Molar polymorphism and variation in tooth number in a semi-aquatic rodent, neusticomys oyapocki (Sigmodontinae, Ichthyomyini)

Abstract

Newly collected specimens of the rare semi-aquatic mouse Neusticomys oyapocki (Rodentia: Sigmodontinae: Ichthyomyini) exhibit individual variation in tooth number. Whereas this Guianan ichthyomyine had previously been described as having only two molars per jaw quadrant, our study examines 31 specimens, of which 12 exhibit from one to four additional third molars. Specimens with at least one third molar come from 6 localities spanning Suriname, French Guiana, and the Brazilian states of Amapa and Para. Third molars reported in this species are tiny (on average 0.55 mm crown length), about half the size of second molars. We show that N. oyapocki has between 8 and 12 molars, which represents extensive variation in the number of teeth. This kind of dental variation is rare in muroid rodents, as only a few such cases have been reported from some carnivorous water rats and moss mice from New Guinea. © 2017, SAREM Sociedad Argentina para el Estudio de los Mamiferos. All rights resreved.