The structure and stratigraphy of Speonesydrion from New South Wales, Australia, and the dentition of primitive dipnoans

New material of Speonesydrion iani, an Early Devonian dipnoan from New South Wales, has provided additional information on the dentition and jaws. Two new partial palates have been found, and X-rays of the parasphe-noid shows that the structure is well preserved. The palatal teeth are well worn even in partly grown material, and they do not originate at a growth point, but at a thickening of the palate. More mandibles have been collected, and thin sections have been prepared to allow a discussion of their histology. On the mandible the teeth are clear, and they are much more defined than they are on the palate. The dental heel is variably developed, and grows in phases by thickening of the dentine at the contact with the bone. Dentine forms on the bone at the base of the heel, partly by solution of the bone and the addition of dentine from the pulp canals, but also by direct growth from the pulp canals dorsal to the bone. In the latter case the dentine and bone are in contact, and the two tissues intermingle. The teeth are also formed on a thickened bone and consist of dentine capped with enamel making a crest. Dentine and bone are related as in the heel. We conclude that the teeth in Speonesydrion are not homologous with the teeth in other dipnoans, and are formed by a different process involving the aggregation of denticles. copy; 2007 E. Schweizerbart'sche Verlagsbuchhandlung.