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The avian nature of the brain and inner ear of Archaeopteryx

Abstract

Archaeopteryx, the earliest known flying bird (avialan) from the Late Jurassic period, exhibits many shared primitive characters with more basal coelurosaurian dinosaurs (the clade including all theropods more bird-like than Allosaurus)1, such as teeth, a long bony tail and pinnate feathers2. However, Archaeopteryx possessed asymmetrical flight feathers on its wings and tail, together with a wing feather arrangement shared with modern birds. This suggests some degree of powered flight capability3 but, until now, little was understood about the extent to which its brain and special senses were adapted for flight. We investigated this problem by computed tomography scanning and three-dimensional reconstruction of the braincase of the London specimen of Archaeopteryx. Here we show the reconstruction of the braincase from which we derived endocasts of the brain and inner ear. These suggest that Archaeopteryx closely resembled modern birds in the dominance of the sense of vision and in the possession of expanded auditory and spatial sensory perception in the ear. We conclude that Archaeopteryx had acquired the derived neurological and structural adaptations necessary for flight. An enlarged forebrain suggests that it had also developed enhanced somatosensory integration with these special senses demanded by a lifestyle involving flying ability4.

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Figure 1: Braincase of the holotype of Archaeopteryx lithographica (BMNH 37001).
Figure 2: Right quadrate of BMNH 37001 in medial view.
Figure 3: Restored endocast of the brain of BMNH 37001 rendered as a shell.
Figure 4: Encephalization index for birds, reptiles and BMNH 37001.
Figure 5: Right inner ear of BMNH 37001.
Figure 6: Comparative proportions of the inner ear of BMNH 37001, selected recent birds, archosaurs and non-archosaur reptiles.

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Acknowledgements

We thank UTCT and NSF for funding CT scanning, and M. Colbert and J. Maisano (UTCT) for technical assistance. L. M. Witmer provided constructive comment and criticism. NERC funded the project, and the NHM Palaeontology Department Research Fund provided supplementary funds for P.D.A. We dedicate this paper to the memory of P. J. Whybrow, formerly Head of the Palaeontology Laboratory at the Natural History Museum London, who died suddenly on 13 February 2004. His skilled preparation work, undertaken long before the advent of CT technology, made this project possible.

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Correspondence to Angela C. Milner.

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Supplementary information

Supplementary Table 1

Data on body mass and endocranial volume of selected modern birds and reptiles in comparison with Archaeopteryx. Data sources are referenced in the main text of the paper. (DOC 313 kb)

Supplementary Table 2

Measurements and proportions of the inner ear of selected modem birds, non-archosaur and archosaur reptiles in comparison with Archaeopteryx. Data sources are referenced at the end of the table. (DOC 101 kb)

Supplementary Movie 1

Braincase reconstruction movie: Movie of the 3D reconstruction of the braincase of Archaeopteryx. The braincase has been restored by reversing the elements of the left side to model the whole structure. (MOV 3424 kb)

Supplementary Movie 2

Quadrate movie: Movie of the right quadrate of Archaeopteryx. The element is depicted exactly as it was preserved in the matrix, partly fragmented but all fragments in natural association. (MOV 553 kb)

Supplementary Movie 3

Brain endocast movie: Movie of the restored endocast of the brain of Archaeopteryx. The endocast has been restored by reversing the elements of the left side to model the whole structure. It has been rendered as a shell since the base of the braincase is absent. (MOV 2779 kb)

Supplementary Movie 4

Inner ear movie: Video clip of the 3D model of the right inner ear of Archaeopteryx lithographica (BMNH 37001). Missing parts of the semicircular canals indicated by blue connecting lines. (MOV 1569 kb)

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Alonso, P., Milner, A., Ketcham, R. et al. The avian nature of the brain and inner ear of Archaeopteryx. Nature 430, 666–669 (2004). https://doi.org/10.1038/nature02706

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