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 to Rhizodus and later to Strepsodus, 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 of Strepsodus, 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 of Strepsodus and Rhizodus evolved within the Rhizodontida, rather than as primitive characters for the group. This new jaw material supports earlier models of mandibular kinesis and shows that the coronoid fangs were oriented such as to facilitate kinesis. Rhizodontid jaw evolution seems to be marked by parallelisms with tristichopterids and early tetrapods.