Mammals were almost destroyed with the dinosaurs 65 million years ago

Watch out! (credit: Donald E. Davis,)

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 have small, localized populations, which are the same animals that are least likely to appear in the fossil record because there were so few of them. So we've underestimated how many mammal species died during the K-T because we didn't account for rare species that lived in small areas at low population size.

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