For more than 20 years, paleontologists have argued mammals began diversifying some 90 million years ago, prior to the disappearance of the dinosaurs.

New research contradicts the narrative, suggesting mammals diversified in the wake of the dinosaurs' departure. Though a few primitive mammal species coexisted with the dinosaurs, it was the ecological space left in the wake of Cretaceous–Paleogene extinction event that allowed the diversification of mammals and the emergence of mammal lineages we recognize today.

Researchers at Queensland University of Technology say biases in evolutionary models have long overestimated the ancient origins of mammal diversification.

"It now appears that the major diversification of placental mammals closely followed the extinction of dinosaurs 66 million years ago, an event that would have opened up ecological space for mammals to evolve into," QUT evolutionary biologist Matthew Phillips said in a news release.

Phillips used the same molecular dating techniques -- measuring the rate of DNA evolution to precisely date fossils -- but took extra precautions against potentially biased data inputs.

"I re-examined fossil calibrations," Phillips explained, "excluding those that were contentious or based on poorly resolved fossil placements and also fossil calibrations from within groups of very large or long-lived mammals, such as whales, for which parallel changes in the rate of DNA evolution in different lineages could distort dating estimates."

"When I took the remaining set of calibrations, the major diversification of placental mammals coincided with the extinction of dinosaurs," he concluded.

Molecular dating has offered a narrative of mammal diversification that doesn't fit with the fossil record. Many of the mammal lineages we know and recognize today -- modern primates, rodents, insectivores, artiodactyls, and carnivores -- can be traced to the fossil records just after the Cretaceous–Paleogene extinction event.

With molecule dating biases accounted for, Phillips and his colleagues say the molecular dating and fossil records now match. The new research is detailed in the journal Systematic Biology.