Mammals alive during the age of the dinosaurs have always been assumed to be tiny creatures, doing their best to stay out from underfoot of the giant reptiles, but a fossil from Madagascar suggests at least one early mammal wasn't afraid to "go big," researchers say.

A nearly complete skull of a mammal looking something like a groundhog suggests the animal was so large, and sporting such strange characteristics, that its existence could never have been predicted, they say.

Examination of the skull suggests it had supersensory abilities, with large eyes and an extremely large part of its brain devoted to smell.

And it was big for its time; almost 20 pounds big, double the size of any other known mammal from the era of the dinosaurs.

"Not only does it have bizarre features, it's bizarre in being so humongous," says vertebrate paleontologist David Krause of Stony Brook University in New York, who reports the find Wednesday in the journal Nature.

Krause, lead author of the study,likens the creature's appearance to an overgrown groundhog. "It's Punxsutawney Phil on steroids," he says with a smile.

The skull of the creature, dubbed Vintana sertichi, was hidden inside a 150-pound piece of Madagascar sandstone brought back to Krause's lab by graduate student Joe Sertich, for whom the creature has been named.

It was fish fossils spread throughout the layers of sandstone that first attracted the scientists; the mammal fossil turned out to be an unexpected bonus.

"We had absolutely no idea it was there," says Krause, who explained that only a CT scan revealed the fossil within. "Paleontology is sometimes pure serendipity."

An analysis of the jaws and teeth of the skull suggests it lived on a diet of seeds, roots or nut-like fruits, researchers say.

The primitive and unique features of the skull are likely the result of its isolation on Madagascar after the landmass that became the large island separated from Africa, Australia and, finally, from the Indian subcontinent.

"Throw together some anatomical features from ancient mammal-like reptiles, Pleistocene ground sloths, an extant rodent, and maybe a few bits and pieces from the Muppets on 'Sesame Street' and you might get something that resembles the cranium of Vintana," Krause says.

Still, of all the unique features of the creature it remains its size that has most surprised researchers, they admit.

"This is enormous for a Mesozoic mammal, most of which were shrew or mouse-sized, living in the shadow of dinosaurs," Krause says.