416-359 million years ago
The skull of Ichthyostega is a mix of primitive and derived features.
In the skull of Ichthyostega, the notochord no longer penetrated the
braincase, making it easier to move the head. The ribs had increased in
length. An olecranon process was present although the arm's movement was
limited. The femur was half the length of the humerus. The anterior tectal
bone around the nostril was lost. After Ichtyostega, Acanthostega, and
the common ancestor of the early tetrapod Whatcheeria, the preopercular
bone which had supported the opercular series of bones was lost; in the
early tetrapods it had already been reduced in size (Clack, 2002).