Abstract
de Beer1 proposed that ostriches and their allies, the ratites, are degenerate birds that evolved, neotenously, from cannates (flying birds). Others2–5 have argued for their primitiveness, but the view that ratites are derived within birds has prevailed6–8. The problem is soluble only when avian phytogeny is understood, and Ostrom9 makes a strong case for the evolution of birds from theropod dinosaurs. Included in his evidence was the ascending process of the astragalus, a feature first shown by Huxley2 to be unique to theropods and birds (Fig. 1). Ostrom's critics10, however, argue that the avian process is not associated with the astragalus, being instead an independent ossification, the pretibial bone11. The present ontogenetic study reveals both processes in birds: the ascending process exemplified by ratites and the pretibial bone by carinates. This implies that, since carinates possess the unique pretibial bone, they could not have given rise to the ratites, which are primitive for this feature.
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McGowan, C. Evolutionary relationships of ratites and carinates: evidence from ontogeny of the tarsus. Nature 307, 733–735 (1984). https://doi.org/10.1038/307733a0
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DOI: https://doi.org/10.1038/307733a0
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