J. Yamashina Inst. Ornith., 14: 86-95, 1982 Fused Thoracic Vertebrae in Birds: Their Occurrence and Possible Significance Robert W. Storer* Abstract 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, MI 48109, USA. 86 Fused Thoracic Vertebrae in Birds 87 and no such determination is attempted here. Instead, I have chosen to consider the different arrangements of fused and unfused vertebrae as units and to compare them as such. In doing this, I have adopted a simple notation in which "S" stands for the synsacrum (included as an aid in orientation) and is followed first by the number of unfused vertebrae and then by the number of fused vertebrae in the notarium. Thus a Table 1. The Distribution of fused thoracic vertebrae. 88 R. W. Storer bird with four fused vertebrae in the notarium and one unfused vertebra between it and the synsacrum would have the formula "S-1-4." In the pelicans, there is a fusion of a few of the posterior thoracic vertebrae with the synsacrum (Barkow 1856: pl. III). This condition is not considered in this paper. The notarium, as considered here, is never fused with the synsacrum there being at least one articulation between it and the synsacrum. The presence or absence of the notarium in the various orders of birds and suborders of the Gruiformes is listed on Table 1. Unless otherwise indicated, the condition listed is the rule in adult birds of the order or suborder. Variation Among and Within Groups The range of variation in the formula is shown in Table 2. The number of unfused vertebrae ranges from 0 in some pigeons and tinamous to 4 in some cranes and that of the fused vertebrae from 2 in several groups to 6 in some Western Grebes (Aechmophorus). Tinamidae. The most common formula in the tinamous was S-1-4, comprising 75 percent of the specimens examined. Formulas involving a total of 6 vertebrae (S-1-5 and S-2-4) were found only 7 times in this family, 5 of them in species of Nothoprocta, which Table 2. Distribution of numbers of fused and unfused thoracic vertebrae . Fused Thoracic Vertebrae in Birds 89 90 R. W. Storer were represented by 9 specimens. This is not related to the size of the birds because members of this genus are of medium size for the family and because the other two specimens with six involved vertebrae were in the genera Nothura and Rhynchotus, small and large birds, respectively. Furthermore, the three birds with but 4 involved vertebrae were in the genera Tinamus (large birds) and in Crypturellus soul, a small bird. Podicipedidae. The greatest number of patterns (nine) was found within this family. In the related genera Tachybaptus and Podilymbus, the most frequent arrange- ment (S-1-4) accounted for 92.5 percent of the individuals examined. In Rollandia the median was S-1-5 (55.6%) and in Podiceps it was S-2-4 (60.0%). In Aechmophorus occidentalis, the situation was more complex, three arrangements being frequent (S-1-5, 42.1%; S-2-5, 28.3%; and S-2-4, 23.8%). There also appeared to be a difference between the sexes (Table 3), S-1-5 being more frequent in males (45.9% to 35.5%) and S-2-4 more frequent in females (29.0% to 20.8%). The trend in the family is for an increase in the numbers both of fused and unfused vertebrae from large to small species (Table 3), although this effect is modified by relationships. For example, there are no significant differences within Podiceps in spite of the rather large range in size of the species, and the species of Podilymbus, which are larger than those of Tachybaptus differ but little from those of that genus. Phalacrocoracidae. The cormorants differ from all the other groups in that the fusion only occurs in approximately half of the species and individuals (42 out of 80 specimens examined) and when it occurs, always involves two vertebrae. In all other genera in which the notarium was found, all adults possessed it. Threskiornithidae. The most frequent arrangement is S-2-3 or 68.3% of the sample. The sample was too small to show trends in the number of fused vertebrae with size of the bird. Phoenicopteridae. S-1-4 was the most frequent formula in all five species exam- ined and accounted for 86% of the total sample. It may be significant that this is not the most frequent formula found in the Threskiornithidae. Falconidae. Omitting the genera Herpetotheres and Micrastur, which lack the notarium, the falconids are nearly equally divided between the formulas S-1-5 and S-1-4, which account for 51 and 47 percent of the specimens, respectively. The remaining genera, except Falco, all have four fused vertebrae. In Falco, 5 species (naumanni, tinnunculus, rupicoloides, cenchroides, and vespertinus) also have four fused vertebrae. Of two specimens of F. biarmicus examined, one had 4 and one, 5 fused vertebrae. All the remaining 18 species of Falco examined had 5 fused vertebrae. This family is unusual in showing a nearly equal division between two arrange- ments and in having genera, or species of Falco showing a single arrangement. In view of this small variation within taxa it is surprising to find the New World Kestrel (F. sparverius, N = 20) differing from the Old World kestrels (F. tinnunculus, N = 2: F. naumanni, N = 1: and F. cenchroides, N = 1) and F. vespertinus (N = 1) from F. amurensis (N = 2), with which it is sometimes considered conspecific. Galliformes. This order is marked by small variation in the formula, 95 percent being S-1-4. Gruidae. The cranes proved highly variable in the pattern of fusion of the thoracic Fused Thoracic Vertebrae in Birds 91 vertebrae and had, on the average, the most unfused vertebrae between the notarium and the synsacrum (67 percent had 3 and 11.5 percent had 4 such unfused vertebrae). In no other group was the latter condition found. Aramidae. The six Limpkins examined all had S-3-3, the most frequent arrange- ment in the Gruidae. Psophiidae. The most frequent formula in the trumpeters was S-2-4, accounting for 71% of the sample. This formula was uncommon (3.8%) in the Gruidae and was not found in the Aramidae, both families considered close to the Psophiidae. Rhinochetidae, Eurypygidae, and Mesoenatidae. The small samples of these three isolated families showed some overlap in arrangements with each other and the Gruidae. Pteroclididae. The small sample of sandgrouse was divided between the two commonest arrangements of found in the Columbidae (S-1-4 and S-1-3) but in the reverse order of frequency. Columbidae. Except for 13 specimens of Columbinae in which the notarium articulated directly with the synsacrum, there appeared to be little difference between that subfamily and the Treroninae. Of the 13 specimens without free vertebrae, 7 were found as variants in 6 different genera, but in Staroenas and Leucosarcia all specimens examined (4 and 2, respectively) had the formula S-0-4.