Dinosaurs did not hold modern mammals back from evolving

A new study from the University of Oxford has revealed that, before they vanished from Earth, dinosaurs were not the main competitors of modern mammals. The research indicates that distinct mammal groups were in greater competition with each other. 

“There were lots of exciting types of mammals in the time of dinosaurs that included gliding, swimming and burrowing species, but none of these mammals belonged to modern groups, they all come from earlier branches in the mammal tree.” said study co-author Dr.  Elsa Panciroli, a researcher from the Oxford University Museum of Natural History. 

“These other kinds of mammals mostly became extinct at the same time as the non-avian dinosaurs, at which point modern mammals start to become larger, explore new diets and ways of life.” 

“From our research it looks like before the extinction it was the earlier radiations of mammals that kept the modern mammals out of these exciting ecological roles by outcompeting them.”

Most of the modern mammal species have origins in groups that expanded in a big way 66 million years ago, after a mass extinction wiped out non-avian dinosaurs. It has been widely assumed that mammals lived in the shadow of the dinosaurs prior to the extinction event. 

Mammals were thought to have been held back by the dinosaurs, remaining small and unspecialized in terms of diet and lifestyle. It appeared that mammals did not begin to thrive until the niches of dinosaurs were made available after the extinction. 

But now, the Oxford team has used new statistical methods to identify the evolutionary “limits” placed on different groups of mammals. The results suggest that closely related mammals were placing the biggest constraints on each other, and not the dinosaurs.

The researchers analyzed the anatomy of all the different mammals which lived alongside dinosaurs, including the ancestors of modern groups, also known as therians. The team examined how frequently new features appeared, such as changes in the size and shape of their teeth and bones, and the pattern and timing of their appearance before and after the mass extinction.

Based on the analysis, the researchers determined that modern mammals were more constrained during the time of the dinosaurs than their close relatives. In other words therians were being shut out of certain niches as their relatives explored larger body sizes, different diets, and new strategies such as climbing and gliding.

“This result makes very little sense if you assume that it was the dinosaurs constraining the therians,” said Dr. Neil Brocklehurst, who led the research. “There is no reason why the dinosaurs would be selectively competing with just these mammals and allowing others to prosper. It instead appears that the therians were being held back by these other groups of mammals.”

According to the experts, the extinction of other mammals helped the most to pave the way for the success of modern mammals.

“Most of the mammals that lived alongside the dinosaurs were less than 100g in body mass – that’s smaller than any non-bird dinosaur. Therefore, these smallest mammals would probably not have been directly competing with dinosaurs,” said study co-author Dr. Gemma Benevento.

Despite this, small mammals show diversity increases after the extinction which are just as profound as those seen in larger mammals, explained Dr. Benevento. 

“Palaeontology is undergoing a revolution. We have greatly expanded the toolkit available to analyse large datasets and directly test our ideas about evolution,” said Dr. Brocklehurst. “Most studies of the mammal radiation have focused on how fast they evolved, but analysing what limits there were on the evolution provides new perspectives. We have had to rethink many of our theories using these state-of-the-art approaches.”

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer