Previous accounts of the dentition of the Carboniferous dipnoan
Uronemus
have 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 Devonian
Uranolophus
and
Griphognathus
and the Carboniferous to Permian
Conchopoma
. Genera with tooth plates have been considered to be a monophyletic group in which tooth plates are a derived character;
Uronemus
has 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 of
Uronemus
has 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 of
Uronemus
is 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 as
Uranolophus
. 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 that
Conchopoma
is a close relative of
Uronemus
is not accepted, and possible new relationships have been proposed. New data on
Scaumenacia
and
Phaneropleuron
, two other genera previously compared with
Uronemus
, are presented.
Rhinodipterus
, a form with elongate lingual ridges, is also discussed.
Phaneropleuron
is shown to have radiate tooth plates and not a marginal row of conical teeth as previously described. It is proposed that the tooth plate of
Uronemus
is 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 extant
Lepidosiren 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 that
Uronemus
was adapted for air breathing. The ‘denticulated’ lineage, which included genera such as
Uranolophus
and
Griphognathus
, shows none of the skeletal features associated with the presence of a tongue, and presumably did not become air breathing.
A basal aïstopod from the earliest Pennsylvanian of Canada, and the antiquity of the first limbless tetrapod lineage
