The Caudal Skeletons of Catostomid Fishes

Abstract

A total of 725 catostomid fishes were examined for interand intraspecific variation in the caudal skeleton. Included were 20 species and 13 genera representing all known catostomid genera. The caudal skeleton was a stable morphological character in this family. The basic pattern consisted of a parhypural, six hypurals, one epurai and one pair of free posterior uroneurals. The hypurapophysis varied interspecifically in length and orientation. Erimyzon was the only genus showing a reduced number of elements; it had neither hypural 6 nor the posterior uroneurals. The caudal skeleton provided no significant phylogenetic information, although the condition in Erimyzon was clearly derivative. Intraspecific variation was studied in a sample of 525 Catostomus commersoni from a wide geographic area. Again the caudal skeleton proved to be a very stable character with only 2.7% of the sample deviating from the expected arrangement. The hypurals showed no tendency to fuse in ontogeny. The adjacent preural region in Catostomus commersoni was highly variable. About 39.2% of the sample had an accessory neural spine on either PU2 or PU3. In addition, 54.7% of the sample had an accessory neural arch on either the compound centrum or PU2. Other catostomids examined also showed a high incidence (24.1% to 68.0%) of accessory neural spines on PU2 or PU3. When the vertebra bearing the accessory spine was counted as one, individuals with accessory spines always had lower mean vertebral counts than those without accessory spines. Vertebral fusion early in larval life probably produced the accessory spine. Although the functional and phylogenetic significance of accessory neural spines is unknown, researchers using the preural region in taxonomic and evolutionary studies should be aware of this marked variability.