The masticatory apparatus of Pampatherium typum, P. humboldtii, and Holmesina paulacoutoi are analysed based on skull, mandibular and dental morphology and the inferred masticatory musculature. Comparison of the apparatus with those of other pampatheres suggests a trend toward increasing ability to process resistant vegetation from Holmesina through Pampatherium. The mechanical design of the apparatus among these pampatheres is nearly identical and has apparently undergone only minor variation since pampatheres first appear in the fossil record, suggesting strong phylogenetic constraint in the form of skeletal and dental elements. Main differences among taxa occur in features associated with the musculature, and thus the primary means for differentiation in masticatory function, which is correlated with diet, were changes resulting in differential force imput. Although the taxonomic status of some specimens is equivocal, indications are that the Plio-Pleistocene paleobiogeographic distribution of pampatheres is correlated with masticatory function (and hence diet), with P. typum, the species best adapted for grinding coarse vegetation, occurring in the more arid Pampean regions of South America. H. occidentalis, although a capable grinder, was the least suited to coarse vegetation. It is known from deposits near the current Peru-Ecuador border, an area of humid lowlands during glacial maxima. H. paulacoutoi and P. humboldtii lie between these extremes, with the latter better adapted for abrasive vegetation. These pampatheres have been found at the same localities, mainly through Brazil, which was characterized by wetter conditions than the Pampas. Their presence together suggests either sympatry or occupation of the same region during different times, with their ranges moving either north (P. humboldtii) or south (H. paulacoutoi) in a transition zone that may have shifted in response to changes in glacial maxima and minima.