes.
The researchers inside Journal of Evolutionary Biology say that animals with wide ranges, living in many habitats, have “about a 40 percent chance of survival,” while those “species that are found in only one place are estimated to have a one percent chance of survival.” To capture the diversity of species across large and small ranges, the researchers analyzed fossils from 145 mammal species found at 23 sites in North America. They concluded that it is very likely that many of those “single locality” mammal species became extinct without leaving a trace in the fossil record.
The researchers also found that current ideas about why dinosaurs didn’t survive the KT mass extinction may also be wrong. A common hypothesis is that dinosaurs became extinct because they were so big. Essentially, they were unable to satisfy their hunger as plant life died out in a world where debris clouds blocked the sun for years. But after the asteroid impact, several mammal species quickly evolved into quite large ones. So it was clearly not just size that drove the dinosaurs to extinction. And it wasn’t a lack of intelligence either, because winged dinosaurs (also called birds) failed to beat mammals despite their intelligence.
“Some other aspect of mammalian biology, for example adaptations for nocturnal foraging, must have prompted mammals to survive where these dinosaurs did not,” the researchers write. “The fact that on each of the different paleocontinents, recovery proceeds in a broadly similar fashion – on each continent, mammals survived and emerged as the dominant land animals – suggests that mammal survival and radiation were determined by their biology, not by a random event.” Of course, mammals with a wide range were naturally more likely to survive than their localized cousins.
That said, some of the first species to bounce back after the mass extinction were rare mammals. As small patches of ecosystem recovered around the world, mammal species diversity flourished. Perhaps this diversification happened precisely because the environment recovered on this patchwork, creating “islands” of life separated by relative wastelands. It is even possible that dramatic diversification in species is a common consequence of mass extinctions in general. From the mass death comes a huge transformation and explosion of life. And somehow, for reasons we don’t yet understand, mammals were in a unique position to succeed.
Journal of Evolutionary Biology2016. DOI: doi: 10.1111/jeb.12882