Repeated Evolution of Fused Thoracic Vertebrae in Songbirds

Abstract

The fusion of two or more thoracic vertebrae, independent of the synsacrum, is more widespread in the Passeriformes than has previously been reported. The bone thus formed is known as a "notarium." I surveyed oscine passerine skeletons and found a notarium with fully fused vertebrae in Chabert's Vanga (Leptopterus chabert), certain woodswallows (Artamidae) and shrikes (Laniidae), the Willie Wagtail (Rhipiduridae, Rhipidura leucophrys), the Phainopepla (Bombycillidae, Phainopepla nitens), the penduline tits (Remizidae) including the Verdin (Auriparus flaviceps), various larks (Alaudidae), the Common Starling (Sturnus vulgaris), the sickle-billed thrashers (Toxostoma spp.), and the crossbills (Loxia spp.). Mapping of character evolution on a supertree suggests that a fully fused notarium has evolved independently at least 12 times in the oscine passerines and that notaria with less extensive fusion of the vertebrae (only the spinous processes fused, for example) are even more widespread phylogenetically. Phenotypic expression of a notarium is fixed in some species and higher taxonomic groups but varies within the species in others. Ontogenetically, the fully fused notarium forms when the bird is immature. The evolutionary development of notaria probably depends on mutations that alter expression patterns of transcription genes (Pax and Hox genes are likely candidates) that control embryological differentiation of the vertebrae. Among the systematic implications of this study are additional support for placement of the Verdin in the Remizidae and for the monophyly of a group of western thrashers (Toxostoma spp.) with strongly decurved bills. Received 2 October 2008, accepted 3 May 2009.