Such disputes have been waging for decades, since researchers first began gleaning evolutionary detail from proteins and DNA. But the skirmish over placental mammals — animals that give birth to live offspring that are in late stages of development, including whales, mice and humans — began with a paper published early last year, arguing that the group diversified only after those dinosaurs that did not evolve into birds went extinct, 65 million years ago.

For that study, Maureen O’Leary, an evolutionary biologist at Stony Brook University in New York, and her team spent several years characterizing and analyzing thousands of traits in dozens of living and fossil mammals. The team combined those characteristics with genetic data to build a giant tree of life, showing how different placental mammals related to one another.

But to establish when the different creatures evolved, the researchers looked only at the fossil record. They concluded that the earliest placental mammals appeared only after the asteroid impact that killed the dinosaurs and marked the end of the Cretaceous period and the beginning of the Palaeogene. After this, the team said, the placentals quickly diversified, and a menagerie of mammals filled the habitat niches left by the dinosaurs.