Caviomorphs are a clade of South American rodents recorded at least since the early Oligocene (>31.5 Ma) that exhibit ample eco-morphological variation. It has been proposed that phylogenetic structure is more important than ecological factors for understanding mandibular shape variation in this clade. This was interpreted as a result of the long-standing evolutionary history of caviomorphs and the early divergence of major lineages. In this work we test this hypothesis through analysis of morphological variation in the mandible of living and extinct species, and compare this information with that obtained through comparative phylogenetic analyses. Our results support the hypothesis of early origin of mandibular variation; moreover, they suggest conservation of early differentiated morphologies, which could indicate the existence of constrained evolutionary diversification.