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Vertebrata |
Department of Geology, The University, Newcastle-upon-Tyne
Department of Biology, Queen Elizabeth College, Campden Hill Road, London S 8
Department of Geology, Royal Scottish Museum, Edinburgh 1, Scotland
Department of Palaeontology, British Museum (Natural History), Cromwell Road, London S W 7
Introduction. The fishes are traditionally divided into two major groups—the Chondrichthyes or cartilaginous fish and the Osteichthyes or bony fish. Watson (1937) proposed that the Acanthodii and Arthrodira (Placodermi) should, by virtue of their possession of a free hyoid, be placed in a new class the Aphetohyoidea, but in view of recent evidence that these animals did not have a free hyoid, this opinion can no longer be sustained (Miles 1964). The systematic position of these two fossil groups is still, however, a matter of considerable controversy. Stensiö (1963) and Jarvik (1960, 1965) unite them with the cartilaginous fish as Elasmobranchiomorphi, whilst Miles (1965) has produced convincing evidence that the acanthodians are related to the bony fish. Ørvig (1960, 1962) on the other hand has indicated that a living group of cartilaginous fish—the Holocephali—were derived from a specialised line of arthrodires, although Patterson (1965) has disputed this. In view of the current state of debate, it is preferable to retain the Acanthodii and Placodermi as independent classes to avoid any unnecessary phylogenetic implications, consequent upon linking either of them to the cartilaginous or bony fish.
With regard to the cartilaginous fish, the Selachii and Holocephali reveal no evidence of common ancestry and they are thus here placed in separate classes. The problem of the bony fish is somewhat comparable, but now seems to be resolved. There are two major divisions—the Actinopterygii or ray-finned fish and the "Sarcopterygii" or lobe-finned fish. This latter group was formerly called the "Choanichthyes", but this is an
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