New Study Finds That Present-Day Mammals Diversified Long After Dinosaurs Went Extinct

A group of researchers has conducted the first species-level analysis of the evolutionary relationships of nearly all living mammals. Using information about the dates of those relationships, the researchers were able to estimate diversification rates and, by working backward, establish when specific groups of mammals first appeared on Earth. The study, which includes work by Ross MacPhee, Curator in the Division of Vertebrate Zoology at the American Museum of Natural History, suggests that, although some early mammals may have benefited from the demise of the dinosaurs, many of those early species 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, are all 75 million years old or older and did not diversify until much later, at least 10 to 15 million years after the mass extinctions at the boundary between the Cretaceous and Tertiary Periods when dinosaurs went extinct. 

The research, which appears in the March 29 issue of the journal Nature and was led by Olaf R.P. Bininda-Emonds of the Technical University of Munich and Andy Purvis of Imperial College in London, brought together a diverse group of scientists to produce the first comprehensive analysis of mammalian biodiversity from about 160 million years ago to the present day. Additional contributors came from the Zoological Society of London, the University of New South Wales, the Royal Botanic Gardens, the National Evolutionary Synthesis Center, the University of British Columbia, and the University of Georgia.

Their work included construction of a so-called "supertree" diagram tracing the lineages and evolution of more than 4,500 extant mammal species. Many paleontologists believe the mass extinction of dinosaurs roughly 65 million years ago marking the transition from the Mesozoic to the Cenozoic Eras permitted the ancestors of modern mammals to flourish and begin the long evolutionary process culminating in the diverse array of species we see today.

"For many years, molecular biologists and paleontologists shared different views about the rise of present-day mammals," said Dr. MacPhee. "Extensive molecular data indicate that our common mammalian roots have to go back 90 to 100 million years, if not more, but many paleontologists have been dubious of this claim given the lack of ancestral-looking fossils until about 50 to 55 million years ago. This new work helps reconcile those differences. Now we know the ancestors of living mammal groups were there, but in very low numbers."

Supertrees are a kind of summary of evolutionary history for a large group of organisms pieced together from many smaller studies for separate groups based on genetic or physical analysis or both. They permit scientists to trace the evolution of groups of species and map the relationships among organisms. With the growth of molecular analytic techniques, the construction and study of supertrees listing thousands of species has required ever-greater computing power, as well as contributions from an increasingly diverse group of scientists. The group publishing in Nature includes bioinformaticians and evolutionary biologists, who gathered and analyzed the data needed to construct the mammalian supertree, as well as paleontologists and ecologists, who helped put the information into context.

The scientists found that although certain mammal groups did diversify immediately after the mass extinction of dinosaurs, almost all of those relatively early groups eventually died out. Instead, it wasn't until a second period of diversification at the beginning of the Eocene Epoch (56 to 34 million years ago) that the ancestors of lineages leading to present-day mammals really began to flourish. The start of this expansion of biodiversity appears to roughly coincide with a rise in global temperatures known as the Cenozoic thermal maximum, though the authors point out that the exact cause of the increased mammalian biodiversity is beyond the scope of their study.

"The big question now is what took the ancestors of modern mammals so long to diversify," said MacPhee. "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. Evidently we know very little about the macroecological mechanisms that play out after mass extinctions."

The study was supported by the National Center for Ecological Analysis and Synthesis (NCEAS), the German Federal Ministry of Education and Research (BMBF), the German Research Association (DFG), the British Natural Environment Research Council (NERC), the Leverhulme Trust, the National Science Foundation (NSF), the Earth Institute at Columbia University, and the Cyberinfrastructure for Phylogenetic Research (CIPRES).

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