Structure and Function of the Shoulder Girdle in Dipnoans: New Material from Dipterus valenciennesi

Abstract

The pectoral girdle, including the "anocleithrum", of Dipterus valenciennesi is described from Caithness, Scotland. This makes it the oldest known dipnoan genus for which the "anocleithrum" is known. The details of the cleithrum, clavicle and scapulocoracoid, are described. The median part of the "anocleithrum" has a pair of long processes for attachment to the endocranial process posterior to bone "I". Problems of the homology of the "anocleithrum" are discussed. The hyomandibular of Chirodipterus australis is used as an example of the Devonian dipnoans. The structure of the opercular and its muscles for the attachment of the hyomandibular process, suggests that the hyomandibular / opercular arrangement was the same kind as in Dipterus valenciennesi. The rotation of the hyomandibular in opening and closing the operculum, makes for a distinctive means for controlling the ventilation of the gill chambers. A comparison of the "anocleithrum" with those of other Palaeozoic dipnoans is presented, and the function of the dipnoan "anocleithrum " is outlined. In dipnoans the gill arches have neither hypobranchials nor pharyngobranchials. We consider that the presence of the "anocleithrum", the movement of the hyomandibular in moving the opercular, and the restriction of the gills, is intimately related. The introduction of these features results from the modification by gene regulation near the beginning of the Devonian. The restriction of these features to the Dipnoi provides a distinctive way to identify the members ofthat group.