[Direct development in the tailed amphibians (salamanders), its origin and evolution]

Abstract

Biphasic life circle including water larvae and terrestrial adults is the primitive character for recent amphibians. Respectively, the larval development type (sensu Zakhvatkin, 1975) is also primitive for them. However, many amphibians possess direct development, in which the most part of ontogenesis takes place in the egg, and a miniature copy of the adult adapted to the terrestrial mode of life comes into the world. Transition from the larval type to the direct one occurs several times independently in Apoda, Urodela, and Anura. Pathways and mechanisms of formation of the direct type, its evolutionary tendencies, and ontogenetic prerequisites of that transition are studied in the plethodontid urodelans herewith in details. It is observed that the entire process of initial formation and subsequent specialization of the direct type involves: 1) progressing displacement of ontogenesis into embryogenesis and loss of larval characters, 2) desynchronization of initially metamorphic transformations and processes with their progressive lost, 3) acceleration of the beginning of functional activity of the thyroid gland, and 4) subsiding of the role of thyroid hormones in the ontogenesis regulation. Transition to the direct development type involves similar kinds of ontogenetic transformations and regulations in both Anura and Urodela despite of their independent evolution. Respectively, mechanisms of that transition are universal for the amphibians. The ontogenetic prerequisites of that transition are shown to be either significant dissociability of the larval and adult stages of ontogenesis (in anurans and plethodontid urodelans), or absence of the extreme larval specializations and respective sharp differences between larvae and adults (in extant Apoda and extinct labyrinthodonts and seymouriamorphs).