Rybczynski Natalia, Dawson Mary R, Tedford Richard H
Canadian Museum of Nature, PO Box 3443 STN D, Ottawa, Ontario K1P 6P4, Canada.
Nature. 2009 Apr 23;458(7241):1021-4. doi: 10.1038/nature07985.
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 ursoids (for example bears) or musteloids (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 epoch. 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.
现代鳍足类动物（海豹、海狮和海象）是半水生的，通常为海洋食肉动物，其四肢已演变成鳍状肢。最近利用形态学和分子证据进行的系统发育研究支持鳍足类动物的单系性，并表明它们与熊型类（如熊）或鼬型类（包括臭鼬、獾、黄鼠狼和水獭的进化枝）是姐妹关系。尽管鳍足类动物在现代食肉动物中的位置似乎已得到较好的确定，但从陆地祖先到现代海洋形态的形态学演变步骤的化石证据一直很薄弱或存在争议。最早有充分代表性的鳍足类动物化石是恩氏海熊兽，这是一种有鳍状肢的海洋动物，在中新世早期出现在北美西北海岸。在此，我们报告在加拿大努纳武特地区一个中新世早期湖泊沉积物中发现了一种新的半水生食肉动物的近乎完整的骨骼，它代表了早期鳍足类动物进化中的一个形态学环节。这个新分类单元保留了一条长尾巴，其前肢和后肢的比例与现代陆地食肉动物更为相似，而与现代鳍足类动物不同。表明半水生适应的形态特征包括前肢肱骨上有突出的三角肌嵴、肩胛后背部扩大、骨盆的髂骨相对较短、股骨缩短以及指骨扁平，暗示有蹼。新化石显示出与鳍足类动物的亲缘关系以及与来自亚洲的渐新世早期两栖兽和来自欧洲的渐新世晚期及中新世的河兽的相似性。这一发现表明鳍足类动物的进化包括一个淡水过渡阶段，可能支持北极是鳍足类动物早期进化中心的假说。
