Arctic Alligators

Ellesmere Island is in the far north of Canada above the Arctic Circle. This is the land of the midnight sun and a rather brisk cold environment. But fifty million years ago it was warmer though still quirky in its day night cycles. 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.

Ellesmere Island is part of the Qikiqtaaluk Region of the Canadian territory of Nunavut. Lying within the Canadian Arctic Archipelago it is considered part of the Queen Elizabeth Islands, with Cape Columbia being the most northerly point of land in Canada. It is today one of the coldest and driest areas on earth. The temperatures range from roughly minus 37 degrees F in winter to 48 degrees F in summer.

Some 55 million years ago, during the early Eocene Epoch, Ellesmere Island was warm and ice-free. It was also home to lush lowland forests and swamps inhabited by alligators, giant tortoises, snakes, lizards, and a host of mammals that included primates, tapirs, hippo-like Coryphodon, and large, rhino-like brontotheres. In the nearby ocean, clams and snails thrived. This extinct environment, well above the Arctic Circle, combined a warm temperate climate with a typical Arctic light regime of bright summers and dark winters.

The new study 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. They also were able to tease out temperature estimates for the warmest and coldest months of the year, critical data that should help scientists better understand past and future biodiversity in the High Arctic as the climate warms, including the geographical ranges and species richness of animals and plants.

The team concluded the average temperatures of the warmest month on Ellesmere Island during the early Eocene were from 66 to 68 degrees F, while the coldest month temperature was about 32 to 38 degrees F. "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.

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

"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, who also is the curator of fossil vertebrates at the University of Colorado Museum of Natural History.

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. Eocene fossil evidence collected there in recent decades by various teams indicate the lush landscape hosted giant tortoises, aquatic turtles, large snakes, alligators, flying lemurs, tapirs, and hippo-like and rhino-like mammals.

The bone and tooth enamel of vertebrate fossils contains biogenic apatite -- a mineral that is fossilized after the death of living organisms and which can be used as a "flight recorder" to infer paleoclimate conditions. Since all of the fossil materials were from the same stratigraphic layer and locality, the oxygen isotope ratios from the animals are linked to the temperatures of both ingested river water and precipitation at the time, allowing them to better estimate temperatures in the Eocene both annually and seasonally, she said.

"We use the water that the animals were drinking as a proxy for paleotemperature," said Eberle. "In mammal fossils, for example, we can analyze the oxygen isotope ratios in a sequence along the length of a large fossil tooth and estimate the warm-month and cold-month averages during the Eocene because teeth grow year round. 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.

The team looked at teeth from a large, hippo-like mammal known as Coryphodon, as well as bones from bowfin fish and shells and bones from aquatic turtles from the Emydidae family, the largest and most diverse family of contemporary pond turtles. While Coryphodon and bowfins grew throughout the year, the turtles exhibited shell growth only during summer months, much like turtles that live today in non-equatorial areas.

"By looking at a host of animals with different physiologies, we were better able to pin down warm- and cold-month temperatures," she said. "Many aspects of biodiversity and species richness are related more to seasonal temperatures and ranges such as cold-month means rather than to mean annual temperature."

Eberle said the new study implies Eocene alligators could withstand slightly cooler winters than their present day counterparts, although data from captive alligators show they are heartier than other members of the crocodilian family and can survive short intervals of subfreezing temperatures by submerging themselves in the water.

In contrast, the existence of large land tortoises in the Eocene High Arctic is still puzzling, said Eberle, since today's large tortoises inhabit places like the Galapagos Islands where the cold month average temperature is about 50 degrees F which is warmer than the extrapolated Eocene Ellesmere temperatures.

This suggests their present range in the Americas does not represent their fullest geographic range as allowed by climate.

For further information: http://www.colorado.edu/news/r/475370cb045bebb0fc62504cc72f9737.html