How do Ontogeny, Morphology, and Physiology of Sensory Systems Constrain and Direct the Evolution of Amphibians?
The evolutionary success of extant amphibians is accompanied by secondary simplification of sense organs and of the nervous system. Strong morphological reduction is found in the lateral line system and in the auditory and visual systems. Canal neuromasts are absent; additional loss of epidermal neuromasts and ampullary organs generally corresponds to terrestrial life. Reduction of the auditory system of some anurans and of many salamanders and caecilians affects middle and inner ear structures as well as central auditory structures. The visual system of caecilians and salamanders is strongly reduced with respect to the number and morphology of retinal ganglion cells and the morphological differentiation of central visual areas, particularly the tectum opticum. The extremes of secondary simplification are found in the salamanders of the plethodontid tribe Bolitoglossini. At the same time, these salamanders are one of the most successful groups of amphibians, and they possess the most derived feeding system and a variety of specializations of the visual system. In amphibians, there is a close correspondence between the degree of secondary simplification on the one hand and genome size (DNA content) and cell size on the other. We hypothesize that this process is the major cause of the observed secondary simplification.