The comparative osteology and taxonomy of the North American cyprinid genera, Richard-sonius, Clinostomus and Semotilus, with notes on other minnows

Abstract

A comparative osteological study was made of the eight chubs and daces of the North American cyprinid genera Richardsonius, Clinostomus and Semotilus. The primary objectives were to approach the whole skeleton as an adaptive mechanism and to study the taxonomy of these fishes. In order to gain perspective, a limited survey was made ~f other North American minnows. This included representatives of the related and more distantly related genera, Gila, Ptychocheilus, Nocomis, Campostoma, Exoglossum, Pimephales, Rhinichthyes and Notropis. Several suckers were also examined. Some controversial aspects of the minnow skeleton have been clarified. The preethmoid is a double ossification which includes lateral and mesal components. The mesal preethmoid is always present. The lateral ossification may or may not occur. When present, it frequently appears only in adults. Both may fuse in certain species. The quadratemetaterygoid fenestra is usually closed by the metapterygoid. In Richardsonius balteatus, this is accomplished by the symplectic. A small fenestra is present in Richardsonius egregious. The greatest length of the lapillus and asteriscus may vary. Either may be the longer or both may be approximately equal in length. The lapillus is usually oriented horizontally in the skull but it may be rarely vertical. Many structural aspects of bones may be interpreted in terms of skeletal stresses. These include cartilaginous areas, thickened regions, and sutural complexity. Adaptations concerned with feeding are the mechanism of jaw protrusion, the means of bite reinforcement, differences in bones relative to mouth position and the avenues for stress dispersal through the skull. Several features of the postcranial skeleton may be correlated with body form and the mode of life. These are the shape of the urohyal. vertebral relations, the shape of the basipterygium and features of the caudal peduncle and tail skeleton. The skeletons, which were examined, do not seem to have many major differences. Bones are rarely rarely absent and there are few fusions. Sensory canals may be unfused to the adjacent bones. The biggest differences are organizational and, if real, proportional but even these tend to be elusive. No taxonomic changes are made. The genera Richardsonius, Clinostomus and Semotilus have been retained. Barbellated, terete minnows and non-barbellated, compressed species may have evolved several times on this continent. There may be much parallelism among the cyprinida. Richardsonius, Clinostomus and Semotilus seem to be related to the western Gila. Clinostomus may have evolved from some early Semotilus rather than Richardsonius. The examination of more cyprinids should precede further taxonomic changes. If osteology is to be used as the basis of cyprinid, generic taxonomy, the skeletons of both near and distant relatives of the group under consideration should be studied. Even so, more work than would be represented by such a study may be necessary in order to justify taxonomic changes.