Evolution: the first tetrapods were not reckless

Amphibians evolved from fish during the Devonian. It is a certainty. However, the environment in which this transition has occurred remains hypothetical. A new study situated the occurrence of tetrapods in wetlands and woodlands. Limb development and mobility of the neck would have allowed them to take advantage of this environment.

Written by Abdullah Al Abbadi/Blue Line News

For Alfred Romer, the fish would be out of the water as a result of drastic changes in environmental conditions. These have caused the drying up of water points (left). According to Gregory Retallack, fish have developed new structures (legs and neck mobile) allowing them to leave the water and enjoy their environment (right). © University of Oregon

The evolution of tetrapods from fish would have taken place during the Devonian, or there is between 390 to 360 million years. The presence of four limbs and a neck differentiates fish from amphibians. Two theories explained so far how and where this crucial stage of evolution could have occurred.

According to Alfred Romer, amphibians have evolved from fish and forced to live out of water after their ponds were drained. Movements would have been necessary to find a new living space. Over time, natural selection would have selected individuals with an effective terrestrial locomotion. Hosting environments to such fishs had to be subjected to drought.
Grzegorz Niedbwiedzki and his colleagues on the other hand proposed intertidal model in 2010. The intertidal zone is the coastal strip subject to the tides, tide pool and open air at low tide. Amphibians are differentiated from fish out of the sea Their theory is based on the discovery of fossilized footprints in a rock that was to be in a lagoon.

Gregory J. Retallack, professor at the University of Maryland, has just published an article describing a third event in the Journal of Geology. He studied footprints and fossilized bones of organisms from the period of transition between fish and amphibians in three sites in the United States. Remarkably, these fossils were still in the soil characteristics of wetlands and forested plains. No trace of dry pond was observed.

According to the work of Gregory J. Retallack, the ancestor of tetrapods was an opportunist rather than a rash venturing out of his pond during a drought. He took advantage of the environment that was open to him to adjust to a new life. This fish was populated in humid regions hosting lakes invaded by roots and tree trunks. Limbs observed by the geologist are mobile and able to enter the wood or roots. The ancestor of tetrapods thus had to take advantage of the presence of roots out of the water.

The mobility of the neck was acquired in order to eat in shallow water. This assumption justifies the development of a more mobile neck out brutal fish of the lakes located in a desert area. To reject the theory of Alfred Romer, Gregory Retallack also relies on the fact that no fish ancestor of tetrapods has never been found in geological structures that may correspond to ancient ponds drained.

Thus, the origin and the release of water from tetrapods would be to link with the development of structures adapted to a new environment. It was not caused by drastic changes in environmental conditions.

Selected source for further read : News Feed Researchers  Published 29-Dec-2011

Interested reader of this article may also want to read 

The evolution of fish: the head before the body


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