Clack J A
University Museum of Zoology, University of Cambridge, UK.
Nature. 1989 Nov 23;342(6248):425-7. doi: 10.1038/342425a0.
The evolution of the middle ear is central to the discussion of how the first tetrapods adapted to life on land as well as their phylogeny. Here I report the discovery of the stapes of Acanthostega gunnari, from the Upper Devonian of east Greenland. This is the earliest tetrapod stapes so far described, and it throws new light on both these aspects of early tetrapod biology. It has been assumed that the common inheritance of all early tetrapods was a light, rod-like stapes associated with a temporal notch in the otic region that was thought to have supported a tympanum, or eardrum. The stapes would have conducted vibrations from the tympanum to the otic capsule. By contrast, the stapes of Acanthostega was stout with a broad distal ramus associated with the temporal notch. I suggest that the temporal notch of Acanthostega and other early tetrapods supported a spiracular opening rather than a tympanum, and that the stapes controlled palatal and spiracular movements in ventilation.
中耳的演化是探讨最早的四足动物如何适应陆地生活及其系统发育的核心内容。在此，我报告在东格陵兰上泥盆统发现的冈纳棘螈（Acanthostega gunnari）的镫骨。这是迄今为止所描述的最早的四足动物镫骨，它为早期四足动物生物学的这两个方面都带来了新的启示。人们一直认为，所有早期四足动物的共同遗传特征是一种轻巧的、棒状的镫骨，它与耳区的一个颞切迹相关联，该切迹被认为支撑着一个鼓膜，即耳膜。镫骨会将振动从鼓膜传导至耳囊。相比之下，棘螈的镫骨粗壮，其远端分支宽阔，与颞切迹相关。我认为，棘螈和其他早期四足动物的颞切迹支撑的是一个鳃裂孔而非鼓膜，并且镫骨在呼吸过程中控制着腭部和鳃裂孔的运动。
