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Mammals' ancestors no overnight success

Species didn't dominate immediately after a killer asteroid hit Earth.

March 29, 2007|From Reuters

WASHINGTON — The asteroid that smacked Earth 65 million years ago wiped out the dinosaurs and paved the way for mammals to dominate, but it took 10 million to 15 million more years for the ancestors of today's mammals to really take over, scientists said Wednesday.

While some mammals seized the day and diversified after the asteroid crashed off the Yucatan Peninsula causing a mass extinction, they largely were evolutionary dead-ends, scientists said.

Researchers led by Olaf Bininda-Emonds of Germany's Friedrich-Schiller-University Jena used DNA from some of the 4,500 species of mammals on Earth and fossils of extinct animals to devise a family tree tracing mammalian evolutionary history. The research is published in today's issue of the journal Nature.

Mammals from the major groups around today arose tens of millions of years before the asteroid struck and survived the calamity. But they remained secondary to now-extinct mammal forms and did not start diversifying and asserting themselves until about 50 million to 55 million years ago, the study found.

The dinosaurs ruled the planet from about 225 million years ago until their demise along with the flying reptiles called pterosaurs, the marine reptiles called mosasaurs and groups of others.

The first mammals appeared roughly 220 million years ago and the first directly related to today's mammals arose about 125 million years ago, scientists said.

But they remained largely an evolutionary afterthought, doing their best to avoid becoming dinosaur lunch, until the arrival of the giant space rock.

"The common perception is that the mammals rose to their current status after the dinosaurs went extinct. While it is true that there is an increase in mammal diversity after this time, and the fossil record shows this quite clearly, it is not in the mammals we see around us today," Bininda-Emonds said.

"Instead, the mammals that seemed to benefit from the death of the dinosaurs belonged to groups that are wholly extinct these days," he said.

Some mammals that flourished shortly after the dinosaurs died included cat-sized, rodent-like Ptilodus, squirrel-like primate relative Plesiadapis, dog-sized meat eaters called creodonts and the lion-sized carnivorous Andrewsarchus, known for its fearsome jaws.

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