Keith Stewart Thomson

The Adaptation and Evolution of Early Fishes

The origin and evolution of some fundamental adaptations of early vertebrates are examined in terms of theoretical considerations and the limited available data on living and fossil chordates. Special emphasis is given to the question of the interrelatedness of apparently widely different functions in general biological activity and in evolutionary processes. Problems in the evolution of calcified systems and the notochord are discussed in the light of recent advances. A new explanation of the mechanical significance of the heterocercal tail is proposed, along with a revision of its evolution, based on new interpretations of the directionality of the thrust produced by the separate parts of the tail. The respiratory patterns of early fishes are discussed with special reference to the question of the origin of auxiliary earial respiration and it is concluded that lungs were probably an extremely early adaptation of fresh water fishes. Jaw mechanisms are examined and a revised scheme proposed for the evolution of the gnathostome condition, based on the assumption that the early agnathan vertebrates possessed a two-phase ventilating pump capable of producing an essentially continuous flow of water over the gills and that this pattern of ventilation was maintained throughout the intermediate stages in gnathostome evolution. Intracranial kinesis is briefly considered. Finally the pattern of nitrogenous respiration is investigated and its evolution related to the most probable sequence of invasion of freshwater and marine habitats and to the pattern of aquatic versus aerial respiration. Throughout, the functional and evolutionary interrelationships of these and other systems are emphasized, together with the necessity for constant revision of the conceptual framework upon which our interpretations of all data is based.