It may seem hard to believe now, but dry, frigid Ellesmere Island in Canada's high North was once teeming with alligators, turtles and rhino-like mammals.

Scientists who have long wondered how the beasts could have survived so far north think they now have the answer. It turns out that the now-glacier-covered island in Nunavut was actually a pretty temperate place back in the early Eocene period, 52 or 53 million years ago.

Researchers from University of Colorado at Boulder estimate that average temperatures back in the Eocene were about 20 degrees Celsius and fell only to about 0 to 3 degrees C in the winter -- which helps to explain how ancient alligators and giant tortoises were once able to thrive there.

Prof. Jaelyn Eberle, of the university's department of geological sciences, said her team came to these conclusions after analyzing the fossils of mammals, fish and turtles found on Ellesmere Island.

The team used a combination of "oxygen isotope ratios" from the fossilized bones and teeth enamel to estimate the average annual Eocene temperature for the site.

"Our data, gathered from multiple organisms, indicate it probably did not get below freezing on Ellesmere Island during the early Eocene, which has some interesting implications," she said in a news release.

"This is arguably the most comprehensive data set for the early Eocene High Arctic, and certainly explains how alligators and giant tortoises could live on Ellesmere Island some 52 to 53 million years ago," said Eberle.

Eberle explained that fossilized bones and tooth enamel contain biogenic apatite -- a mineral that is fossilized after death. This mineral can be used as a sort of "flight recorder" to draw conclusions about paleoclimate conditions.

She says her team looked at teeth from a large, hippo-like mammal known as Coryphodons, as well as bones from bowfin fish and shells and bones from aquatic turtles from the Emydidae family, the largest family of contemporary pond turtles.

They then used evidence found on the teeth to draw conclusions about the temperature of the water that the animals were drinking.

"When it comes to oxygen isotope values in tooth enamel, what we found for these creatures is that you are what you drink," she said.

They found that while Coryphodon and bowfins grew throughout the year, the turtles' shells appeared to grow only during summer months. That's similar to what turtles do today when they live in areas far from the equator.

They also found that Eocene alligators could withstand slightly cooler winters than their present-day counterparts. That's not too surprising, given what scientists know about alligators adaptability.

But the existence of large land tortoises in the Eocene High Arctic is still somewhat puzzling, said Eberle. She noted that today's large tortoises inhabit places like the Galapagos Islands where the cold-month average temperature is about 10 degrees C.

Eberle said the findings not only help to explain how species have evolved and migrated over the millennia, the research also offers "a deep time analogue" for today's rapidly warming Arctic region.

Evidence suggested that temperatures in the Arctic are rising twice as fast as those at mid-latitudes, as greenhouse gases build up in Earth's atmosphere, due primarily to fossil fuel burning and deforestation.

This new study foreshadows the impacts of continuing global warming on Arctic plants and animals, Eberle said.

"It's a means of being able to predict what's in store for Arctic ecosystems as the climate continues to warm, today and into the future," she said.