Adaptive landscapes are a central metaphor for understanding the dynamics of morphological evolution, but geographic landscapes are central to understanding how morphological diversity persists. In this analysis, we revisit a classic paradigm for community assembly in light of the adaptive landscape of desert-adapted rodents (Heteromyidae). Traditionally, these rodents are considered to belong to a single dietary guild, divisible into two functional groups by locomotory mode. According to the model for their community assembly, increases in species richness occur by packing more ecologically similar species into more diverse communities. That model predicts that functional diversity would not consistently increase with species richness, an expectation supported by analysis of phylogenetic community structure, which found that more close relatives are packed into richer communities. Revisiting this case, we integrate an analysis of the adaptive landscape of trophic morphology with an analysis of the geographic patterns of morphological and phylogenetic diversity. We also find that, at a scale of 5 km resolution, richer assemblages consist of closer relatives: as expected, most metrics of phylogenetic diversity negatively scale with species richness. However, the scaling relationships for morphological measures of diversity supply a different explanation: the morphological range and minimum nearest-neighbor distances exhibit contrasting scaling relationships: the range of morphology increases, although not at a constant rate, as the minimum nearest-neighbor distance decreases. With respect to dietary adaptations, local assemblages of heteromyids apparently exhibit a combination of niche-expansion and finer partitioning.