|The Skeleton: Part 2. Vertebrae, Ribs, and Sternum|
MULTIPART CENTRA IN THE TETRAPOD LINE
The centra of rhipidistian fishes are multipart structures lying to the outside of the notochord. Not all of these centra pieces are necessarily joined to the neural or hemal arches, and are therefore somewhat like that of some actinopterygians (bowfin, sturgeon). This type of vertebra, with divided elements as found in the rhipidistian fishes and early fossil tetrapods, is called a rachitomous vertebra.
It consists of the neural spine and its base, the hypocentrum (a hemal spine and its base occur in the tail), and the pleurocentrum which is found on either side and posterior to the dorsal corners of the hypocentrum.
Evidence from fossils of various tetrapods indicates that the pleurocentrum is greater in prominence in some later amphibians and reptiles and that the hypocentrum is smaller in most later fossil reptiles and the tuatura. It is assumed that the centrum of most modern amniotes is the homologue of the pleurocentrum.
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