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On the Origin of Rhipidistian Vertebrae

Joshua Laerm
Journal of Paleontology
Vol. 53, No. 1 (Jan., 1979), pp. 175-186
Stable URL: http://www.jstor.org/stable/1304047
Page Count: 12
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
On the Origin of Rhipidistian Vertebrae
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Abstract

Paleontological evidence for the developmental origin of rhipidistian vertebrae is reviewed and suggests rejection of notions that either Gadow's arculia theory or William's resegmentation theory is applicable to rhipidistians. A new morphogenetic plan for the ontogenetic and phylogenetic origin of rhipidistian vertebrae is proposed. Two vertebral patterns, each with variations, are observed in rhipidistians. Apsidospondylous vertebrae (e.g. Eusthenopteron, Osteolepis) are characterized by three separate paired vertebral elements: the neural arches, intercentra, and pleurocentra. This pattern is believed to be primitive for rhipidistians. In the holospondylous condition (e.g. Megalichthys, Ectosteorhachis) complete ring-like vertebrae are present. It is suggested that the holospondylous vertebrae are derived ontogenetically from a fusion of separate apsidospondylous anlagen. Each vertebra results from three sites of anlagen induction. The neural arches which are intersegmental in position form from arch anlagen at the level of the myosepta and are probably contributed to by sclerotome cells from two adjacent segments. The ventral arches are intersegmental in position and form from arch anlagen at the level of the myosepta probably from sclerotome cells derived from two adjacent segments. Both neural arch and ventral arch anlagen probably result from inductive factors that are related to mechanical stresses acting in the myosepta. The expansion of the ventral arch anlagen as intercentra is due to the functional need for vertebral consolidation. The induction of separate pleurocentra as space-filling compression members permit limited torsion and flexion. Fusion of separate anlagen into holospondylous vertebrae in later and larger rhipidistians provides maximum consolidation to limit torsion and flexion. The similarity of the apsidospondylous vertebral pattern and a possible homology to the vertebrae of a number of actinopterygians is discussed. The dorsal and ventral arches of all osteichthyans are homologous; however, the homology of rhipidistian pleurocentra and actinopterygian intercalaries cannot be established.

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