THE GNATHIC APPARATUS OF PERENNIBRANCHIAT E TAILED AMPHIBIANS AND SOME PROBLEMS OF HETEROCHRONOUS EVOLUTION

The gnathic apparatus of Proteidae and Sirenidae is characterized by t he combination of larval and adult features; some of the latter (the c omplicated pinnate structure of the strong external jaw adductor; stru cture and position of pseudotemporal muscles) are specialized and pecu liar to perennibranchiates. Most of the larval characters of gnathic a nd hyobranchial apparates are associated with adaptations to feeding b y small aquatic animals, while the specialized features of the jaw add uctors are the adaptations to seizing and keeping of large prey under water. The mechanisms and terminology of heterochronies preserving the characters of earlier ontogenetic stages until the adult stage are co nsidered. The facultative (modificational) heterochronies - neoteny, a nd the obligate (mutant) ones - pedomorphosis must be discriminated. T he pedomorphoses can be accelerational (progenesis) and retardational (retrogenesis). The retrogeneses can be divided into fetalizations, la rvizations and juvenilizations. The complexes of retrogenic pedomorpho ses (larvizations) as well as the specialized adult features evolved g radually and independently in Sirenidae and Proteidae.