Early Mammals Rapidly Diversified After Dinosaur Extinction

Following the mass extinction of dinosaurs, mammal diversity exploded on Earth, according to a new study from the University College London (UCL). Based on a recent analysis, researchers believe the anatomy of many animals changed dramatically in the10 million years immediately following the event.

"When dinosaurs went extinct, a lot of competitors and predators of mammals disappeared, meaning that a great deal of the pressure limiting what mammals could do ecologically was removed," Dr. Anjali Goswami, senior author of the study from UCL's Genetics, Evolution and Environment department, explained in a news release. "They clearly took advantage of that opportunity, as we can see by their rapid increases in body size and ecological diversity. Mammals evolved a greater variety of forms in the first few million years after the dinosaurs went extinct than in the previous 160 million years of mammal evolution under the rule of dinosaurs."

Researchers specifically investigated the evolution of placental mammals -- the group that includes nearly 5000 modern species, such as humans, elephants, sloths, cats, dolphins and humans. Their work published in the Biological Journal of the Linnean Society details the early evolution of these animals and how they changed following the dinosaur extinction. However, a subsequent study by the same UCL research team details a new tree of life created for placental mammals since so many fossils of placental mammals from the Paleocene epoch differ from their early ancestors. Their study was published in the journal Biological Reviews. 

"The mass extinction that wiped out the dinosaurs 66 million years ago is traditionally acknowledged as the start of the 'Age of Mammals' because several types of mammal appear for the first time immediately afterwards," Dr. Thomas Halliday, first author of both papers, added in UCL news release. "Many recent studies suggest that little changed in mammal evolution during the Paleocene but these analyses don't include fossils from that time. When we look at the mammals that were present, we find a burst of evolution into new forms, followed by specialization that finally resulted in the groups of mammals we see today. The earliest placental mammal fossils appear only a few hundred thousand years after the mass extinction, suggesting the event played a key role in diversification of the mammal group to which we belong."

For their study, researchers examined fossil bones and teeth from nearly 904 fossil species to compare anatomical differences, which they used to create an updated tree of life. 

"Extinctions are obviously terrible for the groups that go extinct, non-avian dinosaurs in this case, but they can create great opportunities for the species that survive, such as placental mammals, and the descendants of dinosaurs: birds," Dr. Goswami concluded. 

Next, the researchers plan to investigate the rate at which these mammals evolved and how greatly their body size varied using DNA evidence. 

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