Figure 8. Comparison between Trinomys lower teeth, which presents a in Relationships among extant and fossil echimyids (Rodentia: Hystricognathi)

Figure 8. Comparison between Trinomys lower teeth, which presents a deep sulcus between hypolophid and hypoconid (indicated by an arrow in the dP4), resulting in a hypoflexid continuous to the metaflexid ('deep main fold' of Moojen, 1948); and Proechimys lower teeth, which do not present a sulcus or have it quite shallow (feature considered in characters 8 and 20). A, Trinomys dimidiatus (MN 4016; left series); B, Proechimys gularis (AMNH 67323, left series). The arrows, in the dP4 of both specimens, show the area between the hypolophid and hypoconid. Note that, although the Trinomys specimen is older (has all teeth already worn), the sulcus is present in all teeth (state 1); by contrast, in Proechimys, only the unworn M3 presents the sulcus, which is already absent in dP4–M2 (state 0).