Abstract
FOR long we have accepted as well-established and equivalent the five classes of vertebrate animals, but recent zoological research, particularly on the palaeontological side, has modified many old conceptions of relationship and suggests that there may be need for readjustment in the major groups. An attempt at a new classification which will give due weight to recent discoveries has been made by G. Save-Soderbergh (Arkiv. zoologi, 26, No. 17; 1934). Its main suggestions are that the present class Pisces is a medley of two of the three main stocks of Gnatho-stomes and parts of a third one. This third stock (Choanata) gave rise to the higher vertebrates, but probably by two routes, the ancestors of the Dipnoi leading to the Urodela, of the Crossopterygii to the Anura by a devious route. The Amphibia also must be looked upon as a mixed assemblage, which includes the two stocks just mentioned, but also an offshoot of the reptilian Reptiliomorpha, the Anthracosauria. Finally, birds and mammals belong to a richly branching part of the vertebrate phylogenetic tree, most of the branches being grouped as reptiles, while two equivalent branches are given unequal status as the independent classes Aves and Mammalia. The author regards it as absurd that equal systematic value should be given to these classes as to the fundamental group Pisces composed of two entire stocks of Gnathostome vertebrates, and half of the third stock. The writer's first reaction to this interesting and revolutionary view of vertebrate phylogeny, in which birds and mammals are grouped with reptiles and Anthracosauria as equal divisions of the Reptiliomorpha, is the thought that systematic classification is not entirely a matter of equivalents, and that even when phylogeny is known, weight must be given to outstanding novelties in evolution which have originated decisive lines of development. Thus the ‘invention’ of warm-bloodedness, which by adding to the adaptability of vertebrates has enabled them to conquer land surfaces far beyond the reptilian range, seems worthy, in association with the structures which made it possible, of a distinctive classificatory label.
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The Vertebrate Evolutionary Tree. Nature 135, 18 (1935). https://doi.org/10.1038/135018b0
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DOI: https://doi.org/10.1038/135018b0
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