Gegenbaur based this idea on the fin of Ceratodus and its similarity to the gill-region in Elasmobranchs.
He suggested that the pentadactyl limb of modern tetrapods was derived from one side of the archipterygium.
He suggested that the tetrapod limb or cheiropterygium differed in its origins from that of the lungfish and that the two may have diversified from a true ancestral archipterygium.
[1][2] An alternate origin for tetrapod limbs was identified in the lateral fins by Francis Balfour.
[4] Although the idea of the archipterygium is outdated, it was one of the first major applications of evolutionary morphology and development.