Morphological characters of well-established taxonomic utility are infrequently examined for their relative phylogenetic consistency. Second only to characters of reproductive anatomy, jaw morphology and dentition commonly are employed as diagnostic characters for hirudiniform leeches, yet these features are highly variable across the group. Patterns of change were investigated for number of jaws and number of denticles per jaw in a phylogenetic context across 17 hirudiniform leeches representing three families. Phylogeny reconstruction employed 16 morphological characters, as well as two nuclear and two mitochondrial loci, and was evaluated with parsimony and likelihood. Rather than constrain the ancestral number of denticles to extant states, this meristic was optimised with squared-change parsimony. The degree to which dentition patterns were explained by phylogenetic relationships was assessed against a null distribution defined by permutation of extant states across terminals. Dentition was found to be non-randomly explained by phylogeny and, thus, corroborative of relationships among hirudiniform leeches as well as of the uniqueness of a new species of Oxyptychus described here from the Peruvian Amazon.