Acanthostega is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs. It was anatomically intermediate between lobe-finned fishes and those that were fully capable of coming onto land. They had eight digits on each hand linked by webbing. They lacked wrists and were generally unfit to come on land. They had both lungs and gills. About 0.7 m long and 10-20 kg. They had stubby legs. It had fish-like teeth and a lateral line organ (a system of epithelial cells, or hair cells, that detect movement in nearby water).
Rivers and shallow, weed-choked swamps. Probably ate fish. May have spent most of its time in shallow water.
In general, they were poorly adapted for going on land (for example, joints were not very mobile, shoulder and forearm were very fish-like, ribs were too short to support the chest out of water). The eight digits on each hand perhaps indicates that Acanthostega may have been an evolutionary dead end.
The fossilized remains are generally well preserved, with the famous fossil by which the significance of this species was discovered being found by Jennifer A. Clack in East Greenland in 1987, though fragments of the skull had been discovered in 1933 by Gunnar Säve-Söderbergh and Erik Jarvik.