With 14 species recorded, the Miocene La Venta bat fauna is the most diverse bat palaeocommunity in South America. They include the oldest plant-visiting bat in the New World, and some of the earliest representatives of the extant families Phyllostomidae, Thyropteridae and Noctilionidae. La Venta’s Notonycteris magdalenensis is an extinct member of the subfamily Phyllostominae, a group of modern Neotropical animalivorous bats, and is commonly included in studies of the evolution of Neotropical bats, but aspects of its biology remain unclear. In this study, we used multivariate dental topography analysis (DTA) to reconstruct the diet of N. magdalenensis by quantitatively comparing measures of molar complexity with that of 25 modern noctilionoid species representing all major dietary habits in bats. We found clear differences in molar complexity between dietary guilds, indicating that DTA is potentially an informative tool to study bat ecomorphology. Our results suggest N. magdalenensis was probably an omnivore or insectivore, rather than a carnivore like its modern relatives Chrotopterus auritus and Vampryum spectrum. Also, we reconstructed the body mass of N. magdalenensis to be ~95 g, larger than most insectivorous bats, but smaller than the largest carnivorous bat (V. spectrum). Our results confirm that N. magdalenensis was not a specialised carnivore. It remains to be demonstrated that the specialised carnivory ecological niche was occupied by the same lineage of phyllostomines from at least the middle Miocene. Combining our diet and body mass reconstructions, we suggest that N. magdalenensis exhibits morphological pre-adaptations crucial for the evolution of specialised carnivory.