Limbs – welcome!

Reconstructed skeleton of Acanthostega gunnari. Image source: Superoceras.

By Jozef Klembara

The interrelationships among individual elements of nature have attracted the attention of scientists for a very long time. From this point of view, the study of the periods of the Earth which had caused an extinction or at least created very difficult conditions for life is very important. 

The fossil record shows that life in the valleys of the big Caledonian mountains, which had existed on the former European continent 500-370 million years ago, was very favorable for life and the evolution of a large variety of plant and animal species. As for vertebrates, this ecological “Paradise” was inhabited mostly by fish. Among them, Eusthenopteron played the key evolutionary role because its skeleton had displayed already many tetrapod (four-legged animal) features. But, such a favorable environment also enabled the evolution of big fish predators – up to five meters long. At the end of the Caledonian epoch the climate had started to deteriorate and become warmer and warmer. The dying branches and stems of the big fern Archeopteris had fallen into the waters on the ground. Decomposing these plants, bacteria had consumed a lot of oxygen present in water. We suppose, Eusthenopteron had developed lungs, an inevitable organ to survive in this drastically changing environment poor in oxygen. It is also supposed that early populations of Eusthenopteron could not compete against big fish predators and thus (one part of its population) started to inhabit shallow waters close to the river banks. However, these shallow waters contained a lot of plant material which made the space difficult for swimming.

It was exactly in such an environment the scientists found the fossils of a tetrapod called Acanthostega (in 370 million year old deposits, like those containing Eusthenopteron). The Acanthostega has four limbs, each with eight fingers, and a head with internal gills like fish. The anatomy of its girdles shows that this “fish-tetrapod“ animal could not walk on land. It is supposed that it used its limbs for effective movement in the shallow waters full of plant material. All studies strongly indicate that the slow, drastic deterioration of life conditions at the same time produced a distinct progress, the evolution of anatomical novelties: limbs with digits, i.e., the organs used for walking on land and so gaining access to new life territories.

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