2 SPINAL-CORDS IN BIRDS - NOVEL INSIGHTS INTO EARLY AVIAN EVOLUTION

Birds can be subdivided into two large superordinal assemblages based on differences in the dorsal horn of the spinal grey matter. Palaeogna ths (i.e. ratites and tinamous), along with a few other orders of neog nathous birds, exhibit the primitive dorsal horn state characteristic of other amniotes wherein cutaneous nerves form a single map of the bo dy surface across the dorsal horn. In contrast, the vast majority of n eognaths exhibit a novel, distinctly bifid dorsal horn wherein cutaneo us nerves form not one, but two separate maps of the skin, each lying side-by-side. This unusual dorsal horn organization, which has been hi ghly conserved and represents the derived state in birds, may identify a novel, major avian clade. These findings shed new light on historic ally problematic taxa and the early evolutionary branching sequence am ong living birds. Most notably, they reveal that the traditional order s Gruiformes, Columbiformes, Cuculiformes and Piciformes are unnatural assemblages. Further, in addition to palaeognaths, these findings sug gest that most gruiforms, including buttonquails and mesites, as well as pigeons, cuckoos, woodpeckers and songbirds, represent ancient line ages whose ancestry predates the majority of 'modern' birds. The phylo geny of living birds may thus be likened more to a dense bush than the traditional tree, with more than half of all living species arising f rom a basal side branch.