Fused Thoracic Vertebrae in Birds: Their Occurrence and Possible Significance

The notarium, a group of fused thoracic vertebrae, is characteristic of birds of five orders and is found in one or more families of five more orders of non-passerine birds . Sixteen patterns of variation in the number of vertebrae in the notarium and of unfused vertebrae between it and the synsacrum were found. The occurrence of these patterns in the groups having a notarium is tabulated. Selective factors favoring the evolution of fusion of these vertebrae may have included the shock induced by landing on hard substrates or of striking prey and the prevention of downward bending of the ends of the thoracic portion of vertebral column while the birds are in flight. Phylogenetic implications of the presence of a notarium in several groups are discussed. The notarium of Os dorsale of birds is a group of fused thoracic vertebrae usually separated by one or more unfused vertebrae from the anteriormost vertebra of the synsacrum (Baumel 1979: 93, 112). Although it was described and figured as early as 1856 (Barkow 1856: pl. II), I have found little more than casual mention of it in the literature. It therefore seems worthwhile to survey the distribution of this structure among the orders and families of birds, to describe the variation in the numbers of vertebrae in the notarium and between it and the synsacrum, to speculate on its possible adaptive significance, and to discuss phylogenetic implications of its presence. This study is based on examination of skeletons in the U. S. National Museum of Natural History, the Field Museum of Natural History, and the University of Michigan Museum of Zoology. Skeletons of all families of non-passerine birds, as well as those of all the suborders and superfamilies and most of the families of passerine birds, were examined. I am grateful to Dr. Richard L. Zusi for permitting me to use the first collection and for other assistance in the project, to Dr. John Fitzpatrick for permission to use the collection of the Field Museum, and for Steven M. Goodman for obtaining data from the Field Museum and the U.S. National Museum of Natural History, as well as for helpful criticism of the manuscript. It is an honor to offer this paper to the Journal of the Yamashina Institute for Ornithology for its Jubilee number and a pleasure to compliment the Institute on the fine morphological work done there. In birds, the number of vertebrae varies both between and within species, ranging from 39 in some passerines to 63 in some swans (Portmann 1950: 79). This makes it all but impossible to determine homologies between thoracic vertebrae of different species, * Museum of Zoology and Division of Biological Sciences , The University of Michigan, Ann Arbor,