It appeared in the late Devonian period (Famennian age) about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and those that were fully capable of coming onto land.

[3] The front limbs of Acanthostega could not bend forward at the elbow, and therefore could not be brought into a weight bearing position, appearing to be more suitable for paddling or for holding on to aquatic plants.

[2] Acanthostega is seen as part of widespread evolutionary radiation in the late Devonian period, starting with purely aquatic finned tetrapodomorphs, with their successors showing increased air-breathing capability and related adaptions to the jaws and gills, as well as more muscular neck allowing freer movement of the head than fish have, and use of the fins to raise the body of the fish.

[2] These features are displayed by the earlier Tiktaalik, which like Ichthyostega showed signs of greater abilities to move around on land, but is thought to have been primarily aquatic.

[2] In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish –like Eusthenopteron– exhibited a sequence of adaptations: Panderichthys, suited to muddy shallows; Tiktaalik with limb-like fins that could take it onto land; stem-tetrapods in weed-filled swamps, such as Acanthostega, which had eight-digited feet; and Ichthyostega, with full limbs.

Jennifer A. Clack interprets this as showing that Acanthostega was primarily an aquatic animal descended from fish that never left the sea, and that the specializations of the tetrapod lineage were exaptations: features which would later be useful for terrestrial life, even if they originated for a different purpose.

Some individuals reached sexual maturity (based on a fully ossified humerus) at more than six years of age, and adult fossils are much rarer than juveniles.