THE ORIGIN OF THE TETRAPODS

Abstract

Summary1. Only Crossopterygii* and Dipnoi can be considered as possible tetrapod ancestors. Early tetrapods converge on Stegocephalia and allied groups in early Carboniferous time. These and the Devonian fishes are therefore the key‐material.2. The skull structure of all tetrapods is based on a common plan which can only be derived from osteolepid Rhipidistia. The Dipnoan skull is divergently modified, and the holoptychiid and coelacanthid Crossopterygii are too specialized for consideration. Various attempts to derive some tetrapods (urodeles) from Dipnoi are untenable.3. Analysis of the skull structure shows that great changes in proportion took place during the evolution of a primitive tetrapod from an osteolepid structural pattern. There is also considerable modification of the dermal bone pattern of the anterior skull roof. This has led to erroneous homologies. The ‘orthodox’ homologies, and those of Säve‐Söderbergh and Allis, are incorrect and misleading. A straightforward bone‐to‐bone homology between osteolepids and tetrapods is probable, except for the frontals and nasals of the latter, which represent concentrations from the long series of osteolepid ‘nasals’. The tetrapod parietal is homologous with the element usually called ‘frontal’ in osteolepid and other fishes. The differential fusion of elements postulated by Save‐Soderbergh has no basis in fact.4. The kinetic skull of osteolepid Rhipidistia is no bar to the belief that they include the tetrapod ancestors.5. The teeth of early tetrapods and of Rhipidistia are based on similar patterns.6. The histology of the dermal bones of Stegocephalia is similar to that of large Rhipidistia, but the cosmine layer is lost in the former.7. The morphogenetic significance of the latero‐sensory system in relationship to dermal bone formation is discussed. It is apparently of great value where it is developed as enclosed canals, but pit‐line grooves and superficial sulci are of little importance, as shown by the migration of the sulci over the dermal skull of Stegocephalia.8. New work throws much light on the hyomandibular‐stapes problem and the early evolution of the tetrapod ear and tympanic cavity.9. The phylogenetic modification of the external nostrils in osteolepids and Stegocephalia is remarkably similar to the ontogenetic development of the face even in mammals. There may be two bones confused under the name ‘septomaxillary’ in tetrapods.10. The vertebral column and ribs are considered. Many features of adult early tetrapods are explicable on mechanical grounds as correlated with change of habitat and loss of aquatic buoyancy. The problem presented by the pleural ribs of coelacanths and Dipnoi is indicated.11. Recent work on the origin of the tetrapod limb (to be reviewed and discussed in a separate publication) is briefly summarized. It seems probable that the digits and their carpals or tarsals are new developments (neo‐podium), the remainder of the cheiropterygium (arche‐podium) being derived with comparatively small change from the rhipidistian paddle. The metapterygial axis is represented in tetrapods by humerus (femur)–ulna (fibula)–intermedium–two centrals, and is directed between the podials of digits I and II. Both urodeles and other tetrapods show the same basic pattern.12. The early radiation of tetrapods is analysed, and it is shown that there is no good evidence against a monophyletic origin of a single basal type from which all others were derived.13. The dynamic significance of the emergence of tetrapods lies largely in locomotion mechanics and the loss of the buoyancy effect on leaving the water. The effects on limbs, skull, neck region and vertebral column are considered.14. The physiological changes in respiration, excretion and ‘special senses' of hearing, vision, etc. are briefly noted.15. The process of emergency of tetrapods was probably comparatively quick, incipient in the earliest late Devonian and completed before the early Carboniferous.This review is greatly influenced by the opportunities for study in the laboratory of Prof. D. M. S. Watson, F.R.S. and in various museums in Britain, and in museums and other institutions of Scandinavia and North America, made possible by the tenure of a Senior Research Award of the D.S.I.R. (1934‐7), by grants from the Geological Society of London and the Royal Society (1937) and by grants from the two last‐mentioned bodies for field work in eastern Canada and in Scotland.