10 mars 2017

La préhistoire de l'Arctique


Il y a 53 millions d'années, l'Arctique était bien différent du désert glacé que nous connaissons.

Extraits de l'article:

(...) More than 53 million years ago, the Arctic was warm, and covered with extensive jungles and swamps. It was a far cry from the icy carpet that covers the region today.

Neither Gastornis nor Presbyornis are new animals in the fossil record. (...) One was a graceful, long-legged wader while the other was a bulky and flightless nutcracker with a huge beak. Now though, a series of new and surprising fossil fragments has been receiving extra attention.

(...) The location of the new find was the Eureka Sound Group of Ellesmere Island, one of the larger islands in the Arctic Ocean and Canada’s third largest island. It is one of the coldest and driest places in the world today, lying somewhat above the Arctic Circle. Much of it is locked within glaciers and ice caps these days, supporting only the traditional Arctic fare of polar bears, seals, marine birds and whales.

(...) Stidham and team had uncovered the remains of a lost forest, one that stretched from Germany and France, all the way up to England and to Ellesmere itself. (...) The plants found here include dawn redwoods, fast-growing deciduous trees up to sixty meters in height, as well as swamp cypresses. At the time it was more like a wetland in Louisiana or Florida but somewhat milder, with summer temperatures as much as 20 degrees Celsius. Yet the lights still went out during winter, even if it probably never got too far below freezing.

The Eocene world was humid during periods of extended warmth and the earliest mammals were just beginning to enlarge into somewhat recognizable forms. (...) The ancestors of horses, rhinos, cats and dogs were all part of this fauna and today, and might seem like just your generic little mammals. Only a few were as big as or bigger than a tapir or a pygmy hippo.

The Ellesmere fossils also show us that crocodiles, turtles and freshwater fish existed here too.

(...) When the earliest fossils of Gastornis were described in 1855 on the basis of remains found close to Paris, they were thought to be relatives of the modern ostriches and emus. (...) Other fossils from around the Northern Hemisphere though, revealed that it was a relative of ducks and geese, but unlike any other that ever lived. For one, it stood as tall as a man and was unable to fly. Its body and legs were very stocky and its head was topped with a massive beak. Earlier thought to be a fierce predator that preyed on early horses, it is now known to be a herbivore. That massive beak was heavy and lacked the hooked tip of a predatory bird and the bird even lacked sharp talons. It was certainly happier cracking the nuts of palm trees and other tough vegetation.

This made it like a giant, flightless parrot and not just a typical duck. Being taller than the contemporary mammals, it could probably browse from small trees and thus had a great impact on its environment. Thus instead of killing the mammals it lived with, it would be feeding alongside them. This left only the crocodiles and alligators of the time as the apex predators of the Eocene forests.

It was not, however the only big bird found here. Beside Gastornis was another well-known duck named Presbyornis, a somewhat smaller and less impressive creature. Presbyornis was roughly as big as a swan and was able to fly well. Its long legs and neck meant that it was thought to be a flamingo when it was first described in 1926. This bird was a wader, striding through swamps and rivers as it searched for fish or other aquatic meals. Being a flying aquatic bird, it was also probably a migrant that spent the winter at Eureka Sound.

This makes sense as aquatic birds migrate between the Arctic, Eurasia and North America. Plenty of ducks do this today and it makes sense that Presbyornis did the same thing 53 million years ago. Certainly Presbyornis has been discovered all over the Northern Hemisphere while the Ellesmere fossils show the northernmost extent of these creatures. The new remains consist of upper arm bones, with Stidham himself saying how similar they were to the bones found in Wyoming.

(...) Ironically enough the Ellesmere fossils themselves also show us a very fragile ecosystem that would die out in just a few million years. The world would cool down and the glaciers would overtake the north at last, destroying the beautiful Arctic forests for good.

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L'oiseau géant de l'Arctique



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