A long-standing question in community ecology needs an evolutionary perspective to answer: How can so many species coexist in such unproductive environments as deserts? Ecologists have proposed assembly rules that predict how species could be added to communities, framed in terms of functional groups within guilds, specifically, that as species richness rises, new functional groups are added to a community until all are present before any are repeated. The challenge is to classify functional groups within guilds, using an objective and quantitative method. We propose that functional groups can be characterized as distinct adaptive peaks. Heteromyid rodents are a classic model system for testing rules, but only a narrow part of their range has been sampled intensively. It is not clear whether (or how) to generalize the postulated functional groups beyond the southwestern US. Here we analyze the adaptive landscape of Heteromyidae, taking as a starting point the idea that all belong to the same dietary guild, within which resources are partitioned mainly according to body size, but locomotory behaviors also partly determine access. We find that dietary adaptations are far more complex. Our analysis reveals five adaptive peaks for mandibular shape, and four for size. The peaks for the two traits are not concordant, mainly because of heterogeneous adaptations of the bipeds. Miniature bipeds converge towards mid-sized quadrupeds in shape, one giant biped also diverges in that direction but most are similar to mid-sized bipeds and have the same size but not the same shape as the mid-sized quadrupeds. Although locomotory mode obviously affects limbs and posture, it also has consequences for bullar morphology and thus for jaw shape, weakening bite which is differentially compensated in the giants and miniatures.