The delayed rise of present-day mammals

It took 10 to 15 million years after the dinosaurs were wiped out before modern mammals - including our ancient human ancestors - were able to diversify and rise to their present-day prominence across the globe, a landmark new study has found.

The surprise finding overturns the widely held belief that the ancestors of modern mammals were able to quickly evolve and spread to fill many of the empty niches left behind following the mass extinctions of dinosaurs and many other large animals when a huge asteroid crashed into the Earth about 65 million years ago.

An international research team reached that conclusion after analysing the evolutionary links of some 4,500 mammals, creating for the first time a "supertree" of family relationships between almost all species of mammal alive today.

Armed with the information about those relationships, the researchers used DNA data and the fossil record to estimate diversification rates and work backward to establish when specific groups of mammals first appeared on Earth.

The study, which included work by Robin Beck, a PhD student in the UNSW School of Biological, Earth and Environmental Science, suggests that while some early mammals may have benefited from the demise of the dinosaurs, many were on branches of the family tree not closely related to present-day mammals and died off early on.

They also found that modern mammal orders, such as primates, rodents, and hoofed animals, did not diversify until much later, at least 10 to 15 millions years after the mass extinctions at the boundary between the Cretaceous and Tertiary periods when dinosaurs went extinct.

This period of diversification, around 50-55 million years ago, represents the beginning of the Eocene epoch, and appears to coincide with a peak in global temperatures known as the Cenozoic thermal maximum. However, the authors point out that the exact cause of the increased mammalian biodiversity will re

Contact: Robin Beck
University of New South Wales

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