You've heard the story about how an astroid smashed into the Gulf of Mexico roughly 65 million years ago, lighting fires on the ground and sending sun-blocking debris high into the atmosphere. In the millennia that followed, harsh environmental conditions wiped out over 75 percent of species on the planet. Most dinosaurs met their demise, and mammals rose in their ashes. This dark period of die-outs is called the K-T mass extinction, and it marks the boundary between the Cretaceous and Tertiary periods in the geological record. But a new study challenges that picture by suggesting that mammals were killed off at rates similar to those of the dinosaurs. Mammals simply recovered better than their counterparts among the Dinosauria.

Writing in the Journal of Evolutionary Biology, a group of British biologists offers a portrait of the K-T mass extinction that diverges from conventional wisdom in a couple of ways. First, their reassessment of fossil evidence shows that mammal species suffered just 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 were thriving in as little as 200 thousand years after the asteroid impact. Previous studies have estimated that it took at least a million years for diverse ecosystems to return.

The researchers say our understanding of the catastrophe 65 million years ago has been warped both by an incomplete fossil record and observation bias. The animals that are 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 just one place. So we've very likely underestimated how many mammal species died during the K-T because we didn't account for all these rare species that lived in small areas at low population size.

The researchers in Journal of Evolutionary Biology say that animals with large ranges, who live in many habitats, "have a roughly 40 percent chance of surviving," while those "species occurring in just a single locality are estimated to have a one percent chance of surviving." To capture the diversity of species in large and small ranges, the researchers analyzed fossils from 145 mammal species found in 23 locations across North America. They concluded that it's extremely likely that many of those "single locality" mammal species went extinct without leaving a trace in the fossil record.

The researchers also found that current ideas about why dinosaurs didn't survive the K-T mass extinction might be wrong, too. One common hypothesis is that dinosaurs died out because they were so huge. Essentially they were unable to slake their appetites as plant life died out on a world where debris clouds blocked the sun for years. But after the asteroid impact, several mammal species rapidly evolved to be quite large. So obviously it wasn't size alone that killed off the dinosaurs. And it wasn't lack of smarts either, because winged dinosaurs (aka birds) didn't manage to out-compete mammals, despite their intelligence.

"Some other aspect of mammalian biology, for example adaptations for nocturnal foraging, must have predisposed mammals to survive where these dinosaurs did not," the researchers write. "The fact that on each of the different palaeocontinents the recovery plays out in a broadly similar way—on each continent, mammals survived and emerged as the dominant land animals—suggests that the survival and radiation of mammals were determined by their biology, not a random event." Also, of course, mammals with a broad range were 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 across the globe, mammal species diversity bloomed. Perhaps this diversification happened precisely because the environment recovered in this patchwork fashion, creating "islands" of life that were separated from each other by relative wastelands. It's even possible that dramatic diversification in species is a common outcome of mass extinctions in general. Out of mass death there evolves a massive transformation and explosion of life. And somehow, for reasons we don't yet understand, mammals were uniquely positioned to succeed.

Journal of Evolutionary Biology, 2016. DOI: doi: 10.1111/jeb.12882