Emergence of terrestrial vertebrates – a crucial event in life’s evolution on our planet, which took place in the second half of Devonian Period (about 380-390 million years ago) – has never happened all at once. Amphibians, most ancient and primitive terrestrial vertebrates, are still repeating this marvellous transformation in their every lifecycle. Frogs, toads and salamanders are known to spend only second half of their lifecycle on the ground, while being aquatic dwellers during larval stage.

Scientists from laboratory of evolution morphology problems (Severtsov Institute of Ecology and Evolution Problems) have compared transformations of various primitive amphibians of Urodela order – they wanted to find out how metamorphosis changed in the course of evolution. Tailed amphibians show following changes during metamorphosis: back and caudal fins, external branchiaes and labial wrinkles are reduced; branchial clefts disappear; skin and skin gland structure changes; palatine zone is reorganized in the skull; branchial arch skeleton components are partly reduced; dental arrangement and structure is changed; larval hemoglobin is replaced with adult one; external lid and lacrimonasal duct are formed, and ammonium as a final product of nitrogen metabolism is replaced by urea. Scientists have shown that listing existing species of tailed amphibians from the most primitive to the most advanced ones helps to find direct changes of several metamorphosis characteristics, which can be considered to be general course of tailed amphibians’ metamorphosis evolution.

An interesting fact – primitive salamanders have several regulators for various body parts’ transformations. As for evolutionally advanced (tailless) amphibians (frogs and toads), they have thyroid hormones regulating their metamorphosis. Thus, metamorphosis of higher salamanders became fast and “compact”, and previously unified development programme has divided into two almost independent parts – larval and adult ones. Such division opened new evolution perspectives for salamanders – larval and adult stages could have independent development courses. Larval salamanders have better adaptation for aquatic way of life and adult ones – for living on the surface and breathing air.

The work of Russian scientists is a perfect illustration for one important evolution rule, misunderstanding of which may lead some scientists and philosophers to mistakes. The wrong concept is that the more complex is organism’s organization, the higher is his place in the hierarchy. When system’s parts are united in a whole network (like in case of primitive salamanders), evolution changes are not easily performed. However, progressive evolution may lead not only to growth of integrity, but to absolutely opposite thing – dissociation, which can be observed in case of higher salamanders, whose development now has stages, capable for independent evolution.

Source:
    http://elementy.ru/
    http://www.sierradebaza.org
    http://www.vernalpools.enaturalist.org

Kizilova Anna