The Devonian tetrapod Ichthyostega

The material of Ichthyostega Säve-Söderbergh 1932 collected by the Danish expeditions to East Greenland (1929-1955) led by Lauge Koch is described and documented by photographs of all important specimens. The ichthyostegids are related to the osteolepiforms, and comparisons with the Devonian osteolepiform fish Eusthenopteron and post-Devonian stegocephalians are made. In some respects the ichthyostegids are fish-like, whereas in others they have reached the tetrapod level. However, they also show specializations unparalleled in later tetrapods and belong to a sidebranch of the Osteolepipoda. Several new structures, such as canals for the occipital arteries, the prominent sacculus vesicle, the parotic crest of the palatoqudrate, the attachment areas for a paired basicranial muscle, and an arcual plate dorsal to the notochord are described. Special problems, e.g., the terminology of the dermal bones of the skull roof, cranial kinetism, resegmentation, the metapterygial stem, and the origin of the tetrapod limbs, are diseussed. It is concluded that the hindlimb in Ichthyostega is pentadaetyl and that pentadactyly is primitive for the Osteolepipoda.