Some 50 million years ago, the Arctic was thick with plants. Flying lemurs jumped between trees as big as giant sequoias, and steamy swamps hosted alligators and giant tortoises. But one thing about the Arctic hasn't changed: the relentless darkness that blankets it for nearly half the year. New research reveals that, despite the lack of sun, at least one of the region's inhabitants stuck around all winter long.
Scientists are divided over what the Arctic's ancient residents did during the dark winters. Some think that mammals migrated south of the Arctic Circle, where there was enough daylight for plants to grow.
But other evidence points to them staying: Fossils suggest that animals slowly moved across North America, Europe, and Asia via the Arctic over thousands to millions of years using land bridges that were present during that time. That must have meant that some of them set up shop in the far north, giving them the time required to slowly disperse into new areas. But even if the creatures did stay, did they hibernate or keep on kicking through the winter?
Vertebrate paleontologist Jaelyn Eberle of the University of Colorado, Boulder, has been vexed by these questions ever since she started working with fossils in the high Arctic 7 years ago. Recently, she paired up with geochemists and began looking at the incisors of an ancient hippolike creature called Coryphodon.
Here's why: Teeth contain signals of past climate and diet. Differences in temperatures and atmospheric circulation patterns cause water vapor to have more oxygen-18 (a version--or "isotope"--of oxygen) than oxygen-16 in the warm summer than in the cold winter. This ratio of isotopes transfers to rainwater. When Coryphodon drank, the oxygen atoms became incorporated into its tooth enamel.
When the researchers analyzed the enamel, they found that changes in the ratio of oxygen-18 to oxygen-16 over time matched the seasonal changes in wood of trees in the area. (The isotopic ratio is incorporated as the tooth grows, leaving a record over time similar to tree rings.) The evidence points to Coryphodon as a permanent resident of the Arctic, the team reports  in this month's issue of Geology.
More clues to Coryphodon's lifestyle came from an analysis of ratios of carbon-13 to carbon-12, which exists in different ratios in various types of plants. Eberle discovered that Coryphodon fed on fresh Arctic foliage during sunny times and dead Arctic leaves, wood, and fungi during the dark periods. A less-detailed isotopic analysis suggested that tapirs and rhinolike creatures also spent their winters in the Arctic.
This study gives us a "reasonably convincing" mechanism for year-round inhabitants of the Arctic, says John Flynn, a vertebrate paleontologist at the American Museum of Natural History in New York City. Scott Wing, a paleobiologist at the Smithsonian National Museum of Natural History in Washington, D.C., says the study highlights the ability of animals to inhabit northern latitudes as the world warms, which is important now as temperatures rapidly increase in the Arctic. Although today's north is far from having a subtropical climate, he notes, "the consequences of what we are doing now are going to ... ultimately result in conditions that are totally outside of that range of anything that we've seen."