RATITAE (from Lat. ratis, a raft), the name given by B. Merrem (Abh. Ak. Wiss., Berlin, 1812-1813; Phys. Kl., p. 259) to the " flat-breasted birds," in opposition to the Carinatae,or those which normally possess a keeled sternum. In thus dividing the birds into two great equivalent groups, he was followed only by C. L. Nitzsch (1829), T. H. Huxley (1867), P. L. Sclater (1880), A. Newton (1884), R. B. Sharpe (1891), whilst in most of the other numerous classifications the Ratitae (vicariously named Struthiones, Cursores, Brevipennes, Proceres) were treated as of much lower rank.
A diagnosis covering all the Ratitae (struthio, rhea, casuarius, dromaeus, apteryx and the allied fossils dinornis and aepyornis) would, be as follows (i) terrestrial birds without keel to the sternum, absolutely flightless; (ii) quadrate bone with a single proximal articulating knob; (iii) coracoid and scapula fused together and forming an open angle; (iv) normally without a pygostyle; (v) with an incisura ischiadica; (vi) rhamphotheca compound; (vii) without apteria or bare spaces in the plumage; (viii) with a complete copulatory organ, moved by skeletal muscles.
The separation of the Ratitae from the other birds, and their seemingly fundamental differences, notably the absence of the keel and of the power of flight, induced certain authors to go so far as to derive the Ratitae from the Dinosaurian reptiles, whilst Archaeopteryx (q.v.) and the Ca$natae were supposed to have sprung from some Pterosaurian or similar reptilian stock. Such vagaries require no refutation. But it is quite another question, whether the " Ratitae " form a natural group. Sir R. Owen was the first (Comp. Anat. and Physiol. of Vertebrates, ii. 1866) to indicate that the various Ratitae might be referable to various natural groups of the Carinatae. A. W. Forbes likewise had doubts about them. B. Lindsay (P. Z. S., 1885, pp. 684-716, pis. lii.-lv.) found vestiges of a keel in a young rhea, and apteria in the embryonic ostrich, and she concluded that they were descendants of birds which originally possessed the power of flight. This has been settled by M. Fiirbringer (Untersuchungen . . . 1888), and there is now no doubt that the absence of the power of flight is a secondary, not primitive, feature in the Ratitae as well as in the flightless bona fide Carinatae, e.g. Didus, and penguins. It had already been understood that the various genera of the Ratitae were the representatives of so many different groups, each of which was at least equivalent to ordinal rank, and that therefore, if the Ratitae were still to be considered a natural group, this common ancestry must be referred to a remote geological epoch. Furbringer, however, separated Apteryx with Dinornis from the rest, combining his " Apteryges " with Crypturi and Galli as Alectorornithes, the latter being practically A. H. Garrod's Galliformes, of which his " Struthiones " form part together with the Tinamidae or Crypturi. Relationship of this otherwise typically carinate, neotropical family with the Ratitae had already been insisted upon by T. H. Huxley; hence his term Dromaeognathae for the Crypturi. L. Stejneger (Standard Nat. Hist., iv., Boston, 1885) applied this term in a new wider sense to all the Ratitae, and recently W. P. Pycraft has revived this notion by his division of the Neornithes into Dromaeo- and Neognathae. At any rate we begin to see that some of the Ratitae, namely the Rheidae, may possibly be an early and then much modified offshoot of such of the Carinatae as are now represented by the Crypturi, whilst in another part of the world, and at a much later time, kiwis and moas have sprung from a somewhat more Galliform stock, which points to a descent from a still undivided GalliformTinamiform mass. Further, it is the opinion of competent ornithologists that there is affinity of the Australian emeus and cassowaries with the New Zealand moas and with the Malagasy Aepyornis. Struthio alone still stands aloof, possibly because it is the oldest and most specialized form. This genus was already typically developed in late Miocene times, and with a very wide geographical distribution (see BIRD, Fossil), but of the affinities of the other mid- and early tertiary flightless birds we know nothing, and it must be emphasized that we should probably not be able to classify a truly ancestral Ratite, namely, a bird which is still to a certain extent carinate and not yet ratite. It is impossible to give a satisfactory diagnosis of such intermediate forms.
All the recent Ratitae still possess a considerable number of rather primitive characters, e.g. they are typically nidifugous; the simple structure of their neossoptiles; quintocubital; compound rhamphotheca; holorhinal nares imperviae; basipterygoid processes; simple articular facet of the quadrate; configuration of the palatal bones, including the large vomer; incisura ischiadica; simple hypotarsus; the thigh muscles; the copulatory organ.
We restrict the origin of the Ratitae to that great branch of still primitive Carinatae which, after separation of the Ratitae, has further developed into the legion of the Alectoromorphae, notably Tinami- and Galliformes, together with still low Gruiformes (see BIRD, Classification). From such a rudis indigestaque moles, after it had attained an almost world-wide distribution, have arisen the various Ratitae, independently at various epochs and in various countries. Most of them are now restricted to widely separated countries of the southern hemisphere. Although loss of flight (correlated with more or less reduction of the wings and the sternal keel, and often compensated by stronger hind limbs) has occurred, and is still taking place in various groups of birds, it is quite impossible that a new Ratite can still come into existence, because the necessary primitive substratum, whence arose the true Ratitae, is no longer available. Consequently we are justified in retaining " Ratitae " in our classification, although they are a heterogeneous, not strictly monophyletic, assembly. (H. F. G.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)