73 million-year-old fossil of a tiny mouse found in Alaska

The fossil is from the Gypsonictopidae, a family of mammals.
Sejal Sharma
The 'ice mouse' fossil
The 'ice mouse' fossil

Eberle et al 

A bunch of paleontologists were working at a remote site along the banks of the Colville River on Alaska's northern coast when they came across fossils of a tiny mouse belonging to the now-extinct family of mammals called Gypsonictopidae.

"I always like working at the ends of the Earth," said Jaelyn Eberle, leader of the team and professor at the University of Colorado Boulder. "You never know what you're going to find, but you know it's going to be new."

The site is part of the Prince Creek Formation and is so remote that the team had to travel about 75 miles by snowmobiles or a bush plane, according to a report by Phys.Org.

The research team consisted of paleontologists from the University of Alaska Fairbanks and Florida State University.

The mammal is being described as an ‘ice mouse’

The scientists have given the fossil a fitting name—Sikuomys mikros—from "Siku," an Iñupiaq word for "ice," and "mys" and "mikros," the Greek words for "mouse" and "little."

The team claims in their paper that the tiny fossil mammal dates back to 73 million years ago when it survived in one of the coldest conditions ever endured by living organisms on Earth.

"These guys probably didn't hibernate," said Eberle. "They 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."

"Seventy-three million years ago, northern Alaska was home to an ecosystem unlike any on Earth today," said study co-author Patrick Druckenmiller, director of the University of Alaska Museum of the North. "It 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."

A new window into ancient Alaska

Dinosaurs leave behind large bones, so it’s easy for paleontologists to discover more about them. To recover other fossils, the remnants of which may only be a few fragments of jaws and teeth, paleontologists collect buckets of dirt from the riverbanks, wash away the mud and observe them under a microscope in a lab.

"You look under the microscope and see this perfect little tooth," said Eberle. "It's so tiny."

One mysterious detail about the ‘ice mouse’ is that its cousin species at higher altitudes and cooler climates tend to get bigger. But the ice mouse is the opposite. Paleontologists have found related species living thousands of miles to the south that were three to five times larger than Sikuomys mikros, said the Phys.Org report.

Scientists estimate that these tiny ice mouses may have had a better chance at surviving the meteorite crash 66 million years ago that wiped out dinosaurs from Earth.

The study was published in the Journal of Systematic Palaeontology.

Study abstract:

A new eutherian, Sikuomys mikros gen. et. sp. nov., is described from Upper Cretaceous (upper Campanian) strata of the Prince Creek Formation cropping out along the lower Colville River in northern Alaska, USA. The taxon represents the northernmost occurrence of a Mesozoic eutherian (palaeolatitude 80–85°N). The Alaskan taxon differs morphologically from Gypsonictops in having: weak pre- and postcingula on P5, upper molars with small conules, narrower pre- and postcingula, and a postcingulum that extends lingually past the protocone, p5 lacking a paraconid, lower molars with a metaconid that is taller than, or subequal in height to, the protoconid, and a less anteroposteriorly compressed trigonid. Phylogenetic analysis recovers S. mikros as the sister taxon to Gypsonictops in the strict consensus tree. A regression equation for predicting body mass of insectivorans utilizing lower molar area estimates the mass of S. mikros at ∼10.8 grams, approximately one-third to one-fifth that of other gypsonictopids. The occurrence of this lilliputian eutherian – the smallest terrestrial vertebrate known from the high palaeolatitude Prince Creek Formation – provides insight into its overwintering strategies. The pattern in which the smallest species of a lineage occurs at the highest latitudes suggests that S. mikros did not hibernate, but rather was active year-round, akin to extant shrews (Soricidae). From a palaeobiogeographical standpoint, the occurrence of S. mikros is evidence that some leptictidans lived year-round in the Arctic, a probable prerequisite for dispersal between North America and Asia, hypothesized by others to be the place of origin for Leptictida.

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