Incisor Enamel Microstructure of Some Neogene Muroid Rodents of India: Taxonomic, Evolutionary and Biomechanical Implications

The fossil muroid rodent record of the Indian subcontinent is deep-rooted and is based mainly on isolated molars. However, rodent incisors from the Neogene terrestrial sequences of the subcontinent have rarely been studied for their taxonomic, evolutionary, and adaptive applications. Based on a detailed analysis of their enamel microstructure, we assign the rodent incisors from the middle Miocene Siwaliks of Ramnagar (J&K), to murine (cf. Antemus), cricetine and rhizomyine (cf. Kanisamys) and those from the late Miocene deposits of Kachchh (Gujarat) to murine (cf. Progonomys), rhizomyine (cf. Kanisamys), ctenodactyline (cf. Sayimys) and sciurine (cf. Tamias). Early and late Pliocene Siwalik rodent incisors have been assigned to murine (cf. Nesokia, cf. Millardia, cf. Mus, cf. Bandicota, cf. Golunda), gerbilline (cf. Abudhabia) and rhizomyine (cf. Rhizomys). Our study reveals that the Siwalik cricetid incisors differ from those of other murine incisors in possessing surface ornamentation. Murines are characterized by having smooth enamel surface with transversely to partly transverse and partly diagonally arranged HSBs with angled to perpendicular IPM. Compared to this, gerbillines show wide grooves/sulci on the enamel surface while rhizomyines have three to four-layered enamel structures. In Antemus-Progonomys-Mus lineage, Antemus shows round prism cross-section, transverse HSBs and angled IPM, while Progonomys and Mus show oval prism cross-section, transverse and diagonal HSBs, and almost perpendicular IPM. Biomechanically, cf. Antemus and a few other murines with thin modified radial enamel may suggest a diet of hard substances. Rhizomyines like cf. Kanisamys and cf. Rhizomys, and murines such as cf. Nesokia and cf. Bandicota shows thicker outer enamel indicating digging and burrowing behaviours.