A research team has discovered a previously unknown species of marsupials living in Alaska's arctic arc during the time of the dinosaurs, adding a vibrant new detail to a complex ancient landscape.
The thumb named Unnuakomys hutchisoni lived in the Arctic about 69 million years ago in the late chalk period. Its discovery, led by researchers from the University of Colorado and the University of Alaska Fairbanks, is outlined in an article published in Journal of Systematic Palaeontology .
The discovery adds the image of an environment that scientists found surprisingly different. The little animal, which is the northernmost swamp ever discovered, lived among a unique variety of dinosaurs, plants and other animals.
Alaskan's northern slope, which was about 80 degrees north latitude, when U. hutchisoni lived there, was once thought to be a barren environment during the late chalk. This view has gradually changed since the dinosaurs were discovered along the Colville River in the 1
Finding a new mammal species in the far north adds a new layer to the evolving prospect, said Patrick Druckenmiller, director of the University of Alaska Museum of the North.
"Northern Alaska was not only inhabited by a wide variety of dinosaurs, but we are actually finding there were also new species of mammals that helped fill in the ecology," Druckenmiller, who has studied dinosaurs in the region for more than a decade. "With all new species, we paint a new image of this ancient polar landscape."
Marsupials are a type of mammal that carries underdeveloped offspring in a bag. Kangaroos and koalas are the most well-known modern marsupials. Old relatives were much smaller during the late chalk, Druckenmiller said. Unnuakomys hutchisoni was probably more like a small opossum feeding on insects and plants while surviving in the dark for as many as four months every winter.
The research team, whose project was funded by a National Science Foundation grant, identified the new marsupial by a careful process. With the help of numerous graduate and undergraduate students, they collected, washed and screened old river sediment collected on the northern slope and carefully examined it under a microscope. For many years they were able to find many fossilized teeth that were almost the size of a grain of sand.
"I look like searching for proverbial needles in harvester – more stones than fossils," says Florida State University paleobiologist Gregory Erickson, who contributes to the paper.
Jaelyn Eberle, fossil vertebrate curator at the University of Colorado Museum of Natural History, led the effort to examine these teeth and a few small jawbones. Their analysis revealed a new species and relatives.
Mammalian teeth have unique cusps that differ from species to species, making them slightly like fingerprints for long-term organisms, says Eberle, lead author of the study.  "If I had to go down to the Denver Zoo and open the mouth of a lion and look in – which I do not recommend – I could tell you his genus and probably its kind, based only on its cheek teeth," Eberle said .
The name Unnuakomys hutchisoni combines the Iñupiaq word for "night" and the Greek word "mys" for the mouse, a reference to the dark winters animal endured and a tribute to J. Howard Hutchison, a paleontologist who discovered the fossil rich place where the teeth were found.
Other co-authors of Journal of Systematic Palaeontology describe William Clemens of the University of California, Berkeley; Paul McCarthy, from the UAF; and Anthony Fiorillo, from the Perot Museum of Nature and Science.