Abstract
Norell et al.1 describe a Velociraptor ‘wishbone’ which they interpret as a new piece of evidence for the theropod origin of birds. The bone fits a pattern of furcula-like structures that have been discovered in certain late Cretaceous theropods, including Oviraptor, Ingenia and possibly others2, some of which possess clavicles — usually not fused. But this interpretation gives rise to problems of chronology, structure and function.
Main
First there is the temporal paradox: the earliest-known bird, Archaeopteryx, occurred 75-80 million years earlier than the Velociraptor and other late Cretaceous theropods that had ‘wishbones’. Supposed Jurassic examples are from carnosaurs, a group generally thought to be distant from birds — although the cladistic analysis of Thulborn3 places Tyrannosaurus closer to modern birds than to Archaeopteryx.
Second, the furcula-like structure in the specimen of Velociraptor has a round cross-section1, while that of the primitive Mesozoic birds — Archaeopteryx, Confuciusornis and the enantiornithines — is dissimilar, being grooved postero-dorsally along almost its entire length4.
Also, the articulation of the arms of Velociraptor's furcula-like structure along the entire margin of the coracoid (Fig. 1 of ref. 1) is unlike the articular relationship of the furcula in birds. However, it is similar to the relationship of the interclavicle bone to the coracoids found in primitive diapsids5. This raises the possibility that theVelociraptor and theropod ‘furculae’ are nonhomologous to that of birds — the view of Bryant and Russell2. Fig. 1 of ref. 1 does not show an avian articulation of the ‘wishbone’ to the rest of the shoulder girdle and resembles the restoration of Barsbold for Oviraptor and the mounted specimen of Ingenia.
Third, in most flightless birds the furcula degenerates into two clavicular splints6, similar to the clavicles reported in some dinosaurs2. This strongly indicates that flight arose as an original function in birds, not as a modification of a structure already present before flight evolved1.
Fourth, the presence of a furcula-like structure in Longisquama6, a primitive Triassic archosaur with long feather-like scales and postulated arboreal habits, indicates that structures of this type have developed more than once in the archosauromorphs, which means they are weak evidence for a bird link to theropods.
References
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Feduccia, A., Martin, L. Theropod-bird link reconsidered. Nature 391, 754 (1998). https://doi.org/10.1038/35777
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DOI: https://doi.org/10.1038/35777
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