unga-bunga!

Dinos' demise spurred rise of the mammals, new fossil suggests

Wed Jun 20, 3:25 PM ET

PARIS (AFP) - A fossil discovered in the Gobi Desert has unlocked the most emphatic evidence to date that the mass extinction of the dinosaurs helped placental mammals -- of which Homo sapiens is a member -- become masters of the planet.

The fossilised skull of a shrew-like creature was uncovered in Mongolia in 1997 but only now have scientists become aware of its importance, according to a paper published on Thursday by Nature, the weekly British science journal.

A big debate in paleontology is when our mammalian forebears first appeared on the scene.

Some experts, using a "molecular clock" based on the rate of DNA mutation, say placental mammals may have popped up as early as the start of the Cretaceous era, some 145 million years ago.

Others put the key event at 65 million years ago, when some cataclysm brought the curtain down on both the Cretaceous and the long reign of the dinosaurs.

The Mongolian fossil, a well-preserved relic of a species that has been dubbed Maelestes gobiensis, lived in the Late Cretaceous, between 71 and 75 million years ago, when famous dinos such as Velociraptor and Oviraptor strode the Earth.

Researchers led by John Wible, a curator at the Carnegie Museum of Natural History in Pittsburg, Pennsylvania, used the fossil as a benchmark for evolutionary change, comparing shifts in morphology among 69 living and extinct mammals.

Their revamping of the mammalian family tree places the emergence of the placental varieties near the end of the Cretaceous era, when there was an extraordinary "explosion" in the range of these animals.

"Our research gives credence and weight to the traditional paleontological view of placental mammals appearing 65 million years ago, when the dinosaurs died off," Wible said.

"When dinosaurs became extinct, ecological niches emerged that gave modern placentral mammals opportunities to thrive and diversify."

What killed the dinosaurs is an eagerly-explored mystery. A common theory is that a gigantic space rock -- an asteroid or a comet -- whacked into the Earth in what today is Mexico's Yucatan peninsula.

The energy released from the impact set off firestorms and winds that hauled a thick pall of ash into the sky, according to this hypothesis.

The dust so shrouded the planet that the global temperature plummeted, killing off the vegetation on which the dinosaurs (herbivores and the carnivores which fed on them) depended.

That left the door open for smaller creatures that, unlike reptiles, were able to regulate their own body temperatures and whose energy needs were less -- the reign of the mammals had begun.

Of the 5,416 species of mammals alive today, 5,080 are placental; the remainder are marsupials, whose babies develop in pouches, and so-called monotremes (mammals that lay eggs), such as the platypus.