SKIN AND BLOOD-VESSELS OF THE SNOUT OF THE AUSTRALIAN LUNGFISH, NEOCERATODUS-FORSTERI, AND THEIR SIGNIFICANCE FOR INTERPRETING THE COSMINE OF DEVONIAN LUNGFISHES

Improved structural and functional interpretations regarding the dermal skeleton of Paleozoic lungfishes (Dipnoi) can be derived from a direct comparison of Recent and fossil tissues. In particular. skin from the snout of adult Australian lungfish (Neoceratodus forsteri) contains horizontal plexuses and vertical capillary loops which resemble in structure, size and density components of the cosmine layer in such Paleozoic lungfishes as Dipterus valenciennesi and Chirodipterus australis. In addition to these dermal papillae. the skin of the snout also contains ampullary electroreceptors, goblet cells, compound mucus glands, and terminal branches and openings of the mechanoreceptive lateral line system. Pore canal systems of fossil lungfishes previously have been interpreted as housing electroreceptors or other cutaneous sense organs of the lateral line system. In contrast, we regard pore canal systems as evidence of a complex cutaneous vasculature involved in the deposition of mineralized tissues. Prevailing ideas on the structure and biological role of cosmine are reinterpreted, including the theory that electroreceptors played an important part in the origin of the dermal skeleton.