The relationships ofUronemus: A Carboniferous dipnoan with highly modified tooth plates

Abstract

Previous accounts of the dentition of the Carboniferous dipnoanUronemushave stressed the significance of the scattered small denticles. These, together with the marginal teeth and ridges, have been interpreted as primitive characters of the dipnoan dentition shared with three other genera: the DevonianUranolophusandGriphognathusand the Carboniferous to PermianConchopoma. Genera with tooth plates have been considered to be a monophyletic group in which tooth plates are a derived character;Uronemushas been excluded from this group in all previous investigations dealing with the significance of the dentition for determining relationships among dipnoans. The macromorphology of the dentition ofUronemushas been re-examined and correlated with the histology of all the dental tissues. Optical study of thin sections and scanning electron microscope study of the adjacent cut surfaces has shown that the hard, wear-resistant dentine of the teeth and ridges is petrodentine. The arrangement, growth, wear and histology of the dental tissues have been compared with those of denticulated and tooth-plated genera. The arrangement of new teeth relative to the tooth ridge, the pattern of wear along the ridge, and the type of dentine and its growth indicate that the dentition ofUronemusis best interpreted as a tooth plate with one long lingual tooth ridge and reduced lateral tooth rows. Therefore the marginal tooth ridges are not considered to be homologous with those of denticulate dipnoans such asUranolophus. The presence of petrodentine, a tissue type only found in forms with tooth plates, is consistent with the view that the dentition is derived by modification of a radiate tooth plate. The denticles covering restricted regions of the palate and lower jaw are considered to have been a secondary acquisition. The suggestion thatConchopomais a close relative ofUronemusis not accepted, and possible new relationships have been proposed. New data onScaumenaciaandPhaneropleuron, two other genera previously compared withUronemus, are presented.Rhinodipterus, a form with elongate lingual ridges, is also discussed.Phaneropleuronis shown to have radiate tooth plates and not a marginal row of conical teeth as previously described. It is proposed that the tooth plate ofUronemusis derived from a dipterid type of plate. A discussion of some of the other factors involved in determining the relationships of the genus is given. From an examination of the use of the tongue for respiration and feeding by the extantLepidosiren paradoxa, it is concluded that many features of dipnoan evolution in the tooth-plated lineage result from the adoption of air breathing after an early evolutionary phase of gill respiration, and thatUronemuswas adapted for air breathing. The ‘denticulated’ lineage, which included genera such asUranolophusandGriphognathus, shows none of the skeletal features associated with the presence of a tongue, and presumably did not become air breathing.