PNAS：化石表明大象祖先可以水陆两栖
英美科学家进行的一项化石研究显示，大象3700万年前的祖先曾生活在水中。生活方式类似河马。这项研究发表在最新一期美国《国家科学院院刊》（PNAS）上。
据英国广播公司4月15日报道，英国牛津大学和美国纽约州立大学石溪分校的科学家对从埃及北部出土的两种牙齿化石进行了分析，它们同属于一个远古哺乳动物家族的两个物种。这一远古哺乳动物家族的动物都是大象的祖先，它们的外形介于马和犀牛之间。
研究人员说，这两个物种分别叫做钝兽和始祖象，它们生活在3700万年前的埃及北部。当时埃及北部被亚热带雨林和沼泽地覆盖。这两种动物大体上是水生的哺乳动物，以河流或沼泽中的淡水植物为食。（来源：新华网 葛秋芳）
生物谷推荐原始出处：
（PNAS），vol. 105 | no. 15 | 5786-5791 ，Alexander G. S. C. Liu, Erik R. Seiffert, and Elwyn L. Simons
Stable isotope evidence for an amphibious phase in early proboscidean evolution
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*Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, United Kingdom; Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794-8081; and Division of Fossil Primates, Duke Lemur Center, 1013 Broad Street, Durham, NC 27705
Contributed by Elwyn L. Simons, February 7, 2008 (received for review December 19, 2007)
The order Proboscidea includes extant elephants and their extinct relatives and is closely related to the aquatic sirenians (manatees and dugongs) and terrestrial hyracoids (hyraxes). Some analyses of embryological, morphological, and paleontological data suggest that proboscideans and sirenians shared an aquatic or semiaquatic common ancestor, but independent tests of this hypothesis have proven elusive. Here we test the hypothesis of an aquatic ancestry for advanced proboscideans by measuring 
18O in tooth enamel of two late Eocene proboscidean genera, Barytherium and Moeritherium, which are sister taxa of Oligocene-to-Recent proboscideans. The combination of low 18O values and low 18O standard deviations in Barytherium and Moeritherium matches the isotopic pattern seen in aquatic and semiaquatic mammals, and differs from that of terrestrial mammals. 13C values of these early proboscideans suggest that both genera are likely to have consumed freshwater plants, although a component of C3 terrestrial vegetation cannot be ruled out. The simplest explanation for the combined evidence from isotopes, dental functional morphology, and depositional environments is that Barytherium and Moeritherium were at least semiaquatic and lived in freshwater swamp or riverine environments, where they grazed on freshwater vegetation. These results lend new support to the hypothesis that Oligocene-to-Recent proboscideans are derived from amphibious ancestors.
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