Cutaneous respiration and the origin of the modern Amphibia

Abstract

SUMMARY Though they already possessed lungs, the ancestors of the modern Amphibia, or ‘Lissamphibia’, evolved an accessory respiratory surface, reducing the scales and using the moist, naked surface of the body for this purpose. This can be explained on the assumption that the ancestral lissamphibian had not evolved the costal method of ventilating the lungs, and relied solely on hyoid ventilation, as do the lungfish and the Lissamphibia themselves. It seems unlikely that this method would have been adequate for active land life. The adoption of accessory cutaneous respiration would then have been wholly advantageous in allowing colonization of the land, even though the resulting high rate of water loss from the skin would restrict the range of terrestrial habitats that could be exploited. The Palaeozoic amphibians are re-examined in search of a group which might be ancestral to the Lissamphibia. It appears that Romer's ‘typical microsaurs’ can now be divided into at least two groups, the microbrachids and the gymnarthrids. The gymnarthrids possess all the necessary primitive characters required in an ancestral lissamphibian; they also possess a number of other characters which are found in the modern Amphibia, including one or two which are diagnostic of that group. Though many specialized lissamphibian cranial characters are not found in the gymnarthrids, these may have appeared at a slightly later stage of lissamphibian evolution. Finally, the earliest gymnarthrids are of Mid-Carboniferous age, well before the earliest known lissamphibians. In view of all these facts it seems very likely that the modern amphibians are descended from the gymnarthrids, or from some similar, closely related group of microsaurs. If this theory proves correct, then the microsaur-lissamphibian line of evolution diverged from the labyrinthodont-reptilian line at a very early stage in tetrapod history. In this case, many of the peculiar features of the Lissamphibia may have little or no relevance to the evolutionary history of the Reptilia.