Radical transformation of the skull characterizes bird evolution. An increase in the relative size of the brain and eyes was presumably related to the loss of two bones surrounding the eye, the prefrontal and postorbital. We report that ossification centres of the prefrontal and postorbital are still formed in bird embryos, which then fuse seamlessly to the developing nasal and frontal bones, respectively, becoming undetectable in the adult. The presence of a dinosaur-like ossification pattern in bird embryos is more than a trace of their evolutionary past: we show how persistent modularity of ossification centres has allowed for evolutionary re-organization of skull architecture in evolution. Our findings also demonstrate that enigmatic mesodermal cells forming the posterior region of the avian frontal correspond to the ossification centre of the postorbital, not the parietal, and link its failure to develop into an adult bone to its incorporation into the expanded braincase of birds.
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We wish to thank B.-A. Bhullar for kindly allowing us to examine and photograph embryos of A. mississippiensis. Special thanks go to M. Sallaberry and J. Mpodozis at Universidad de Chile. This work was funded by grants Anillo ACT172099 and Fondecyt 1150906 (Conicyt, Government of Chile) to A.O.V. This work is dedicated to the memory of Professor Juan Fernández Hidalgo.
The authors declare no competing interests.
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Smith-Paredes, D., Núñez-León, D., Soto-Acuña, S. et al. Dinosaur ossification centres in embryonic birds uncover developmental evolution of the skull. Nat Ecol Evol 2, 1966–1973 (2018). https://doi.org/10.1038/s41559-018-0713-1
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