A new genus of rhizodontid (Sarcopterygii, Tetrapodomorpha) from the Lower Carboniferous Horton Bluff Formation of Nova Scotia, and the evolution of the lower jaws in this group
Letognathus gen. nov. is described from the Lower Carboniferous (Tournaisian) Horton Bluff Formation. Included in this genus is the species Letognathus hardingi (Dawson 1868), which was originally assigned toRhizodus and later toStrepsodus, but it is distinct from these genera in tooth morphology and the presence of several more primitive rhizodontid characters. By contrast with these latter more derived genera, the parasymphysial plate contacts the first coronoid; the first coronoid bears a row of lingually deflected teeth, the tooth striations occur around the entire circumference of the crown and are much finer and less regular than those ofStrepsodus, and marginal tooth row reaches anteriorly to the dentary symphysis. Phylogenetic analysis indicates that rhizodontid jaws primitively bear symphysial tusks; slender recumbent teeth; an anterior projection of the dentary over the anteromesial depression for the median symphysial plate and shows the highly derived jaws ofStrepsodus andRhizodus evolved with...