A uniquely specialized ear in a very early tetrapod
A uniquely specialized ear in a very early tetrapod
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DOI：
10.1038/nature01904
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发表时间：
2003-09-04
期刊：
影响因子：
64.8
通讯作者：
Ketcham, RA
中科院分区：
文献类型：
--
作者：
Clack, JA;Ahlberg, PE;Ketcham, RA
泥盆纪晚期鱼石螈是几十年来已知最早的四足动物，也是介于鱼类和陆地脊椎动物之间过渡形式的唯一代表。然而，尽管自1932年以来，人们就从大量标本中了解了它（参考文献1），但它的形态仍然是个谜，也不是人们所期望的非常原始的四足动物(2)。其明显的特化使其被认为是四足动物家族树的“盲分支”或“侧分支”，最近的分支学分析对其在四足动物茎群中的确切系统发育位置（5-8）存在分歧。特别是，它的脑壳和耳朵区域无法解释，以至于传统的解剖学术语似乎不适用(4)。使用1998年收集的新材料（参考文献9），准备早期收集的材料，以及高分辨率计算机断层扫描，我们在这里识别并解释了这些有问题的解剖结构。现在可以看到它们是高度特化的耳朵的一部分，可能是在水中使用的听力装置。这代表了四足动物区域在结构和功能上的独特修饰，不同于随后的四足动物进化中所见的任何修饰。与它同时代的棘螈（Acanthostega）一样，鱼石螈也有深槽状的鳃条（10），这表明鱼石螈可能比以前认为的更能适应水生环境。
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(2). Its apparent specializations led it to be considered as a "blind offshoot"(3) or "side-branch"(4) off the tetrapod family tree, and recent cladistic analyses have disagreed about its exact phylogenetic position(5-8) within the tetrapod stem group. In particular, its braincase and ear region defied interpretation, such that conventional anatomical terms seemed inapplicable(4). 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(10) suggest that Ichthyostega may have been more aquatically adapted than previously believed.