The new species—dubbed Cronopio dentiacutus for its narrow snout and long fangs—was about 8 to 9 inches (20 to 23 centimeters) long and likely used its pointy teeth to hunt and eat insects.
The second oldest mammal skull ever recovered from South America, C. dentiacutus existed when dinosaurs still roamed Earth—providing a tantalizing glimpse into the history of early mammals, experts say.
Paleontologists found the mostly complete skull in 2002 outside a rural village in northern Argentina. At the time, however, the skull was mostly hidden in rock, and its identity remained a mystery.
So in 2005 the scientists sent the skull to a technician, who spent three years removing the rock from around the fossil—finally revealing a saber-toothed, squirrel-like creature.
"When [the movie] Ice Age came out, we thought the squirrel character in it looked ridiculous, but then we found something like it," said study leader Guillermo Rougier, a paleontologist at the University of Louisville in Kentucky.
"This animal looks very peculiar, with long snout and canines, and it highlights that we know so little. Surprises like this are bound to happen."
Ancient Mammal Fossils Extremely Rare
Both mammals and dinosaurs appeared near the end of the Triassic period, some 220 million years ago.
When dinosaurs disappeared about 65 million years ago, mammals thrived. But ancient-mammal fossils are still exceedingly rare, mostly because of their small sizes.
"Getting on your hands and knees, or going flat on the ground and crawling your way across, is how you find small-mammal fossils," Rougier said. "It is not very glamorous. You basically roll in the dirt all day."
As a result, paleontologists know of roughly one genus of mammal for every million years between 65 million and 220 million years ago—making for a woefully incomplete record.
"Imagine trying to reconstruct the history of life with that information," Rougier said. (See "Platypus Genome Reveals Secrets of Mammal Evolution.")
"We're certain there were hundreds of [genuses], but for now it's like trying to reconstruct the brilliance of James Joyce with just ten of his words."
The "saber-toothed squirrel" study was published today in the journal Nature.