Evolution Diary

The asteroid that finished off the dinosaurs 65 million years ago opened up niches for the majority of today’s living mammals, according to a new study.

The finding is the latest volley in a long-simmering debate over when and where the direct ancestors of everything from whales to rats to humans first arose.

Such mammals are called placentals, which give birth to live young after a long pregnancy, as opposed to pouched marsupials like kangaroos, which give birth after a short pregnancy, and monotremes like platypuses, which lay eggs.

The new report contradicts a DNA study of living mammals published this March that argued the lineages that led to modern placentals began to arise millions of years before the dinosaur die-off and continued well after.

The new study compares fossils from the past 150 million years with living mammals. Its findings support the view that modern placental lineages first appear around 65 million years ago in the Northern Hemisphere.

“This is related to the demise of the dinosaurs,” said John Wible, the curator of mammals at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

“The dinosaurs go extinct, and these various sorts of niches that were occupied by the dinosaurs all of a sudden become open, and it was into those niches that opportunistic placentals evolved.”

Wible is the lead author of the new study published today in the journal Nature.

Side Branches

Wible and colleagues performed their analysis after discovering a well-preserved shrewlike creature among fossils collected in the Gobi desert of Mongolia.

The skull of a 70-million-year-old mammal (left) resembles the skull of a modern mole (right), but the ancient animal is from a long-dead side branch of the mammal family, according to a new study.

The study, which compared mammal fossils from the past 150 million years with living mammals, found that the modern placental mammals first appeared after the extinction of dinosaurs.

The fossils were discovered in 1997 and stored at the American Museum of Natural History in New York City. In 2003 Wible noted the creature’s unique teeth and realized it represented a new species.

The mammal, named Maelestes gobiensis, is between 71 and 75 million years old. Dinosaurs like the fast-running Velociraptor and the parrot-beaked Protoceratops were its contemporaries.

While Maelestes gobiensis looked generally like a shrew, it is not the direct ancestor of any living mammal, Wible explained. Rather, it represents a side branch of the group of mammals that includes placentals.

“The new fossil is one of these extinct relatives that falls outside of the main body of the living placental mammals,” Wible said.

By comparing the new fossil with fossils from living and extinct mammals, Wible’s team found that a few mammal forms dated to about 90 million years ago dead-ended rather than spawning modern placentals, as previous research suggested.

Animals that old lived in a period called the Cretaceous, which ran from 144 to 65 million years ago.

“All the things we’re finding in the Cretaceous, they’re all just extinct end-branches. They’re not giving rise to any modern-day groups,” Wible said.

The finding contrasts with previous research led by David Archibald, a biologist at San Diego State University in California, which suggested that lineages to modern mammals appeared before the dino die-off.

“We think some of the Cretaceous [mammals] were harbingers of things to come,” Archibald said of his study.

“A few, not a lot, but a few lineages were showing up at 90 million years.”

Archibald added that the new analysis may change his mind about the newfound crop of Cretaceous mammals.

“Theirs is the most complete study so far,” he noted of Wible’s team.

Molecular Contrast

The new study more sharply contrasts with a suite of DNA studies that pegged the origin of placental mammals before the disappearance of dinosaurs.

The study published in March—led by Olaf Bininda-Emonds, a zoologist at Germany’s Friedrich Schiller University—concluded that placentals originated perhaps 100 million years ago and had a second burst of diversification about 35 million years ago.

That finding supports what Archibald and colleagues called the short-fuse theory of mammal evolution and diversification—an explosive origin followed by a long time lag before the various forms evolved into distinct niches.

Other studies have pegged the origin to as early as 140 million and as recent as 80 million years ago.

The difference, according to Wible, is that different teams use different genes for their analysis and different fossils to calibrate their so-called molecular clocks.

Paleontologists, by contrast, are limited by a spotty fossil record. A Cretaceous placental may exist, but it has just not yet been discovered.

“I would not be surprised if I went out tomorrow and in some strata that’s around 80 million years I found a placental,” Wible said.

“But I would be surprised if I went out 125 million years ago and found a placental, which is what some of the molecular papers are telling me as a paleontologist I should be able to do.”

John Roach
for National Geographic News
June 20, 2007

Story reposted from National Geographic