Adaptations to life in freshwater for Mioceratodus gregoryi, a lungfish from Redbank Plains, an Eocene locality in southeast Queensland, Australia

Abstract

Few Cenozoic lungfish fossils consist of articulated, associated bones and tooth plates. Mioceratodus gregoryi from the Paleogene (Eocene) deposit of the Redbank Plains Formation in southeast Queensland is unusual in this respect because the fossil includes tooth plates and elements of the skull. An analysis of the material and reconstruction of the skull and associated skeletal material provides new insights into the fish and its environment. The fish has a mandible with a wide separation between the lower tooth-bearing bones, and a strong ceratohyal bone. This suggests that, like the extant Australian lungfish, Neoceratodus forsteri, the fossil fish had a moveable basihyal that could be inserted between the prearticular bones to seal the oral cavity. This would have allowed the fish to draw food, air and water into the mouth, and dig holes by sucking mud into the oral cavity and blowing it out again, all useful attributes for a fish that lived in a shallow freshwater lake. The living Australian lungfish has similar structures in the mandible and hyoid apparatus, and performs comparable actions. The occipital ribs, also preserved in the Redbank Plains fossil, are embedded in hypaxial muscles and not moveable. It is unlikely that these ribs have any influence on the suctorial process in these two species.