抖阴旅行射

Paleontologists working in northern Alaska have discovered a tiny fossil mammal that thrived in what may have been among the coldest conditions on Earth about 73 million years ago.

The researchers, led by Jaelyn Eberle of 抖阴旅行射 Boulder, described the Late Cretaceous animal in a study in the "Journal of Systematic Palaeontology."

They gave it the scientific name Sikuomys mikros鈥攆rom 鈥淪iku,鈥� an I帽upiaq word for 鈥渋ce,鈥� and 鈥渕ys鈥� and 鈥渕ikros,鈥� the Greek words for 鈥渕ouse鈥� and 鈥渓ittle.鈥澨�

It鈥檚 a fitting title. While the little ice mouse wasn鈥檛 actually a mouse, instead belonging to a now-extinct family of mammals called Gypsonictopidae, it was certainly tiny. The furry critter may have looked a bit like a modern-day shrew and weighed an estimated 11 grams, or less than an empty aluminum soda can. It also lived year-round in northern Alaska, which at the time lay much farther north, above the planet鈥檚 Arctic Circle. There, the ice mouse likely weathered as much as four months of unending darkness in the winter and temperatures that fell below freezing.听

鈥淭hese guys probably didn鈥檛 hibernate,鈥� said Eberle, curator of fossil vertebrates at the 抖阴旅行射 Museum of Natural History and professor in the Department of Geological Sciences. 鈥淭hey stayed active all year long, burrowing under leaf litter or underground and feeding on whatever they could sink their teeth into, probably insects and worms.鈥�

She and her colleagues had to be equally tenacious to discover the fossil animals: The researchers identified the new species from only a handful of tiny teeth, each about the size of a grain of sand.

鈥淚 always like working at the ends of the Earth,鈥� Eberle said. 鈥淵ou never know what you鈥檙e going to find, but you know it鈥檚 going to be new.鈥�

Those minute fossils are giving researchers a new window into ancient Alaska, said study co-author Patrick Druckenmiller, director of the University of Alaska Museum of the North.听

鈥淪eventy-three million years ago, northern Alaska was home to an ecosystem unlike any on Earth today,鈥� he said. 鈥淚t was a polar forest teeming with dinosaurs, small mammals and birds. These animals were adapted to exist in a highly seasonal climate that included freezing winter conditions, likely snow and up to four months of complete winter darkness.鈥澨�

Venturing north

Getting to the ends of the Earth isn鈥檛 always easy.

The researchers, including paleontologists from the University of Alaska Fairbanks and Florida State University, unearthed the fossils from sediments along the banks of听the Colville River鈥攏ot far from the Beaufort Sea on Alaska鈥檚 northern coast. The site, part of what's called the Prince Creek Formation, is so remote the team travels the 75 or so miles from Deadhorse, Alaska, by snowmobile or bush plane.听

鈥淥ur team's research is revealing a 鈥楲ost world鈥� of Arctic-adapted animals,鈥� said Gregory Erickson, a co-author of the study at Florida State University. 鈥淧rince Creek serves as a natural test of these animal's physiology and behavior in the face of drastic seasonal climatic fluctuations.鈥�

The late William Clemens of the University of California, Berkeley听was also a co-author of the new research.

Unlike dinosaurs from the same time period, which left behind large bones, the only fossils remaining from the region鈥檚 mammals are a few teeth and fragments of jaws. To recover these precious specimens, the group collects buckets of dirt from the riverbanks. In the lab, the researchers wash away the mud and sort what remains under a microscope.

鈥淵ou look under the microscope and see this perfect little tooth,鈥� Eberle said. 鈥淚t鈥檚 so tiny.鈥澨�

Safety underground

In the case of the ice mouse, those perfect little teeth have inspired a perfect little mystery.

For many groups of mammals on Earth, species tend to get bigger at higher latitudes and cooler climates. The ice mouse and its close cousins seem to follow the opposite pattern. Paleontologists have found related species living thousands of miles to the south that were three to five times larger than Sikuomys mikros.听

Eberle suspects the ice mouse was so small because there was so little to eat during the winter in Alaska.听

鈥淲e see something similar in shrews today,鈥� she said. 鈥淭he idea is that if you鈥檙e really small, you have lower food and energy needs.鈥�

Sikuomys mikros听may have spent the cold months in Alaska underground. In the end, such a subterranean lifestyle may have been a blessing for animals like the ice mouse. Burrowing mammals may have stood a better chance of surviving the harsh conditions that followed the meteorite crash that killed the dinosaurs 66 million years ago.