The peculiar architectonics of contour feathers of the emu (Dromaius novaehollandiae, Struthioniformes)

Abstract

The emu ( Dromaius novaehollandiae ) now belongs to a monotypic genus, because the other two species, D. ater and D. baudinianus , which were smaller (the size of a bustard) and lived, respectively, on Kangaroo and King islands, were exterminated by humans in the early 19th century. The emu (Dromaiidae), cassowary (Casuariidae), and the extinct Dromornithidae families are either combined into the suborder Casuarii of the order Struthioniformes or regarded as a separate order Casuariformes. Here, we follow the former classification. Struthioniformes is one of two modern orders of paleognaths (Palaeognathae), a relict group that diverged from the main phylogenetic line of birds (class Aves) no later than the early Cretaceous period, although their oldest fossil remnants are dated to the early Cenozoic. Their closest known phylogenetic relationship is with the order Tinamiformes, together with which Struthioniformes form the infraclass Palaeornithes (“ancient birds”). Although the current species diversity of Struthioniformes is poor, the branched taxonomic structure of the order (ten species, six genera, five families, and four suborders) definitely indicates an ancient origin of these birds. The organization of Struthioniformes combines archaic characters that have disappeared in more advanced taxa of birds and signs of extreme specialization for terrestrial locomotion, without flight. The archaic characters are the paleognathic anatomy of the palate, which is immovably joined with other skull bones (in contrast to the neognathic type in all other living birds); a primitive structure of leg muscles; and a compound rhamphotheca consisting of several pieces instead of a single sheath. The secondary primitive characters (that are assumed to develop in Struthioniformes as flightless birds) are a heavy, apneumatic skeleton; a flat sternum (without a keel); flattened and shortened bones of the wing, especially the metacarpals; a reduced furcula; fused scapula and coracoid; the absence of apteria and the coccygeal gland; etc. The wing skeleton of the emu is considerably simplified; however, there still are remnants of the clavicles.