The Relationships and Evolution of the Rollers: Families Coraciidae, Brachypteraciidae, and Leptosomatidae

THE order Coraciiformes has long been considered by many to be a heterogeneous assemblage of families. Indeed some workers have concluded that few of the families usually included in the order are related to one another, and they have recognized as many as five separate orders (Stresemann, 1959). Despite the extreme ordinal splitting of some authors, all have accepted the close relationship of three distinct taxa: the Coraciidae, or true-rollers; the Brachypteraciidae, or ground-rollers; and the Leptosomatidae, or cuckoo-rollers. The interrelationships and evolution of the rollers is the subject of this paper; their affinities to the remaining families of the order will be discussed in the future. The Coraciidae consist of two genera, Coracias with eight species, and Eurystomus with three (Peters, 1945). The family is distributed over Africa, southern and eastern Europe, southern Asia, the islands of southeastern Asia, and Australia. The Brachypteraciidae include three genera, Brackypteracias and Atelornis with two species each, and Uratelornis with one. All are confined to Madagascar. The third family of rollers, the Leptosomatidae, has been erected for the monotypic genus, Leptosomus, found on Madagascar and the nearby islands. The taxonomic arrangement of the rollers has varied with different authors. Some have put the Brachypteraciidae and Coraciidae in one family and Leptosomus in a separate family (Fiirbringer, 1888; Peters, 1945; Verheyen, 1960); some would place all three taxa in a single family (Sharpe, 1871; Dresser, 1893; Beddard, 1898; MayrandAmadon, 1951); and a third group would recognize three separate families (Stresemann, 1927-1934, 1959; Storer, 1960; Wetmore, 1960). Because of the arbitrary nature of delimiting family-rank taxa, I do not believe we shall ever reach universal agreement on the ranks to be used for the rollers. Therefore in this paper I want to emphasize the degree of relationship and the evolutionary patterns as shown by morphological criteria, in particular the osteology, rather than on the hierarchical arrangement. The anatomy of the true-rollers has never been studied exhaustively. Except for Giebel's (1857) comments on Nitzsch's work on Coracias garrulus, no other investigation has been directed specifically to the Coraciidae. Milne-Edwards (1876, plates 81-82) figured the skeleton