POLYDACTYLY IN THE EARLIEST KNOWN TETRAPOD LIMBS

NEW specimens of the earliest known tetrapod limbs shows them to be polydactylous. The forelimb of Acanthostega has eight digits and the hindlimb of Ichthyostega has seven. Both of these come from the Upper Devonian of East Greenland, complementing the only other known Devonian tetrapod limb, that of Tulerpeton from Russia1, which has six digits. The morphology of the specimens suggests that limbs with digits may have been adaptations to an aquatic rather than a terrestrial environment. The pattern of digits corresponds to a recently proposed model for limb development2 in which digit number is unspecified, rather than earlier models3-10 which are rejected because they postulate a fixed number of elements in the ancestral limb. We challenge pentadactyly as primitive for tetrapods3,11. The form of these limbs suggests early specialization in the evolution of the tetrapod limb bud. © 1990 Nature Publishing Group.