Acanthostega gunnari, 60 cm.
Supervised by Per Ahlberg, University of Uppsala.
Acanthostega gunnari 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 walking on land.
There is one main difference of living in water vs. living on land: In a lake or sea, the water buoyancy supports the organism, but on land the organism needs to counter the force of gravity by its skeletal structure in order to move around.
Going from water to land requires that the organism develop a shift in locomotory dominance from the pectoral girdle to the pelvic girdle, making the pelvic girdle the weight-bearing structure of the skeleton. Acanthostega is the earliest stem-tetrapod to show this shift in skeletal structure, and this specialization would later be useful for terrestrial life for the tetrapod lineage.
Acanthostega was primarily an aquatic animal probably living in shallow, weed-choked swamps. It descended from fish that never left the sea, but Acanthostega was adapted for what they call terrestrial-style feeding, as the morphology of the skull indicate that it was able to bite prey at or near the water's edge.
It grew to 60 cm and had 8 digits in each hand linked by webbing. It had both gills and lungs, but its ribs were too short to support its chest cavity out of water.