Mesotheriidae includes small to medium-sized herbivores that belong to the so-called South American native ungulates. Mesotheriids radiated from the early Oligocene to the early–middle Pleistocene and were traditionally grouped into two subfamilies: Trachytheriinae and Mesotheriinae. Beyond the conspicuous fossil record of the group, little is known about its palaeobiology and, particularly, the dietary habits. So far, different studies suggest grazing habits, similar to extant ungulates, but recent contributions are challenging this vision to some degree. In this contribution, we explore dietary preferences in mesotheriids, focusing on food intake and oral processing strategies. We performed a shape analysis of the snout and reconstructed ancestral shapes to assess evolutionary trends within the clade. Also, we considered other morphological variables linked to the masticatory apparatus (zygomatic plate, incisive foramina, cheek teeth morphology) to perform a functional characterization. Our results allowed us to recognize three snout configurations: Trachytherus, "Intermediate", and Mesotherium morphotypes that can be associated with different food intake capabilities and, considering also other morphological proxies, to consume different food items. Taking into account the information about palaeoenvironmental conditions, the evolution of the snout anatomy and associated features were interpreted in the context of a consumption gradient from coarse and tough foods (root, tuber, tough leaves) to grasses per se. We propose that during the ecomorphological evolution of mesotheriids, a first phase during the late Oligocene-Miocene took place, in which specialization to- and diversification within an ecomorphospace linked to the consumption of hard-to-break food items occurred. A second phase incipiently started during the late Miocene, and well-defined during Pliocene-Pleistocene, characterized by an ecomorphological displacement towards grazing habits. Inferred habits in Mesotheriidae could give light to the palaeoecological evolution and niche partition within South American herbivorous mammalian communities.