Acanthostega gunnari

Acanthostega (meaning "spiny roof") is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs. 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.

The 60 cm (24 in) Acanthostega had eight digits
 on each hand (the number of digits on the feet is unclear) linked by
webbing, it lacked wrists, and was generally poorly adapted for walking
on land. It also had a remarkably fish-like shoulder and forelimb. The
front foot 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. Acanthostega is the earliest stem-tetrapod to show
the shift in locomotory dominance from the pectoral girdle
 to the pelvic girdle. There are many morphological changes that allowed
 the pelvic girdle of Acanthostega to become a weight-bearing structure.
 In more ancestral states the two sides of the girdle were not attached.
 In Acanthostega there is contact between the two sides and fusion of
the girdle with the sacral rib of the vertebral column. These fusions
would have made the pelvic region more powerful and equipped to counter
the force of gravity when not supported by the buoyancy of an aquatic
environment. It had internal gills that were covered like those of
fish. It also had lungs, but its ribs were too short to support its
chest cavity out of water.
It has been inferred that Acanthostega
probably lived in shallow, weed-choked swamps, its legs being not an
adaptation to walking on land. 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 in the tetrapod lineage developed features which would
later be useful for terrestrial life. At that period, deciduous
 plants were flourishing and annually shedding leaves into the water,
attracting small prey into warm oxygen-poor shallows that were difficult
 for larger fishto
 swim in; Clack remarks on how the lower jaw of Acanthostega shows a
change from those of fish that have two rows of teeth, with a large
number of small teeth in the outer row, and two large fangs and some
smaller teeth in the inner row.

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.

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. 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.
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. Their descendants also included pelagic lobe-finned fish such as coelacanth species.