Figure 13. Comparison between Trinomys upper teeth, which presents a in Relationships among extant and fossil echimyids (Rodentia: Hystricognathi)

Figure 13. Comparison between Trinomys upper teeth, which presents a deep sulcus between protoloph and the protocone region in little-worn teeth (indicated by an arrow in the dP4), resulting in a hypoflexus continuous to the paraflexus ('deep main fold' of Moojen, 1948), and Proechimys upper teeth, which do not present a sulcus or have it quite shallow (character 32). A, Trinomys dimidiatus (MN 4016, left series); B, Proechimys gularis (AMNH 67323, right series reversed to the left side). The arrows, in the dP4 of both specimens, show the area between the protoloph and the protocone region. Note that, although the Trinomys specimen is older (has all teeth already worn), the sulcus is present in all teeth; by contrast, in Proechimys, only the unworn M3 presents the sulcus, which is already absent in dP4–M2.