Researchers at the University of New South Wales say their study suggests mammals had already begun to diversify long before the asteroid-linked extinction event thought to have killed off most dinosaurs.
While there have been two major spikes in the otherwise steady evolution of mammals, they said, both appear to be independent of the dinosaur wipe-out.
One occurred around 93 million years ago, when the major divisions of living mammals -- placentals, marsupials and monotremes, such as the platypus -- began to appear.
Most of those mammals belonged to lineages that are either extinct or have dwindled drastically in numbers, researchers said.
"It was other groups of mammals, not those we see today, that took advantage of the extinction of the dinosaurs," research team member Robin Beck said.
The second evolutionary spike in mammalian history didn't occur until about 10 million to 15 million years after the dinosaurs' demise, about 55 million to 34 million years ago, when the preponderance of mammals, especially the ancestors of many groups alive today -- such as primates, rodents and hoofed animals -- really took off, the study found.
"The big question now is what took the ancestors of modern mammals so long to diversify," study team member Ross MacPhee, curator of the American Museum of Natural History, said. "It's as though they came to the party after the dinosaurs left, but just hung around while all their distant relatives were having a good time."
The researchers theorize the peak in the diversification of modern mammalian groups is associated with a period of global warming and the subsequent emergence of flowering plants but say more research is needed to link the events.