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Evolutionary origins of the avian brain

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Abstract

Features that were once considered exclusive to modern birds, such as feathers and a furcula, are now known to have first appeared in non-avian dinosaurs1. However, relatively little is known of the early evolutionary history of the hyperinflated brain that distinguishes birds from other living reptiles and provides the important neurological capablities required by flight2. Here we use high-resolution computed tomography to estimate and compare cranial volumes of extant birds, the early avialan Archaeopteryx lithographica, and a number of non-avian maniraptoran dinosaurs that are phylogenetically close to the origins of both Avialae and avian flight. Previous work established that avian cerebral expansion began early in theropod history and that the cranial cavity of Archaeopteryx was volumetrically intermediate between these early forms and modern birds3,4. Our new data indicate that the relative size of the cranial cavity of Archaeopteryx is reflective of a more generalized maniraptoran volumetric signature and in several instances is actually smaller than that of other non-avian dinosaurs. Thus, bird-like encephalization indices evolved multiple times, supporting the conclusion that if Archaeopteryx had the neurological capabilities required of flight, so did at least some other non-avian maniraptorans. This is congruent with recent findings that avialans were not unique among maniraptorans in their ability to fly in some form5,6.

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Figure 1: Coelurosaur phylogeny and partitioned endocranial casts.
Figure 2: Bivariate plots of log-transformed body-mass data.
Figure 3: Bivariate plots of log-transformed total-endocranial-volume data.
Figure 4: Principal components analysis plot of neuroanatomical region volumes.

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Acknowledgements

Funding for this project was provided by a NSF DDIG (DEB 0909970) to A.M.B. and M.A.N., NSF IIS-0208675 and EAR-0948842 to T.B.R. and a Columbia University International Travel Fellowship to A.M.B. The University of Texas Computed Tomography Facility and the AMNH MIF helped with computed tomographic scanning and processing imagery. M. Colbert, P. Gignac, D. Ksepka, J. Flynn and J. Meng read and provided useful comments on the text.

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Contributions

A.M.B., G.S.B. and M.A.N. designed the study. A.M.B. wrote the paper, performed data entry and analytical work, and prepared figures. G.S.B. assisted in data interpretation and helped to write the paper. T.B.R. contributed computed tomography data and assisted in data interpretation. M.A.N. provided computed tomography data and assisted in writing the paper.

Corresponding author

Correspondence to Amy M. Balanoff.

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The authors declare no competing financial interests.

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Balanoff, A., Bever, G., Rowe, T. et al. Evolutionary origins of the avian brain. Nature 501, 93–96 (2013). https://doi.org/10.1038/nature12424

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