Assemblage-level studies of mean trait variation are common in macroecology. However, how phylogenetic relationships among species affect trait-based macroecological patterns is still unresolved. I used an approach based on variation partitioning analysis using environmental and phylogenetic lineage variation as predictors to investigate whether variation in mean trait values among Neotropical sigmodontine rodent communities is best explained by macroecological adaptation, biogeographical history, or joint effects of both – the latter resulting in phylogenetic niche conservatism (PNC) at the metacommunity scale. Metacommunity PNC best explained mean variation in body size and skull/mandible shape across assemblages, and the pattern of metacommunity PNC suggests that influence of environmental factors on mean trait variation relies heavily on spatial biogeographical clade sorting. This suggests that biogeographical lineage distribution should be taken into account in analyses seeking to correlate environmental variables with mean trait variation.