Clack J A, Ahlberg P E, Finney S M, Dominguez Alonso P, Robinson J, Ketcham R A
University Museum of Zoology, Cambridge, Downing Street, Cambridge, CB2 3EJ, UK.
Nature. 2003 Sep 4;425(6953):65-9. doi: 10.1038/nature01904.
The Late Devonian genus Ichthyostega was for many decades the earliest known tetrapod, and the sole representative of a transitional form between a fish and a land vertebrate. However, despite being known since 1932 (ref. 1) from a large collection of specimens, its morphology remained enigmatic and not what was expected of a very primitive tetrapod. Its apparent specializations led it to be considered as a "blind offshoot" or "sidebranch" off the tetrapod family tree, and recent cladistic analyses have disagreed about its exact phylogenetic position within the tetrapod stem group. In particular, its braincase and ear region defied interpretation, such that conventional anatomical terms seemed inapplicable. Using new material collected in 1998 (ref. 9), preparation of earlier-collected material, and high-resolution computed tomography scanning, here we identify and interpret these problematic anatomical structures. They can now be seen to form part of a highly specialized ear, probably a hearing device for use in water. This represents a structurally and functionally unique modification of the tetrapod otic region, unlike anything seen in subsequent tetrapod evolution. The presence of deeply grooved gill bars as in its contemporary Acanthostega suggest that Ichthyostega may have been more aquatically adapted than previously believed.
晚泥盆世的鱼石螈属在几十年间一直是已知最早的四足动物，也是鱼类与陆地脊椎动物之间过渡形态的唯一代表。然而，尽管自1932年（参考文献1）以来就已通过大量标本对其有所了解，但其形态仍然成谜，并不符合人们对一种非常原始的四足动物的预期。其明显的特化特征使其被视为四足动物谱系树的“旁支”或“侧枝”，并且最近的分支系统学分析对于它在四足动物干群中的准确系统发育位置也存在分歧。特别是，其脑壳和耳部区域难以解释，以至于传统的解剖学术语似乎并不适用。利用1998年收集的新材料（参考文献9）、对早期收集材料的整理以及高分辨率计算机断层扫描，我们在此识别并解释了这些有问题的解剖结构。现在可以看出它们构成了一个高度特化耳朵的一部分，可能是一种用于水中的听觉装置。这代表了四足动物耳部区域在结构和功能上的独特变化，与后续四足动物进化过程中所见的任何情况都不同。像其同时代的棘螈一样，鱼石螈存在深深凹陷的鳃弓，这表明鱼石螈可能比之前认为的更适应水生环境。
