Osteichthyan vertebrae

Abstract

The earliest osteichthyan vertebrae are composed of neural and haemal arches supported by the notochord. The arches develop from one xelerotome in the position of the myosepta. Sclerotomic tissue anterior or posterior to the arch aatagen may form separate intercalaries, part of the arch bases, or, in teleosts, the zygapophyses. Centra were formed independently in a number of actinopterygian groups, in some dipnoans and in the osteolepiform rhipidistians. They ossify in the perichordal tube in either a segmentai or intersegmental position. In some teleosts, at least, perichordal ossification is preceded by calcification of the fibrous notochordal sheath. Experimentation in centrum design was greatest in actinopterygians below the teleost level. The so-called pleurocentra of some osteolepiforms may be intercalaries. Modifications in mode of development and in adult design probably reflect relatively minor changes in a fairly uniform morphogenetic pat tern that has existed throughout the history of osteichthyan vertebrae. The absence of sclerotomic resegmentation in the tetrapod sense suggests that this phenomenon arose during the rhipidistian—amphibian transition.