New Take On Dinosaur Extinction Aftermath
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- Date: Fri, 30 Mar 2007 08:12:44 +0800
New Take On Dinosaur Extinction Aftermath
New Study Challenges Theory That Dinosaurs' Demise Spurred Evolution In Mammals'
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NEW YORK, March 28, 2007
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In an undated photo provided by Professor Rodolfo Coria, a dog sits by a replica
of the head of a Mapusaurus roseae at the Carmen Funes Museum in Plaza Huincul,
Argentina. The dinosaur was discovered in the Patagonia region of Argentina and
appears to be one of the biggest meat-eating dinosaurs known. (AP)
Fast Fact
The study's family tree includes 4,510 species, more than 99 percent of mammal
species covered by an authoritative listing published in 1993.
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(AP) The big dinosaur extinction of 65 million years ago did not produce a
flurry of new species in the ancestry of modern mammals after all, says a huge
study that challenges a long-standing theory.
Scientists who constructed a massive evolutionary family tree for mammals found
no sign of such a burst of new species at that time among the ancestors of
present-day animals.
Only mammals with no modern-day descendants showed that effect.
"I was flabbergasted," said study co-author Ross MacPhee, curator of vertebrate
zoology at the American Museum of Natural History in New York.
At the time of the dinosaurs' demise, mammals were small, ranging in size
between shrews and cats. The long-held view has been that once the dinosaurs
were gone, mammals were suddenly free to exploit new food sources and habitats,
and as a result they produced a burst of new species.
The new study says that happened to some extent, but that the new species led to
evolutionary dead ends. In contrast, no such burst was found for the ancestors
of modern-day mammals like rodents, cats, horses, elephants and people.
Instead, they showed an initial burst between 100 million about 85 million years
ago, with another between about 55 million and 35 million years ago, researchers
report in Thursday's issue of the journal Nature.
The timing of that first period of evolutionary development generally agrees
with the conclusions of some previous studies of mammal DNA, which argue for a
much earlier origin of some mammal lineages than the fossil record does.
The second burst had shown up in the fossil record, MacPhee said. But he said
the new study explains why scientists have been unable to find relatively
modern-looking ancestors of the creatures known from that time: Without any
evolutionary boost from the dinosaur demise, those ancestors were still
relatively primitive.
Some experts praised the large scale of the new evolutionary tree, which used a
controversial "supertree" method to combine data covering the vast majority of
mammal species. It challenges paleontologists to find new fossils that can shed
light on mammal history, said Greg Wilson, curator of vertebrate paleontology at
the Denver Museum of Nature & Science.
William J. Murphy of Texas A&M University, who is working on a similar project,
said no previous analysis had included so many mammal species.
But, "I don't think this is the final word," he said.
The study's approach for assigning dates was relatively crude, he said, and some
dates it produced for particular lineages disagree with those obtained by more
updated methods.
So as for its interpretation of what happened when the dinosaurs died off, "I'm
not sure that conclusion is well-founded," Murphy said.
John Gittleman, a study co-author and director of the University of Georgia
Institute of Ecology, said the researchers considered a range of previously
reported dates for when various lineages split. They found the overall
conclusions of the study were not significantly affected by which dates they
chose, he said.
Researchers should now look at such things as the rise of flowering plants and a
cooling of the worldwide climate to explain why ancestors of present-day mammals
took off before the dinosaurs died out, Gittleman said. The cause of the later
boom is also a mystery, he said.
The study's family tree includes 4,510 species, more than 99 percent of mammal
species covered by an authoritative listing published in 1993. (Nearly 300
species have since been added to the listing, but the researchers said that
doesn't affect their study's conclusions.) To construct it, the researchers
combined previously published work that relied on analysis of DNA, fossils,
anatomy and other information.
S. Blair Hedges, an evolutionary biologist at Pennsylvania State University,
said the new work "pushes the envelope in the methods and data, and that's
really important."
He said the demise of the dinosaurs may have affected mammal evolution by
influencing characteristics like body size rather than boosting the number of
new species created. Such changes wouldn't be picked up by the new study, he
noted.
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