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The Origin of the Larva and Metamorphosis in Amphibia

1957; University of Chicago Press; Volume: 91; Issue: 860 Linguagem: Inglês

10.1086/281990

1537-5323

Henryk Szarski,

Ichthyology and Marine Biology

All larvae of Ichthyopsida show a great similarity in many important traits. The amphibian larvae, however, have a considerable number of characteristics, which can be considered as comparatively new caenogenetic adaptations. They are most numerous in larvae of Anura. In many fishes having a larval stage in development there is also a metamorphosis more or less similar to the amphibian one. The influence of the thyroid on fish metamorphosis is very variable and never so pronounced as in Amphibia. The dependence of metamorphosis on the thyroid secretion is greatest in Anura. It is much smaller in Urodela and Gymnophiona. In these two groups metamorphosis is a slow and poorly coordinated set of processes. The following conclusions can be drawn from these facts. The Crossopterygian ancestors of Amphibia must have had a larval stage in their development, very similar to the larvae of recent Dipnoi and Urodela. A great evolutionary gain was achieved by Amphibia through the prolongation of the duration of the larval stage, by evolving larval adaptations and by a shortening and synchronization of metamorphosis. Probably some of the morphogenetic processes, forming part of metamorphosis of recent amphibian larvae were dependent on the thyroid hormone secretion in Crossopterygians, just as they depend on it in recent fishes. Other processes developed the dependence on the thyroid comparatively recently. A striking example is the resorption of the tail in Anura. The thyroid gland could not have been a factor in the origin of Tetrapoda from their fresh-water ancestors.