Flowering plants and hippo-like creatures once thrived in the Arctic, where the tundra and polar bears now prevail.
New research, detailed in the June issue of the journal Geology, is shedding light on the lives of prehistoric mammals on Canada's Ellesmere Island 53 million years ago, including how they survived the six months of darkness during the Arctic winter.
Today, Ellesmere Island, located in the high Arctic (about 80 degrees north latitude), is a polar desert that features permafrost, ice sheets, sparse vegetation and a few mammals. Temperatures there range from minus 37 degrees Fahrenheit (-38 Celsius) in winter to plus 48 degrees F (9 Celsius) in summer. It is one of the coldest and driest places on Earth.
But 53 million years ago, the Arctic had a completely different look.
The findings have implications for understanding how ancient animals dispersed across North America and what might lie in store for modern mammals that are moving northward because of climate change.
Arctic then and now
In 1975, researchers discovered that a surprising menagerie of Arctic creatures lived on Ellesmere during the early Eocene epoch (55 million to 50 million years ago). A team led by Mary Dawson of the Carnegie Museum of Natural History in Pittsburgh discovered fossil alligator jaw bones. Since then, fossils of aquatic turtles, giant tortoises, snakes and even flying lemurs — one of the earliest forms of primates — have also turned up.
The new research team analyzed the signatures of carbon and oxygen in the fossil teeth of three types of mammals that once dwelled in the Arctic — a hippo-like, semi-aquatic creature known as Coryphodon; a second, smaller ancestor of today's tapirs (pig-like animals found in South America and Southeast Asia); and a third rhino-like mammal known as brontothere — to find out what the Arctic environment was like in the past.
The results point to warm, humid summers and mild winters with temperatures probably ranging from just above freezing to near 70 degrees F, said study team member Jaelyn Eberle of the University of Colorado at Boulder.
The region was probably similar to swampy cypress forests in the Southeast United States today, Eberle said. Fossils of tree stumps as large as washing machines can still be found there.
In the winter, the prehistoric mammals would have experienced the same perpetual night that Arctic denizens do now. Come summer, they would have lived in constant sunlight.
The analysis of the fossil teeth also told researchers what the animals munched on during their lifetimes, and answered the question of whether or not they hibernated or migrated during the winter darkness. (The same questions exist for Arctic dinosaurs, Eberle said.)
Coryphodon and its contemporaries were similar to those living in Wyoming and Colorado at the time.
"They do not appear to be a unique, 'specialized' group of beasts adapted to High Arctic conditions," Eberle told LiveScience in an email.
So many scientists thought these creatures might go on long migrations, like today's barren ground caribou, which travel more than 600 miles (1,000 kilometers) to over-winter in subarctic forests, Eberle said.
The new study found that this was not the case.
The teeth records indicated that the creatures dined on a summer diet of flowering plants, deciduous leaves and aquatic vegetation. But in winter's twilight they apparently switched over to foods like twigs, leaf litter, evergreen needles and fungi, Eberle said.
"We were able to use carbon signatures preserved in the tooth enamel to show that these mammals did not migrate or hibernate," Eberle said. "Instead, they lived in the high Arctic all year long, munching on some unusual things during the dark winter months."
Fossils of babies and juveniles of the mammal species, as well as their sturdy builds, also make it likely that these animals stayed put during the winter, Eberle added.
Migration, past and future
The year-round residence of mammals such as Coryphodon was a "behavioral prerequisite" for their eventual dispersal across high-latitude land bridges that geologists believe linked Asia and Europe with North America, Eberle said.
"In order for mammals to have covered the great distances across land bridges that once connected the continents, they would have required the ability to inhabit the High Arctic year-round in proximity to these land bridges," Eberle said.
The fossils also suggest that creatures such as tapirs originated in the Arctic, as that is where the earliest known fossils of these groups are found, and later migrated southward.
"Until geologically older fossils of tapirs and brontotheres are found elsewhere in the world, the Arctic is very much in the running as a place of origin," Eberle said.
The animals likely made their way south from the Arctic in minute increments over millions of years as the climate shifted and cooled, the researchers suggest.
"This study may provide the behavioral smoking gun for how modern groups of mammals like ungulates — ancestors of today's horses and cattle — and true primates arrived in North America," Eberle said.
The new study, funded by the National Science Foundation, also foreshadows the impacts of continuing global warming on Arctic plants and animals, Eberle said.
Temperatures in the Arctic are rising twice as fast as those at mid-latitudes with global warming. Air temperatures over Greenland have risen by more than 7 degrees F since 1991, according to climate scientists.
"We are hypothesizing that lower-latitude mammals will migrate north as the temperatures warm in the coming centuries and millennia," Eberle said. "If temperatures ever warm enough in the future to rival the Eocene, there is the possibility of new intercontinental migrations by mammals."
"Let's face it — when the climate changes, an organism has three choices — adapt, move/disperse, or go extinct," she added.
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Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.