 In 2006 , Marcelo Weksler published a large @-@ scale cladistic analysis of Oryzomyini ( " rice rats " ) , the tribe to which O. polius belongs . He used both morphological data and molecular characters from the IRBP gene . In all of his analyses , O. polius was found to be part of clade D , one of four large groups within Oryzomyini , as the sister group to a clade containing all the other species of clade D. Clade D was supported by two shared derived ( synapomorphic ) molecular characters and by seven morphological synapomorphies — the tail has a different color above and below ; the parietal bone extends to the side of the skull ; the incisive foramina ( openings in the palate ) extend back between the first molars ; the posterolateral palatal pits ( perforations of the palate near the third molars ) are complex ; the sphenopalatine vacuities ( openings in the mesopterygoid fossa , the gap behind the end of the palate ) are large ; the pattern of the arterial circulation in the head is derived ; and the posteroloph ( a crest at the back ) is present on the third upper molar . Two other molecular synapomorphies supported the clade of all members of clade D except O. polius , coupled with three morphological traits — in these species , but not in O. polius , the first upper molar has an additional small root at the outer ( labial ) side ; the first lower molar has additional small roots ; and the second upper molar has the mesoflexus ( one of the valleys between the cusps and crests ) divided in two . 
