The caudal skeleton of the catfishes, order Siluriformes. American Museum novitates ; no. 2398

49 p. : ill. ; 24 cm.Includes bibliographical references (p. 47-49)."To achieve a better understanding of the evolution of catfishes, comparative studies of single character complexes throughout the entire order is believed to be a rewarding approach. A survey of the caudal skeleton of the Siluriformes reveals 10 basic features which, taken together, distinguish catfishes from other fishes. Of these the most diagnostic are: 1) bases of hypurals 3 and 4 fused with a distinct U[subscript]2 chordacentrum in the young and with a usually reduced second ural centrum in the adults; 2) a secondary hypurapophysis; 3) principal rays of the caudal fin fewer than 10+9, with upper principal rays equal to, or fewer than, the lower rays. Within the Siluriformes four features of the caudal skeleton are found to exhibit group specific patterns of variation and trends from primitive to advanced conditions, and may thus be useful in determining relationships: 1. In the trend from the primitive condition of six separate hypurals to the most advanced condition of complete fusion of caudal elements, various groups have reached different structural levels. In this process the sixth hypural is lost. 2. The trend toward elaboration of the sites of caudal muscle origin (hypurapophysis and secondary hypurapophysis) has involved the formation and elaboration of shelves from originally distinct projections, and a subsequent dorsal shift of these sites. 3. While the most primitive principal caudal fin ray number in siluriforms is 9+9, most groups have 8+9. The trend toward a reduction of principal rays always involves loss of an upper ray before loss of a lower so that upper principal rays are never more numerous than lower ones. 4. A separate U[subscript]2 chordacentrum is present in the young of all Ostariophysi except the Loricariidae, Plotosidae, and probably the Chacidae. In the adults of the majority of catfishes a reduced second ural centrum fused with one or more hypurals lies in the cavity on the posterior face of the compound centrum, PU[subscript]1+ U[subscript]l. In some groups the second ural centrum fuses to the compound centrum. In the Loricariidae and Plotosidae the second ural centrum is fused with PU[subscript]1+U[subscript]l, in early development. A separate, well-developed second ural autocentrum occurs in some members of four specialized and unrelated families. This is interpreted as independent redevelopment of a presumedly primitive pre-ostariophysan condition. The advanced conditions of each of these four features of the caudal skeleton tend to occur together in forms which are also regarded as advanced in most other parts of their anatomy. The primitive character states of these features tend to be retained together in a number of families, i.e. Diplomystidae, Ictaluridae, Bagridae, Cranoglanididae, Schilbeidae, Pangasiidae, and Cetopsidae. Advanced features in the caudal skeleton indicate a relationship between the Clariidae and Heteropneustidae, the Doradidae and Auchenipteridae, the Loricariidae, Astroblepidae, and Callichthyidae, and the Plotosidae and Chacidae. The siluriform caudal skeleton shares many features with that of the cypriniforms but it is consistently more advanced. The ostariophysan caudal skeleton is similar to that of the clupeoids, but it resembles the caudal skeleton of the Gonorynchiformes more closely than that of any other group"--P. 45-46