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عدد المساهمات : 1147
تاريخ التسجيل : 05/02/2013
 | | موضوع: The Giant Camels of the Prehistoric High Arctic الأربعاء مارس 06, 2013 10:33 pm | |
|  Fossils can be astonishingly beautiful testaments of evolution and prehistoric time. There are few things I like better than wandering a museum’s fossil hall, admiring the skeletal architecture of a sauropod’s neck or the polished curl of an ammonite’s shell. But many, many fossils are ugly, hopelessly broken pieces of organisms past. The showroom pieces in museum halls only represent the rare, visually-magnificent few; or at least animals that paleontologists can reconstruct to the point where the remains take on an appealing cast of near-vitality. Nevertheless, even fossil crumbs have stories to tell if we know how to draw them out. One such skeletal remnant has just revealed the presence of an enormous camel that wandered forests of Canada’s High Arctic when the world was warmer.  The shards of the Ellesmere camel tibia. Photo © Martin Lipman, via the Canadian Museum of Nature. The fossil is the focus of a new Nature Communications paper by paleontologist Natalia Rybczynski and colleagues. From a purely aesthetic standpoint, the bone looks terrible. The thirty tan shards don’t even make up a full element. Altogether, laced together digitally, the scraps constitute part of a tibia from some sort of hoofed mammal. If they had been found in another deposit, the cracked fossils may not have even merited collecting, but these were the first bones found among the ancient plants of the Fyles Leaf Bed, and were apparently all that was left of a huge and hitherto undiscovered herbivore. But what kind of beast was it? In the paper, Rybczynski and coauthors point out that the anatomical landmarks on the pieces only narrow the fossil down to the level of artiodactyl – hoofed mammals with an even number of toes such as deer, cows, and camels. The size of the bone suggested that the tibia fragment came from a camel. At the time the bone was buried, about 3.4 million years ago, the largest artiodactyls in North America were camels. By using the proportions of dromedary and Bactrian camels as a proxy, the researchers estimated that the complete tibia would have been about 22 inches long, or about 29% longer than the same bone in the extant animals. Of course, that assumed that the bone truly did belong to a camel. Morphology alone did not solve the mystery. The solution was contained within the bone itself. Collagen is a major component of bone – it’s the primary protein that makes up the flexible part of skeletal elements. Not only is collagen able to survive for a long time in the fossil record, aided in this case by the cold and dry conditions that have since developed in the Arctic, but protein profiles of the material can help distinguish mammals at the genus level. With this in mind, Rybczynski and coauthors sampled prehistoric collagen from the Ellesmere camel and compared the signature of the protein with collagen from 37 other mammal species. The collagen from the High Arctic fossils most closely resembled the profiles of dromedary camels and prehistoric camel bones found in the Yukon, thought to be members of the genus that spawned modern camel species, Paracamelus. And the Ellesmere fossil was also a record-breaker – the bone belonged to a camel that lived about 750 miles further north than any other camel found on the continent. The environment the humped herbivore foraged over was quite a bit different than those of Ellesmere Island today. Around 3.4 million years ago, the global climate was about 35 degrees Fahrenheit warmer than at present, and the High Arctic habitats of Pliocene Ellesmere Island might have been as much as 70 degrees Fahrenheit hotter. The high latitude habitat still experienced chilly winters and almost six months of darkness, but the cold was not so harsh in the Pliocene. And, based on the plant fossils found at the same site, the ancient camels appear to have lived in boreal forests that were on the edge of the tundra. The existence of a huge camel in northern Canada might seem strange, but, in fact, the real oddity is that there are no longer any native camels in North America. The first camelids evolved in North America about 45 million years ago, and the herbivores proliferated into a variety of forms and sizes. By about 12,000 years ago, though, the last of North America’s camels were wiped out in the continent’s megafaunal extinction, leaving only two lineages present elsewhere in the world – the dromedary and Bactrian camels of Africa and Asia, and the llamas, alpacas, guanacos, and vicuñas of South America. In the big picture of camel evolution, the Ellesmere animal was on the dromedary and Bactrian branch and relatively close to the origin of those still-living species. Rybczynski and coauthors refrain from identifying the Ellesmere camel down to genus or species. Based on the collagen evidence, the herbivore might have been a northern population of Paracamelus related to the population preserved in the younger Yukon deposits , but additional bones from both sites are needed to be sure. Still, the paltry remains of the Yukon and Ellesmere camels indicate that the hebivores were capable of surviving in northern forests, and hint that camels as we know them today carry traits that evolved to help them survive in such cold habitats. The low-crowned teeth of modern camels might be an inheritance of ancestors that browsed in northern forests, Rybczynski and coauthors suggest, and a fatty hump that lets camels withstand harsh desert environments would have been just as advantageous in high latitude Pliocene habitats where the sky was dark for half the year. Adaptations that allow camels to thrive in deserts might have evolved in cool forests first, a testament to the flexibility of the wandering artiodactyls despite their ultimate extinction on the continent of their birth. | |
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