Ichthyostega A genus of amphibians that, together with Ichthyostegopsis and Acanthostega, first appeared in the Upper Devonian. Ichthyostega shows refinements of the skull, and the development of strong limbs. It retained a long, rather fish-like tail, and links with the Crossopterygii are confirmed by the presence of teeth with a labyrinthine infolding of the enamel, as well as a notochordal tunnel (see NOTOCHORD) underlying the brain case and a lateral-line system. Ichthyostega grew to just under 1 m in length. An unexpected finding about Ichthyostega and its relatives, from recently discovered, more complete specimens, is that they had more than the five digits per limb that characterizes modern tetrapods: Ichthyostega had seven on the hind feet (the forefeet are still unknown), and Acanthostega had eight on both fore and hind feet.

Labyrinthodontia (class Amphibia) A group (or subclass) comprising amphibians that were primitive in character and from a few centimetres to several metres long. The bones forming the palate often carried the folded teeth that give the subclass its name. They lived during the Palaeozoic and Triassic. Labyrinthodonts show affinities with Crossopterygii and with reptiles, and are connected to other amphibians via such fossil groups as the Loxomammatids, Nectrideans, and Temnospondyla. The modern tendency is to regard the labyrinthodonts as an informal assemblage of primitive amphibians, rather than as a formal taxonomic group.

Ichthyostega Amphibians in the form of Ichthyostega and Ichthyostegopsis first appear in the Upper Devonian. Ichthyostega shows refinements of the skull, and the development of strong limbs. It retains a long, rather fish-like tail, and links with the crossopterygian fish are confirmed by the presence of teeth with a labyrinthine infolding of the enamel. Ichthyostega grew to just under 1 m in length. An unexpected finding about Ichthyostega and its relatives, from recently discovered, more complete specimens, is that they had more than the five digits per limb that characterize modern tetrapods: Ichthyostega had seven on the hind feet (the forefeet are still unknown), and Acanthostega had eight on both fore-and hind feet.

Labyrinthodontia Subclass of the Amphibia, primitive in character, and ranging in length from a few centimetres to several metres. They were the Palaeozoic and Triassic amphibians. There are no known connections with other amphibians, but they show affinities with crossopterygian fish and with reptiles.

Acanthostega See ICHTHYOSTEGA.