Earliest known tetrapod braincase and the evolution of the stapes and fenestra ovalis

Abstract

ACANTHOSTEGA gunnari, from the Upper Devonian (Famennian) of East Greenland, is the most primitive known tetrapod, and retains many fish-like characters^1–4. I report here the discovery of further well preserved specimens that show the earliest known tetrapod braincase, and shed light on the history of the tetrapod ear region. The fenestra ovalis is shown to be derived directly from the vestibular fontanelle^5,6, a hole in the sidewall of the braincase of fishes seen in their embryology and in primitive fossil fish adults. The hole is not a uniquely tetrapod character^7,8. A specialized auditory fenestra ovalis may have evolved more than once among tetrapods. As in other tetrapods, the stapedial footplate of Acan-thostega fitted into the fenestra ovalis, but instead of being free to vibrate as part of an ear, was firmly held there, forming a major component of the braincase wall. It was the only component linking the otic capsule to the palate. Though the stapes may have carried muscles operating a spiracular valve, the new material suggests that it was not a mobile component of the skull as previously suggested⁴. The stapes, spatially replacing parts of the fish brain-case including the process carrying facets for the hyomandibular articulation⁹, has a footplate which incorporates both heads of the sarcopterygian hyomandibula.