Study Re-Evaluates Mammals' Evolution

Mar 29, 01:51 PM

Current Headlines: By John Noble Wilford

The mass extinction that wiped out dinosaurs and other life 65 million years ago apparently did not, contrary to conventional wisdom, immediately clear the way for the rise of today's mammals.

In fact, the ancestral branches of most mammals, including primates, rodents and hoofed animals, emerged long before the global extinction and survived it more or less intact. But it was not until at least 10 million to 15 million years afterward that the lineages of living mammals began to flourish in number and diversity.

Some mammals did benefit from the extinction, but these were not closely related to extant lineages and most of them soon died off.

Those are the surprising conclusions of a comprehensive study of molecular and fossil data on 4,510 of the 4,554 mammal species known to exist today. The researchers reporting the findings in the Thursday issue of the journal Nature said this was the first virtually complete species-level study of existing mammals.

Writing in the journal, the leaders of the project said the "fuses" leading to the explosive expansion of mammals "are not only very much longer than suspected previously, but also challenge the hypothesis that the end-Cretaceous mass extinction event had a major, direct influence on the diversification of today's mammals."

They said their analysis of more than 40 lineages of existing mammals showed that diversification rates "barely changed" in the aftermath of the extinctions at the boundary of the Cretaceous and Tertiary periods. The transforming changes started 10 million years later and lasted until about 35 million years ago.

Other scientists said the so-called long-fuse model opened a door to a better understanding of the evolutionary history of mammals and would force a re-examination of the ecological and other causes underlying the more recent proliferation of mammals.

The international team that produced the new "supertree" of mammalian evolution was led by Olaf Bininda-Emonds of the Technical University of Munich and Andy Purvis of Imperial College London. Other members included paleontologists, mammalogists, evolutionary biologists and other researchers from Australia, Canada and the United States.

In another article in Nature, David Penny and Matthew Phillips of Massey University in New Zealand, who were not involved in the research, wrote: "Inferring a good tree of such scale is groundbreaking, and the methods will be used as a model for tree-of- life studies - whether of birds, flowering plants, invertebrate groups or other organisms."

They also noted that a similar analysis for birds, published recently in the journal Biology Letters, revealed that more than 40 avian lineages survived the mass extinctions. Most paleontologists now think that birds descended from dinosaurs. So in a sense, even dinosaurs in one form escaped the calamity.

But until now most paleontologists had favored a "short fuse" model in which mammals came into their own almost immediately after the dominant reptiles vacated their habitats. Before the extinctions, most mammals were small nocturnal creatures.

The study confirmed and elaborated on research by molecular biologists indicating that many of today's mammalian orders originated from 100 million to 85 million years ago. The reasons for this evolutionary burst are not clear.

Drawing on molecular and fossil data, the researchers said they found that the "pivotal macroevolutionary events for those lineages with extant mammalian descendants" occurred well before the mass extinction and long after. They emphasized that the molecular and fossil evidence provided "different parts of this picture, attesting to the value of using both approaches together."

But the researchers conceded that much more research would be required to explain "the delayed rise of present-day mammals."

Ross MacPhee, a curator of vertebrate zoology at the American Museum of Natural History who was a team member, said that paleontologists were previously dubious of the claims by molecular biologists of such an early ancestry of today's mammals. The fossil record of mammals in the Cretaceous period, they contended, was too sparse to support such an interpretation.

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