Most land-dwelling vertebrates have rotating limbs terminating in digits, a characteristic useful for land movement  Biologists who assume that this characteristic evolved only after animals abandoned aquatic environments must consider the Acanthostega, a newly discovered ancestor of all land vertebrates  It possessed rotating limbs terminating in digits, but its skeleton was too feeble for land movement
It also breathed using only internal gills, indicating that it and its predecessors were exclusively aquatic
Acanthostega originated as a land-dwelling species, but evolved gills only after moving to an underwater environment.