Friedman Matt
Committee on Evolutionary Biology, University of Chicago, 1025 East 57th Street, Chicago, Illinois 60637, USA.
Nature. 2008 Jul 10;454(7201):209-12. doi: 10.1038/nature07108.
All adult flatfishes (Pleuronectiformes), including the gastronomically familiar plaice, sole, turbot and halibut, have highly asymmetrical skulls, with both eyes placed on one side of the head. This arrangement, one of the most extraordinary anatomical specializations among vertebrates, arises through migration of one eye during late larval development. Although the transformation of symmetrical larvae into asymmetrical juveniles is well documented, the evolutionary origins of flatfish asymmetry are uncertain because there are no transitional forms linking flatfishes with their symmetrical relatives. The supposed inviability of such intermediates gave pleuronectiforms a prominent role in evolutionary debates, leading to attacks on natural selection and arguments for saltatory change. Here I show that Amphistium and the new genus Heteronectes, both extinct spiny-finned fishes from the Eocene epoch of Europe, are the most primitive pleuronectiforms known. The orbital region of the skull in both taxa is strongly asymmetrical, as in living flatfishes, but these genera retain many primitive characters unknown in extant forms. Most remarkably, orbital migration was incomplete in Amphistium and Heteronectes, with eyes remaining on opposite sides of the head in post-metamorphic individuals. This condition is intermediate between that in living pleuronectiforms and the arrangement found in other fishes. Amphistium and Heteronectes indicate that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature.
所有成年比目鱼（鲽形目），包括在美食方面为人熟知的鲽鱼、鳎鱼、大菱鲆和大比目鱼，都有高度不对称的头骨，双眼位于头部的一侧。这种排列方式是脊椎动物中最奇特的解剖学特化之一，它是在幼体发育后期一只眼睛迁移而形成的。尽管对称的幼体转变为不对称的幼鱼这一过程有充分的文献记载，但比目鱼不对称的进化起源尚不确定，因为没有过渡形态将比目鱼与其对称的亲属联系起来。这种中间形态被认为无法存活，这使得鲽形目在进化辩论中占据了突出地位，引发了对自然选择的抨击以及关于跳跃式变化的争论。在这里，我表明欧洲始新世已灭绝的棘鳍鱼类——颈鳍鱼属和新属异眼鱼属，是已知最原始的鲽形目鱼类。这两个类群头骨的眼眶区域都像现存比目鱼一样强烈不对称，但这些属保留了许多现存形态中未知的原始特征。最引人注目的是，颈鳍鱼属和异眼鱼属的眼眶迁移并不完全，变态后的个体眼睛仍位于头部的两侧。这种情况介于现存鲽形目鱼类和其他鱼类的排列方式之间。颈鳍鱼属和异眼鱼属表明，现存比目鱼颅骨深度不对称的进化在本质上是渐进的。
