OSTEOLOGY AND FUNCTIONAL MORPHOLOGY OF THE OSTEOLEPIFORM FISH GOGONASUS ANDREWSAE LONG, 1985, FROM THE UPPER DEVONIAN GOGO FORMATION, WESTERN AUSTRALIA

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Abstract - Gogonasus andrewsae Long is a Frasnian (Late Devonian) osteolepiform which was described from a single ethmosphenoid unit which lacked the cheek. Since the first description in 1985, two more complete specimens have been found, and are described herein. They are of particular significance in that they have been etched from limestone and are virtually undistorted. As a result they provide detailed information on the anatomy of the Osteolepiformes previously available only from interpretations of ground sections, and from etched specimens of the genus Cladarosymblema, which comes from the much later Early Carboniferous (Visean) of Queensland, and Medoevia, which comes from an unknown, probably Late Devonian, locality in Russia. Particular items of significance are the notch in the anterior end of the maxilla defining the posterior margin of the choana; the absence of prenasal fossae; the very high crista suspendens; the foramen for the ophthalmica magna situated at the anterior edge of the basisphenoid process; the pair of large rounded processes at the edge of the fossa Bridgei; the proximal part of the hyomandibular is horizontal; the basibranchial/basihyal is short, broad and flat, making a single unit; and the posterior edge of the palatoquadrate making a high arch.

An attempt is made to show how the total structural complex of the head can be interpreted in functional terms. Gogonasus probably had a very limited degree of intracranial kinesis, and the amount of vertical movement between the maxilla, palatoquadrate complex and the rostral part of the ethmosphenoid was also very small. The high position of the hyomandibular, and its distal end being adjacent to the opercular bone, is here suggested as not being directly related to the high position of the stapes in primitive tetrapods, but to the high shape of the head, and enlargement of the buccal cavity, and as a new means of opening the opercular series. Closure of the mouth was achieved largely through the adductor mandibulae, without a need to involve basicranial muscles for initiation.