Abstract
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 area analyses to members of an oryzomyini clade informally named ‘ D ’ in former studies and included related fossil tetralophodont forms. Based on 98 morphological characters and sequences of five gene fragments, we found that the tetralophodont condition is paraphyletic. Among living taxa, P seudoryzomys is sister to H olochilus, and L undomys is derived from a basal divergence. A clade formed by living H olochilus and the fossils Noronhomys and C arletonomys is sister to H olochilus primigenus , making H olochilus paraphyletic. Therefore, we describe a new genus that accommodates the fossil H . primigenus . Because trans‐Andean taxa currently share a common ancestor with taxa of cis‐ A dean distribution, the northern Andes uplift may have worked as a postdispersal barrier. The tetralophodont lineages diverged during the P liocene from a cis‐Andean ancestor, and the G reat P lains in S outh A merica may have favoured the diversification of tetralophodont forms adapted to open habitats during the P liocene.