海豹、海狮和海象（统称为鳍足类动物）是从陆生食肉动物演化来的，但最早的已知鳍足类动物“海熊兽”已经有了脚蹼。
现在，加拿大北极地区一个新发现的化石让我们有机会看到鳍足类演化的一个更早阶段，它是一种像水獭一样的陆生和水生之间的过渡形式。该化石是一个几乎完整的骨架，适应半水生生活，包括趾骨之间可能形成了蹼。这一发现支持关于鳍足类起源于北极的假说。（生物谷Bioon.com）
生物谷推荐原始出处：
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.
