Comparative morphology of the radial carpal bone of neornithine birds and the phylogenetic significance of character variation

Abstract

The morphology of the radial carpal bone (os carpi radiale) of neornithine birds is for the first time evaluated in a comparative context. An unexpected morphological variation of this bone is documented, and characteristic derived morphologies are identified. One of these characterizes a subclade of Accipitridae, which includes the taxa Harpiinae, Circinae, Melieraxinae, Accipitrinae, Milvinae, Haliaeetinae, Buteoninae, and Aquilinae. Another derived morphology of the radial carpal is found in the Picocoraciae, the clade including Coraciiformes sensu stricto, Alcediniformes, Bucerotes, and Piciformes. This latter morphology is absent in Leptosomidae and Trogonidae and constitutes the first morphological apomorphy of Picocoraciae. A distinctive morphology of the radial carpal is further present in passeriform birds. Character variation of the radial carpal provides new data for the evaluation of conflicting phylogenetic hypotheses, and it is detailed that the morphology of this bone further contributes to a phylogenetic placement of controversial avian taxa in the fossil record.