2016年12月7日，国际著名学术杂志《Cell》子刊《Current Biology》杂志在线发表了美国宾夕法尼亚大学地球和环境学系助理教授Lauren Sallan的一篇研究论文，论文揭示了鱼尾部进化过程。

尽管它们体型存在明显的不同，但大象、蜥蜴和鳟鱼有相似之处。它们的身体末端都有细长、灵活的结构，人们将其称为尾巴。但美国宾夕法尼亚大学古生物学家的新研究揭示，鱼类尾部和四足动物的尾部实际上是完全不同的结构，有不同的进化历史。

宾夕法尼亚大学地球和环境学系助理教授Lauren Sallan分析了一个具有3.5亿年历史的幼鱼化石。这条幼鱼长着一条有鳞的肉质尾巴和一条柔软的尾鳍，这两个部位相互上下挨着。该研究组发现这些尾部结构是完全独立的。

研究人员指出，该鱼类为Aetheretmon valentiacum，属于硬骨鱼。该化石数十年前被发现于苏格兰，并一直被保存在博物馆，但一直未被详细研究。成年Aetheretmon鱼也有非对称的尾部，上部的尾巴长于下部的尾巴，并包含脊椎骨。而现代硬骨鱼的胚胎也有相似的双尾结构。

“3.5亿年前，古老鱼类和现代鱼类有相同的进化起始点。”Sallan说，“并非现代硬骨鱼胚胎上出现了‘祖先的’尾部，而是所有的鱼的尾部都有两个不同的结构，但随着功能和生态要求，它们的尾部出现了调整。”

这个发现推翻了之前的观点：人们一直认为的现代成年鱼的尾鳍只是简单地加到其祖先尾巴的末端。研究人员表示，鱼退去了肉质尾巴，留下了更灵活的尾鳍以提高它们的游水能力。后来有些鱼类进化成半水生动物，然后逐渐演变成陆生动物，它们失去了灵活的尾鳍但保留下了肉质尾巴。而且该发现还有助于解释为什么人类没有尾巴，但都有尾椎骨。

原文链接：

Fish ‘tails’ result from outgrowth and reduction of two separate ancestral tails

原文摘要：

The symmetrical, flexible teleost fish ‘tail’ has been a prime example of recapitulation — evolutionary change (phylogeny) mirrored in development (ontogeny). Paleozoic ray-finned fishes (Actinopterygii), relatives of teleosts, exhibited ancestral scale-covered tails curved over their caudal fins. For over 150 years, this arrangement was thought to be retained in teleost larva and overgrown, mirroring an ancestral transformation series. New ontogenetic data for the 350-million-year-old teleost relative Aetheretmon overturns this long-held hypothesis. The ancestral state consists of two outgrowths with distinct organizers and growth trajectories; a lower median fin turned caudal fin, and an upper vertebrae-bearing tail, equivalent to that of tetrapods. These two tails appear at a shared developmental stage in Aetheretmon, teleosts and all living actinopterygians. ontogeny does not recapitulate phylogeny; instead, differential outgrowth determines final morphology. In Aetheretmon and other Paleozoic fishes, the vertebrae-bearing tail continues to grow beyond the caudal fin. In teleosts, and some others, a stunted tail is eclipsed by the upward-expanding caudal fin, rendering a once ventral body margin as the terminus. The double tail likely reflects the ancestral state for bony fishes. Many tetrapods and non-teleost actinopterygians have undergone body elongation through tail outgrowth extension, by mechanisms likely shared with distal limbs. Teleosts have gone to the other extreme; losing tail outgrowth for functional reasons. Recognition of the tail as a limb-like outgrowth has important implications for the evolution of vertebrate form.