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A uniquely specialized ear in a very early tetrapod

Abstract

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 tetrapod2. Its apparent specializations led it to be considered as a “blind offshoot”3 or “sidebranch”4 off the tetrapod family tree, and recent cladistic analyses have disagreed about its exact phylogenetic position5,6,7,8 within the tetrapod stem group. In particular, its braincase and ear region defied interpretation, such that conventional anatomical terms seemed inapplicable4. 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 Acanthostega10 suggest that Ichthyostega may have been more aquatically adapted than previously believed.

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Acknowledgements

We thank the staff of MGUH and the Demark and Greenland Geological Survey for cooperation, access to material and support in the field; M. Colbert (UTCT) for technical assistance; H. Blom for discussion of the anatomy of Ichthyostega; and NERC and BBSRC for funding the project and additional CT scanning.

Author information

Correspondence to J. A. Clack.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Movie: This shows the skull roof with the braincase and stapes in place, rotating about an approximately parasagittal axis. Note the inflated chamber formed under the skull table area and the separation of this chamber from the adductor fossa. (MOV 673 kb)

Supplementary Movie: This shows a similar view to the above but with the skull roofing bones removed. (MOV 904 kb)

Supplementary Movie: This shows the brain endocast rotating about an approximately parasagittal axis. (MOV 696 kb)

Supplementary Movie: This shows the lateral view of the braincase with the stapes in place, roofing bones and palatoquadrate ossifications removed, rotating about a horizontalplane. (MOV 624 kb)

Supplementary Movie: This shows the braincase and roofing bones with the stapes in place, in lateral view rotating through about 45 degrees along a parasagittal axis. (MOV 659 kb)

Supplementary Movie: This shows the right stapes rotating about a horizontal plane. (MOV 572 kb)

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Further reading

Figure 1: Ichthyostega sp. specimens used for the study.
Figure 2: Models created from CT scan series of MGUH f.n.
Figure 3: Reconstructions of the braincase and otic region.
Figure 4: Cladogram of Devonian and Carboniferous taxa showing skulls and hyomandibula/stapes.

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