|
After dinosaur demise, mammals late to the party
2007-03-31
The asteroid that smacked Earth 65 million years ago wiped out the dinosaurs and paved the way for mammals to dominate, but it took another 10 to 15 million years for the ancestors of today's mammals to really take over, scientists said on 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. 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 55 to 50 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 a bunch of other animals. 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 these furry little creatures remained largely an evolutionary afterthought, doing their best to avoid becoming dinosaur lunch, until the fortuitous intervention of the giant space rock. 'WHOLLY EXTINCT' "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 by e-mail. "Instead, the mammals that seemed to benefit from the death of the dinosaurs belonged to groups that are wholly extinct these days," said Bininda-Emonds, whose research appeared in the journal Nature. Some of the mammals that flourished in the time shortly after the dinosaurs died included cat-sized, rodent-like Ptilodus, squirrel-like primate relative Plesiadapis, dog-sized meat eaters called creodonts and the spectacular lion-sized carnivorous Andrewsarchus, known for its fearsome jaws. Most of the present-day groups of placental mammals arose between 100 and 85 million years ago during the Cretaceous Period, the last act of the Age of Dinosaurs, with the peak around 93 million years ago, the researchers said. Placental mammals give birth to live young and include rodents, carnivores, primates and hoofed mammals. Other types of mammals include marsupials, like kangaroos, and the very rare monotremes like the platypus that lay eggs. Along with the ancestors of today's marsupials, placental mammals took the asteroid in stride. "Even with everything dying around them, it was business as usual for the present-day groups (of mammals) and the whole thing didn't seem to have any positive or negative effect on them," Bininda-Emonds said. Study co-author John Gittleman, director of the University of Georgia's Institute of Ecology, added, "We don't see that there's much of a relationship between the demise of the dinosaurs and the diversification of the modern-day mammals." Co-author Ross MacPhee of the American Museum of Natural History in New York said the big question is what took the ancestors of modern mammals so long to diversify? "What was our lot doing? Sitting on their hands as far as I can determine," MacPhee said.
|  | | | Profile |
News71 | Gallery | Links | |
| |
|
|
| 
