鯨豚的祖先 是超大長腳鼠
轉載自：http://tw.news.yahoo.com/article/url/d/a/071221/2/qbj9.html
2007/12/21 07:30 國際中心／綜合報導
英國「自然」科學雜誌十九日報導，根據在印度喀什米爾發現的化石，鯨魚、海豚與小海豚的祖先，是一種外表近似鹿的長尾無角小型哺乳類動物或超大型長腳鼠。
科學家過去一直懷疑，鯨魚源自四腳的哺乳類偶蹄動物，牠們生活在南亞陸地，後來逐漸適應海上生活，現在科學家總算找到鯨魚從陸地變成水棲動物失落的環節。
美國東北俄亥俄大學醫藥學院解剖學教授席維森等人相信，鯨類的祖先是一種浣熊大小的草食性哺乳動物「印多霍斯」（Indohyus），這種動物看來就像縮小版的鹿，生活在約四千八百萬年前。「印多霍斯」主要生活於陸地，只有在躲避掠食者時，才會遁入水中。
由於ＤＮＡ相似又有類似鯨魚的特徵，河馬過去一直被認為才是失落的環節，因此有些科學家對這項新發現存疑。
席維森在研究數百個印多霍斯化石後，得出上述結論。此外，印多霍斯的骨頭外層，比其他同體型的哺乳動物密實，牙齒的化學構造也有類似水棲動物的氧同位素，顯示這種動物也待在水中。
之前，科學家猜測，鯨魚為了吃海中的魚類，才從肉食性的偶蹄動物演化為水棲動物。但席維森說，根據他們的研究，情況並非如此。「印多霍斯」是為避免遭到掠食而遁入水中，後來生活於水中，最後才從草食變成肉食。
參閱：http://www.sciam.com/article.cfm?id=closest-whale-cousin
2007年12月20日 星期四
Eagle vs. Water Chevrotain
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Closest Whale Cousin—A Fox-Size Deer?
Researchers split on closest evolutionary kin to whales and dolphins
By JR Minkel
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Closest Whale Cousin—A Fox-Size Deer?
Researchers split on closest evolutionary kin to whales and dolphins
By JR Minkel
from:http://www.sciam.com/article.cfm?id=closest-whale-cousin
A group of researchers says that the closest known evolutionary cousin of whales, dolphins and porpoises is not the hippopotamus, as conventional wisdom has it, but an extinct deer-like animal roughly the size of a fox or raccoon. In a new study, the team finds that a fossilized specimen of the extinct, 48-million-year-old mammal Indohyus bears several telling similarities to whales, including dense limb bones for ballast and a middle ear structure found only in the cetaceans, or sea-dwelling mammals, which is thought to help them hear underwater.
"What we think happened is that the ancestors of both Indohyus and whales were animals that looked like a tiny deer," says Hans Thewissen, professor of anatomy at Northeastern Ohio Universities Colleges of Medicine and Pharmacy, who led the study, published in Nature. The modern creature that most resembles Indohyus, however, is the African mousedeer (or chevrotain), which lives on the forest floor but scurries into the water to take cover from predators [see video of eagle pursuing chevrotain]. Similarly, Thewissen says, the common ancestor of whales and Indohyus may have been a herbivore (plant-eater) that took to water to hide out, but eventually switched to a swimming, meat-eating lifestyle, which it passed down to modern cetaceans.
Other experts, however, caution that although the scenario is possible, the ancestry analysis is based on incomplete data. Researchers "really thought the book was closed on this," says Annalisa Berta, an evolutionary biologist at San Diego State University. "To suggest that this fossil somehow is closer than hippos, that's a big deal—I'm just not convinced."
Whatever its relationship with whales, Indohyus was probably not a direct predecessor of them, Thewissen says, because the specimen, unearthed 30 years ago in Kashmir, dates to roughly two million years after the earliest known cetacean fossils.
Over the past 15 years, researchers have uncovered a series of fossils intermediate between whales and land animals, but were still missing a link to landlubbing beasts, which Thewissen says Indohyus now provides.
The disagreement reflects the fact that researchers weren't around to watch successive populations of animals budding off from those that came before, resulting in a bushy pattern of new species over the millennia. Instead they have to infer the bush's branch points. They do this by categorizing extinct and living species together into the evolutionary bush that has the fewest cases of unrelated species evolving the same trait, which is in general less likely than one species passing down a unique trait to another species.
Indohyus, for example, has a half-walnut-shaped tympanic (a bony casing around the middle ear bones) with thicker walls on the sides than in front. The same thickening is found in all fossil and modern cetaceans but not in other mammals, the group notes. The fossil also has dense limb bones that would have weighed it down in water. But in contrast to the pointy molars of dolphins or killer whales, however, its equivalent nchompers are squarish like those of hippos, possibly for grinding plant matter, the group suggests.
The researchers used a computer program to test the possible evolutionary bushes that could have yielded cetaceans along with artiodactyls—the mammalian order made up of two-toed, hoofed animals—which include Indohyus and hippos.
The new analysis does not yet unseat the hippo as cetaceans' kissing cousin, because it only takes into account anatomical features, not molecular ones, says Maureen O'Leary, a professor in the department of anatomical sciences at Stony Brook University on Long Island, N.Y. She says that her own categorization of artiodactyls supports the hippo as the closest relative to cetaceans, but notes that it did not include the features uncovered by the Ohio team.
Nature 450, 1190-1194 (20 December 2007) | doi:10.1038/nature06343; Received 26 June 2007; Accepted 3 October 2007
Whales originated from aquatic artiodactyls in the Eocene epoch of India
J. G. M. Thewissen1, Lisa Noelle Cooper1,2, Mark T. Clementz3, Sunil Bajpai4 & B. N. Tiwari5
Department of Anatomy, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio 44272, USA
School of Biomedical Sciences, Kent State University, Kent, Ohio 44242, USA
Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA
Department of Earth Sciences, Indian Institute of Technology, Roorkee, Uttarakhand 247 667, India
Wadia Institute of Himalayan Geology, Dehra Dun, Uttarakhand 248 001, India
Correspondence to: J. G. M. Thewissen1 Correspondence and requests for materials should be addressed to J.G.M.T. (Email: thewisse@neoucom.edu).
Although the first ten million years of whale evolution are documented by a remarkable series of fossil skeletons, the link to the ancestor of cetaceans has been missing. It was known that whales are related to even-toed ungulates (artiodactyls), but until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth. We also show that a major dietary change occurred during the transition from artiodactyls to whales and that raoellids were aquatic waders. This indicates that aquatic life in this lineage occurred before the origin of the order Cetacea.
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