Oryzomyini is the richest tribe among the S igmodontine rodents, encompassing 32 living and extinct genera and including an increasing number of recently described species and genera. Some O ryzomyini are tetralophodont showing a reduction in the number of molar folds to four, while most taxa in this tribe retain the plesiomorphic pentalophodont state. We applied phylogenetic methods, molecular dating techniques and ancestral...