Paleognathes are named for a characteristic, complex architecture of the bones in the bony palate. Cracraft (1974) defined it with five characters.
- The vomer is large and articulates with the premaxillae and maxillopalatines anteriorly. Posteriorly the vomer fuses to the ventral surface of the pterygoid, and the palatines fuse to the ventral surface of this pterygovomer articulation.
- The pterygoid prevents the palatine from articulating medially with the basisphenoid.
- The palatine and pterygoid fuse into a rigid joint.
- The articulation on the pterygoid for the basipterygoid process of the basicranium is located near the articulation between the pterygoid and quadrate.
- The pterygoid–quadrate articulation is complex and includes the orbital process of the quadrate.
Paleognathes share similar pelvis anatomy. There is a large, open ilio–ischiatic fenestra in the pelvis. The pubis and ischium are likely to be longer than the ilium, protruding out beneath the tail. The postacetabular portion of the pelvis is longer than the preacetabular portion.
Paleognathes share a pattern of grooves in the horny covering of the bill. This covering is called the rhamphotheca. The paleognath pattern has one central strip of horn, with long, triangular, strips to either side.
In Paleognathes the male incubates the eggs. The male may include in his nest the eggs of one female or more than one. He may also have eggs deposited in his nest by females that did not breed with him, in cases of nest parasitism. Only in Ostriches and the Great Spotted Kiwi does the female also assist in incubating the eggs.
The tinamous of Central and South America are primarily terrestrial, though they fly weakly. Tinamous have very short tail feathers, giving them an almost tailess aspect. In general they resemble galliform birds like quails and grouse. Tinamous have a very long, keeled, breastbone with an unusual three-pronged shape. This bone, the sternum, has a central blade (the Carina sterni), with two long, slender lateral trabeculae which curve to either side and nearly touch the keel posteriorly. These trabeculae may also be thought of as the rims of two large foramina that incise the posterior edge of the sternum, and extend almost its whole length. Tinamous have a proper semicircular furcula, with no trace of a hypocleidium. There is an acute angle between the scapula and coracoid, as in all flying birds. The pelvis has an open ilio–ischiadic fenestra that incises the posterior edge between the ilium and ischium, as in all paleognathes. Tinamous have no true pygostyle, their caudal vertebrae remain unfused, as in ratites. Tinamou feathers look like those of volant birds in that they have a rachis and two vanes. The structure of tinamou feathers is unique, however, in that they have barbs that remain joined at their tips. Thus the parallel barbs are separated only by slits between them. Tinamous have uropygial glands.
Ratite birds are strictly flightless and their anatomy reflects specializations for terrestrial life. The term "ratite" is from the Latin word for raft, ratis, because they possess a flat breastbone, or sternum, shaped like a raft. This characteristic sternum differs from that in flighted birds, where the pectoral musculature is disproportionately large to provide the power for wingbeats and the sternum develops a prominent keel, or carina sterni to anchor these muscles. The clavicles do not fuse into a furcula. Instead, if present at all, each is splint-like and lies along the medial border of the coracoid, attached there by a coraco–clavicular ligament. There is an obtuse angle between the scapula and coracoid, and the two bones fuse together to form a scapulocoracoid. Ratites have reduced and simplified wing structures and strong legs. Except in some Rhea wing feathers, the barb filaments that make up the vanes of the feathers do not lock tightly together, giving the plumage a shaggier look and making it unnecessary to oil their feathers. Adult ratites have no preen gland (uropygial gland) that contains preening oil.
Read more about this topic: Paleognathae
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