Tiny 'ice mouse' survived Arctic cold in the age of dinosaurs

A small fossil animal that survived in what may have been among the harshest climates on Earth 73 million years ago has been found by paleontologists working in northern Alaska.

The Late Cretaceous mammal was described by the researchers, who were led by Jaelyn Eberle of the University of Colorado Boulder, in a report that was released this month in the Journal of Systematic Palaeontology.

They gave it the scientific name Sikuomys mikros, which is derived from the Iupiaq word "Siku" for "ice" and the Greek terms "mys" and "mikros" for "mouse" and "little," respectively.

Its title is appropriate. The little ice mouse was really a member of the Gypsonictopidae family of extinct mammals, not a mouse, although it was unquestionably small. The fuzzy creature, believed to weigh 11 grams, or less than an empty metal drink can, may have resembled a modern-day shrew.

Additionally, it spent the whole year in northern Alaska, which at the time was located far further north, above the Arctic Circle. There, the ice mouse most likely endured up to four months of nonstop winter darkness and subfreezing temperatures.

According to Eberle, a professor in the Department of Geological Sciences and curator of fossil vertebrates at the CU Museum of Natural History, "These guys probably didn't hibernate." They were active throughout the year, digging underground or under leaf litter to eat anything they could get their teeth into, most likely insects and worms.

She and her colleagues had to work just as hard to find the extinct animals; they were only able to identify the new species from a small collection of teeth, each about the size of a grain of sand.

Eberle remarked, "I always enjoy working at the very edge of the planet." You never know what you'll find, but you can be sure it will be something fresh.

According to study co-author and director of the University of Alaska Museum of the North Patrick Druckenmiller, those tiny fossils are opening up a new window for research into prehistoric Alaska.

"Northern Alaska was home to an ecosystem unlike any on Earth today, 73 million years ago," he added. It was a polar forest full with birds, tiny animals, and dinosaurs. These creatures were created to survive in a highly seasonal environment with icy winters, snow, and up to four months of total winter darkness.

traveling north

It's not always simple to go to the extremities of the Earth.

The fossils were discovered by the researchers, who also included paleontologists from Florida State University and the University of Alaska Fairbanks, in sediments along the banks of the Colville River, which is located close to the Beaufort Sea on Alaska's northern shore. The crew flies by bush aircraft or snowmobile to the location, which is a portion of the so-called Prince Creek Formation, from Deadhorse, Alaska, a distance of around 75 miles.

Gregory Erickson, a co-author of the study from Florida State University, stated that the research his team is doing is exposing a "Lost world" of species that have adapted to the Arctic. In the presence of extreme seasonal climatic swings, Prince Creek acts as a natural test of these animals' physiology and behavior.

The new study also included co-authors from the University of California, Berkeley, including the late William Clemens.

The only animal fossils still found in the area are a few teeth and bits of jaws, unlike dinosaurs from the same era that left behind massive bones. The crew gathers soil from the riverbanks in buckets in order to retrieve these priceless specimens. The muck is washed away in the lab, and the remaining material is sorted under a microscope.

Eberle remarked, "You look under the microscope and see this perfect little tooth." It is really little.

Protection underneath

Those adorable tiny teeth of the ice mouse have given rise to a charming little enigma.

At higher latitudes and in colder climates, species of several families of mammals on Earth tend to become larger. It appears that the ice mouse and its related species follow the opposite tendency. Paleontologists have discovered closely similar species that were three to five times bigger than Sikuomys mikros and lived thousands of kilometres to the south.

Eberle speculates that the ice mouse's diminutive size was a result of its lack of food during Alaska's winter months.

Today, shrews exhibit a similar behavior, she observed. "The theory is that if you're really small, your energy and food requirements are lower."

It's possible that Sikuomys Mikros spent the chilly months underground in Alaska. In the end, living underground may have benefited creatures like the ice mouse. The severe circumstances that followed the meteorite smash that wiped off the dinosaurs 66 million years ago may have been more hospitable to burrowing animals.