海豹,、海獅和海象（統(tǒng)稱為鰭足類動(dòng)物）是從陸生食肉動(dòng)物演化來(lái)的，但最早的已知鰭足類動(dòng)物“海熊獸”已經(jīng)有了腳蹼,。
現(xiàn)在,，加拿大北極地區(qū)一個(gè)新發(fā)現(xiàn)的化石讓我們有機(jī)會(huì)看到鰭足類演化的一個(gè)更早階段，它是一種像水獺一樣的陸生和水生之間的過(guò)渡形式,。該化石是一個(gè)幾乎完整的骨架,，適應(yīng)半水生生活，包括趾骨之間可能形成了蹼,。這一發(fā)現(xiàn)支持關(guān)于鰭足類起源于北極的假說(shuō),。（生物谷Bioon.com）
生物谷推薦原始出處：
Nature 458, 1021-1024 (23 April 2009) | doi:10.1038/nature07985
A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia
Natalia Rybczynski1, Mary R. Dawson2 & Richard H. Tedford3
1 Canadian Museum of Nature, PO Box 3443 STN D, Ottawa, Ontario K1P 6P4, Canada
2 Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania 15213, USA
3 Division of Paleontology, American Museum of Natural History, New York, New York 10024, USA
Modern pinnipeds (seals, sea lions and the walrus) are semi-aquatic, generally marine carnivores the limbs of which have been modified into flippers. Recent phylogenetic studies using morphological and molecular evidence support pinniped monophyly, and suggest a sister relationship with ursoids1, 2 (for example bears) or musteloids3, 4, 5, 6, 7 (the clade that includes skunks, badgers, weasels and otters). Although the position of pinnipeds within modern carnivores appears moderately well resolved, fossil evidence of the morphological steps leading from a terrestrial ancestor to the modern marine forms has been weak or contentious. The earliest well-represented fossil pinniped is Enaliarctos, a marine form with flippers, which had appeared on the northwestern shores of North America by the early Miocene epoch8, 9. Here we report the discovery of a nearly complete skeleton of a new semi-aquatic carnivore from an early Miocene lake deposit in Nunavut, Canada, that represents a morphological link in early pinniped evolution. The new taxon retains a long tail and the proportions of its fore- and hindlimbs are more similar to those of modern terrestrial carnivores than to modern pinnipeds. Morphological traits indicative of semi-aquatic adaptation include a forelimb with a prominent deltopectoral ridge on the humerus, a posterodorsally expanded scapula, a pelvis with relatively short ilium, a shortened femur and flattened phalanges, suggestive of webbing. The new fossil shows evidence of pinniped affinities and similarities to the early Oligocene Amphicticeps from Asia and the late Oligocene and Miocene Potamotherium from Europe. The discovery suggests that the evolution of pinnipeds included a freshwater transitional phase, and may support the hypothesis that the Arctic was an early centre of pinniped evolution.
