Article: New material of the early tetrapod Acanthostega from the Upper Devonian of East Greenland
J. A. Clack
New material of one of the oldest known tetrapods, Acanthostega gunnari, is described: three skulls, together in one block, in association with postcranial material. This is the first postcranial material to be described for Acanthostega. The skulls show an animal with a broad, closed, denticulated palate in which the pterygoids meet in the mid-line as in loxommatids and Ichthyostega. The ventrally grooved parasphenoid resembles that of some osteolepiform fish rather than that of tetrapods. The basal articulation is tetrapod-like with well-developed basipterygoid processes. The otic capsules appear to be well ossified and the braincase fits flat under the skull table, in contrast to the complex facets in Ichthyostega. No synapomorphies with any particular tetrapod group have been discovered, but one additional character defining all tetrapods (large ornamented interclavicle) and two defining all neotetrapods (presplenial-anterior coronoid suture, surangular contributes significantly to margin of adductor fossa) have been identified. The latter two can be used to establish whether isolated lower jaws belong to fishes or to tetrapods.