416-359 million years ago



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, 2002).
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).