Wednesday, December 24, 2014

A Hypothesis on the Evolution of Tetrapod Limbs

Urodelans, Ichthyostega and the origin of the tetrapod limb




Available information on the development of primitive urodele (Hynobiidae) limbs and limb structure in Devonian tetrapods provide the basis for formulating the hypothesis on the existence of a special phase, the phase of the biserial limb in tetrapod history. The limb of the Devonian amphibian Ichthyostega with two groups of digits, preaxial and postaxial, corresponds well to this phase. Based on the structure of the Ichthyostega limb, it is suggested that the fin type ancestral to terrestrial limbs was asymmetrical biserial, possessed a short axis, unbranched jointed preaxial radials deviating one by one from each mesomere of the axis, and unbranched jointed postaxial radials deviating from the distal mesomeres of the axis in two or more radials.

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