Although Acanthostega was an amphibian, it possessed
a number of primitive fish-like features.
Acanthostega probably possessed at least 3 pairs of gill arches similar
to modern lungfish and it probably supplemented the air absorbed from
its lungs using its internal gills. The atlas was the only vertebrae that
fused across the midline of the body. Although Acanthostega had a neck,
the notochord's penetration of the skull made the neck less mobile. Large
muscles attached to the exoccipitals to support the neck. The neural arches
were reduced in the cervical region (Clack, 2002). Acanthostega's ribs
were shorter and not as thick as in Ichthyostega. The interclavicle could
have functioned in a way similar to the sternum of later tetrapods (Clack,
Acanthostega, like other early tetrapods retained scales on its underside.
The loss of scales on the dorsal surface would have made these tetrapods
lighter and allowed more gas exchange through the skin. Acanthostega also
had evolved a lacrimal gland, lacrimal ducts, eyelids, eyelid muscles,
and a retractor bulbi muscle (Clack, 2002).