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.