The anuran middle ear: developmental heterochronies and adult morphology diversification.

Many anuran related species with similar modes of life and similar mating calls display the occurrence of extreme differences in their middle ear morphology (for example, middle ear present in Microhyla ornata but absent in M. heymonsi; stapes present in Bombina orientalis but absent in B. bombina; the tympanic membrane is thin and transparent in Bufo melanostictus, but covered by thich unmodified skin in B. bufo). As loss or reduction of this system has a profound negative effect on hearing sensitivity, these patterns of middle ear diversification cannot be explained in terms of functional adaptivity. The formation of the anuran middle ear is greatly delayed and retarded, it begins at relatively late premetamorphic stages and progresses through postmetamorphic development. Because of the delayed and retarded morphogeny this system is subjected to paedomorphic underdevelopment through: progenesis (M. heymonsi), neoteny (Bombina species), and post-displacement (B. bufo). The late onset of the middle ear development is predetermined; because of the peculiar anuran pattern of its morphogeny, it can be formed only at or after the end of metamorphosis. The author thinks that the occurrence of non-tympanic routes for airborne hearing decreases the pressure of functional requirements and allows developmental heterochronies to be the main factor in the diversification of the anuran middle ear.