A new study of the High Arctic climate roughly 50 million years ago led by the University of Colorado at Boulder helps to explain how ancient alligators and giant tortoises were able to thrive on Ellesmere Island well above the Arctic Circle, even as they endured six months of darkness each year.

The new study, which looked at temperatures during the early Eocene period 52 to 53 million years ago, also has implications for the impacts of future climate change as Arctic temperatures continue to rise, said University of Colorado at Boulder Associate Professor Jaelyn Eberle of the department of geological sciences, lead author of the study.

The team used a combination of oxygen isotope ratios from fossil bone and tooth enamel of mammals, fish and turtles that lived together on Ellesmere Island to estimate the average annual Eocene temperature for the site. The team concluded the average temperatures of the warmest month on Ellesmere Island during the early Eocene were from 19-20 degrees C, while the coldest month temperature was about 0-3.5 degrees C

A paper on the subject was published in this month's issue of Earth and Planetary Science Letters.

During the Eocene, Ellesmere Island — which is adjacent to Greenland — probably was similar to swampy cypress forests in the southeastern United States today, said Eberle.

The bone and tooth enamel of vertebrate fossils contains biogenic apatite that can be used as a "flight recorder" to infer paleoclimate conditions.

The new study implies Eocene alligators could withstand slightly cooler winters than their present-day counterpartsIn contrast, the existence of large land tortoises in the Eocene High Arctic is still somewhat puzzling, said Eberle, since today's large tortoises inhabit places like the Galapagos Islands where the cold-month average temperature is about 10 degrees C.