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| Lestodon armatus | |
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| Tweet Topic Started: Nov 21 2013, 07:41 PM (3,577 Views) | |
| Taipan | Nov 21 2013, 07:41 PM Post #1 |
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Lestodon armatus![]() Temporal range: Pleistocene Scientific classification Kingdom: Animalia Phylum: Chordata Class: Mammalia Superorder: Xenarthra Order: Pilosa Family: †Mylodontidae Subfamily: †Lestodontinae Tribe: †Lestodontini Genus: †Lestodon Species: †Lestodon armatus Lestodon is an extinct genus of megafaunal ground sloth from South America during the Pleistocene period. Its fossil remains have been found in Argentina, Paraguay, Uruguay, and Bolivia. 4.6 m (15 ft) from snout to tail tip, it is estimated to have weighed between 2 and 4 tons. It was a herbivore and primarily fed on the grasses on the South American plains and is thought to perhaps have used its semi-bipedal stance to obtain foliage from trees. Lestodon is placed as member of the Mylodontidae as indicated by the lobed form of the last tooth in the dentition. The genus name "Lestodon" derives from the Greek for "robber tooth." ![]() Skull of Lestodon armatus at the Harvard Museum of Natural History Evolutionary Adaptations The skeleton morphologically reflects its evolutionary history as well as its daily activities. At the caudal end, the sloth’s head connects to its neck with a class one lever. The skull is two to three orders of magnitude larger than living species of sloth, but the muzzle itself shows highly positive allometry to the size of the skull in comparison to modern sloths and even other megafauna of the same order Pilosa. L. armatus had a large, blunt, and square mandible well adapted for foliage consumption. Its muzzle and dentition show a significant adaptation to this diet. The shape of the muzzle aided the ground sloth in the grazing necessary to sustain the metabolic activity of its large body; square, flat muzzles are associated with bulk feeders while pointed snouts are adaptive features of precision eaters, like the long sloping snout of modern anteaters, which are also in the same class Xenarthra (Bargo et al., 2006). The mandible, a class three lever, has a shape that indicates evolutionary pressure for strength over speed. To sustain its caloric needs with only plant matter, the sloth would have needed to be able to consume a high quantity of food with little precision. The species would dig for its food, and would have accidentally ingested a large amount of gritty dirt and soil particulate. This gives them a large crown height, called hypsodonty, that is unsuitable for strong bite forces. While the mandibular muscles would have been well developed to support the allometrically positive jaw of L. Armatus, they would have also been extremely weak (Bargo et al., 2006). ![]() Phalanges of Lestodon armatus at the Harvard Museum of Natural History Appendicular Skeleton The pursuit of bulk plant matter is reflected in the stance, movement, and total bone structure of the ground sloth. In order to dislodge a large amount of foliage from the earth, L. armatus would have used its flat, spade-like phalanges in a hook-and-pull fashion to turn over soil and vegetation. Once again, the evolutionary design of the giant sloth favors quantity and force to quality and speed in its manus. While the giant sloth was not fully bipedal, its forelimbs show evolutionary pressure for specialized functions over walking and could easily shift its center of gravity. The olecranon of L. Armatus, for example, is longer allometrically than species whose forelimbs are primarily adapted for movement; the longer olecranon on the ulna maximizes the leverage to the moment arm of the triceps, the extensor of the forearm, but is proportionally much longer than is necessary for walking (Bargo et al., 2000). The extension of the forearm would have been very strong, but not very fast. Triceps are the primary muscle activated during the digging motion, but the odd articulation of the scapula to the humerus, and therefore the orientation of the teres major, also strengthens the force during pulling. Compared to humans, the scapula is tilted laterally, with the medial margin at a higher elevation than the acrimonion. This orientation maximizes the power of the teres major during digging. Energy storage is possible in the tendons of the triceps, produces power necessary to execute the digging behavior Lestodon armatus uses to acquire the bulk of its diet. The spinal column of L. Armatus is arranged parallel to the ground, making the angle between the femurs and the majority of the mass of the megafauna slightly more than orthogonal. On the femur, the femoral condyles have a large articulated range, which helps them to extend their posture vertically from their normal quadrupedal position. Extant related species of the magnorder Xenarthra, like anteaters, show a similar design. All species within this order have specialized forelimbs and a center of gravity shifted caudally towards the pelvis from other quadrupeds, but rostrally when compared to fully bipedal humans. The lumbar vertebrae are said to be xenarthrous, the eponymous characteristic of the magnorder, and that means that they come into contact with each other at more points to stabilize the pelvis and the bulk of the caudally shifted mass. It would have been unable to gallop. In humans, pelvic bones are not fused together in a solid block, and this allows us greater variability of motion from side to side. Though there is reduced mobility, xenarthrous fusion provides a stable platform for digging. Were People Killing Giant Sloths in South America 30,000 Years Ago? Bones with marks left by human tools could point to earlier human arrival in the Americas. ![]() A giant sloth bone might hold the key to the peopling of the Americas. PHOTOGRAPH BY MARTIN BATALLES ![]() Map of Uruguay Arroyo by NGM Maps. Helen Thompson for National Geographic PUBLISHED NOVEMBER 20, 2013 When did people get to the Americas? The answer remains a subject of fiery debate. Most scientists agree that humans began arriving in the Americas between 13,000 and 15,000 years ago, and the Clovis people of North and Central America are generally considered the "first Americans." But new fossil evidence from a streambed in southern Uruguay could challenge such theories. Results published November 19 in Proceedings of the Royal Society B suggest the presence at the site of human hunters who may have killed giant sloths and other megafauna. That itself isn't odd, but the site, called Arroyo del Vizcaíno, has been radiocarbon dated to between 29,000 and 30,000 years old—thousands of years before people were thought to be there. "That's pretty old for a site that has evidence of human presence, particularly in South America," said study co-author Richard Fariña, a paleontologist at Uruguay's Universidad de la República. "So, it's strange and unexpected." What's the controversy? Giant sloths, saber-toothed cats, oversize armadillos, and other large mammals once roamed the Americas—a diversity that would easily rival an African savannah today. But by 11,000 years ago, many of the species had disappeared, likely due to climate change or the arrival of human hunters in the New World. But when exactly humans got here, and how they arrived, remains unknown. What's new? In 1997, severe drought forced local farmers to drain a lagoon in Arroyo del Vizcaíno, which exposed a mysterious bed of gigantic bones. After a series of bureaucratic roadblocks, paleontologists excavated the site in 2011 and 2012, unearthing over a thousand fossils. "From the paleontological point of view, that is absolutely marvelous in itself," Fariña said. Many of the bones belong to three extinct ground sloth species, mainly Lestodon armatus. Weighing in at up to four tons, the animals "were the size of smallish elephants," he said. ![]() The giant sloth bones were uncovered at a dig site dated at 29,000 to 30,000 years old. Fossils from other common South American megafauna turned up in the mud as well: three species of glyptodonts, or armadillo ancestors; a hippo-like animal called a toxodon, which has no living relatives; a South American saber-toothed cat (Smilodon populator); and an elephant-like stegomastodon, among others. Some of the bones bear telltale markings of human tools, which suggests the animals were hunted for food. The team also found a potentially human-made scraper that could have been used on dry animal hides, and stone flakes. Why is it important? Clues from the site point to a human presence at Arroyo del Vizcaíno much earlier than accepted theories of migration. Fariña and his team are both excited and cautious about their results. Farino said the strength of the new evidence lies in the team's methodology and the fact that two of the bones they tested for dating also bore markings similar to those made by human tools. "The association can't be closer than it is," he said. The date of Arroyo del Vizcaíno may make some archaeologists cringe: South America's earliest human settlement at Monte Verde in Chile dates to only 14,000 years ago. What does this mean? The study certainly does not prove definitively that humans were killing giant sloths 30,000 years ago in South America. The fossils found at Arroyo del Vizcaíno might simply be a product of nature mimicking human tools, and the authors acknowledge that possibility. "South America played an exceptionally important role in the peopling of the Americas, and I'm pretty sure we have some significant surprises waiting for us," Bonnie Pitblado, an archaeologist at the University of Oklahoma who was not associated with the study, said in an email. "Maybe people killing sloths at [the Arroyo del Vizcaíno site] 30,000 years ago is one of them, maybe it's not—but it certainly isn't going to hurt to have it on our collective radar screen as we continue to contemplate the peopling of the New World." What's next? The Uruguayan team has further excavations and environmental reconstruction studies planned for the site. Fariña estimates that it could yield a thousand more bones, and they plan to build a local museum to house the site's many fossils. http://news.nationalgeographic.com/news/2013/11/131120-giant-sloths-people-americas-ancient-archaeology-science/ |
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