The Caudal Skeletons of Catostomid Fishes

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 epural 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; i t 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 PU, or PU,. In addition, 54.7% of the sample had an accessory nrural arch on either the compound centrum or PU,. Other catostomids examined also showed a high incidence (24.1% to 68.0%) of accessory neural spines on PU, 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. INTRODUCTION Fishes of the superorder Ostariophysi dominate the freshwater fauna on all major continents except Australia. Minnows, suckers, characins and catfishes are among the groups included in this diverse, highly successful superorder. A lack of basic anatomical information has hampered the elucidation of evolutionary relationships within the Ostariophysi. Resolution of this problem awaits the initiation of comprehensive studies aimed at providing taxonomic information, both morphologkal and biochemical, for reasonably sized groups (genera, tribes or families). Examples of family-wide studies on catostomids (suckers) are those of Nelson (1948, 1949, 1961) on the Weberian apparatus, opercular series and swim bladder; Eastman ( 1977) on the pharyngeal apparatus; Lundberg and Marsh (1976) on the pectoral fin rays; Huntsman (1970) on blood sera and muscle extracts; and Bussjaeger and Briggs (1978) on bile salts. This paper deals with the caudal skeleton of the Catostomidae, a primarily North and Central Arnerican group of ca. 65 species. In rnany areas of the United States, catostomids are second in abundance only to cyprinids. Catostomids probably diverged from a cyprinid ancestor in southeast Asia sometime before the Eocene (Uyeno and Smith, 1972). They crossed a Bering Sea land bridge to North America, leaving the relict genus Myxocyprinus in China (Miller, 1958). Information derived from study of the caudal skeleton is commonly employed in taxonomic and evolutionary work (Hollister, 1936, 1937; Gosline, 1960; Cavender, 1966a, 1966b; Patterson, 1968; Greenwood and Rosen, 1971; Rosen, 1973). 1 Present address: Department of Zoology and College of Osteopathic Medicine, Ohio University, Athens 45701.