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DEVONIAN PERIOD

416-359 million years ago

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skull

skull

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).