T he origin and the struc-tural transformation of paired appendages is a fundamental trait of vertebrate evolution. Since the recognition of structural homology among tetrapod limbs, the existence of a unique developmental programme for limb patterning has been proposed. Basic instructions would constitute an informational groundplan on which species-specific neontological customizations are generated. Paleontological and embryological (experimental and comparative) observations have led to various conceptual frameworks for understanding the origin and evolution of tetrapod limbs (reviewed in Refs 1, 2). Yet our knowledge of the transformational sequence between lower and higher vertebrates is still too scarce for the evolutionary history