U. COLORADO (US)—Ancient alligators and giant tortoises were able to flourish on Ellesmere Island well above the Arctic Circle some 50 million years ago, even as they endured six months of darkness each year. Now scientists think they know why.
Findings from a new study, which looked at temperatures during the early Eocene period 52 to 53 million years ago, have implications for the impacts of future climate change as Arctic temperatures continue to rise, says Jaelyn Eberle, the study’s lead author and an associate professor in the geological sciences department at the University of Colorado at Boulder.
Researchers 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 Fahrenheit (19-20 degrees C), while the coldest month temperature was about 32 to 38 degrees F (0-3.5 degrees C).
“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 says.
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,” says Eberle.
During the Eocene, Ellesmere Island—which is adjacent to Greenland—probably was similar to swampy cypress forests in the southeastern United States today, says 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 says.
“We use the water that the animals were drinking as a proxy for paleotemperature,” explains 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 adds.
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 says. “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.”
Bowfins—which have a long dorsal fin and powerful jaws—inhabit a variety of waters today from the Saint Lawrence River drainage in Quebec south to Florida and Texas. The team also compared the ranges of bowfins, aquatic turtles and giant tortoises of today with their ranges in the Eocene to help them estimate temperatures, according to coauthor Michael Newbrey from University of Alberta, Edmonton.
Eberle says 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 somewhat puzzling, adds Eberle, since today’s large tortoises inhabit places like the Galapagos Islands where the cold-month average temperature is about 50 degrees F (10 degrees C.)
But during the late Pleistocene period some 10,000 to 50,000 years ago—when air temperatures were comparable to those today—large land tortoises were found as far north as present-day Pennsylvania and Illinois, Eberle notes.
This suggests their present range in the Americas does not represent their fullest geographic range as allowed by climate. Factors like hunting by early Native Americans and the past extent of glaciers probably are playing a role in today’s distribution of giant tortoises, she adds.
Eberle, who calls the new results “a deep time analogue” for today’s rapidly warming Arctic region, says quantitative estimates of early Eocene climate conditions at high latitudes like Ellesmere Island are rare and often contradictory. Previous estimates of the early Eocene mean annual temperatures have ranged from 39 to 68 degrees F (4 to 20 degrees C), a temperature range equivalent to geographic ranges reaching from Canada to Florida.
The new study foreshadows the impacts of continuing global warming on Arctic plants and animals, Eberle says. 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 human activities like fossil fuel burning and deforestation, according to climate scientists.
Researchers from Colorado College, Colorado School of Mines, and the University California, Berkeley, collaborated on the work, which was funded by the National Science Foundation.
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