Mean adult body mass of mammal taxa is a fundamental ecological variable. Variability in the distributions of body masses of a mammal fauna suggest variability in habitat structure. Mammal remains from the Marmes archaeological site in southeastern Washington State date between 13,200 and 10,400 b.p., during the Pleistocene–Holocene transition (PHT). Known environmental history prompts the expectations that the Marmes PHT mammal remains should represent greater species richness and a larger array of body-mass sizes than modern faunas in the Marmes locale and in open shrub-steppe habitats, and lower species richness and a smaller array of body-mass sizes than modern faunas in closed forest habitats; species richness and the array of body-mass sizes should be similar to that for a mixed habitat of cool shrub-steppe with scattered conifers. The Marmes PHT cenogram meets these expectations. Body-mass clumps displayed by the Marmes PHT mammal fauna fall between those of closed forests and open shrub-steppe habitats in terms of clump richness and breadth, and in terms of gap width. Marmes PHT body-mass clumps are very similar to those for the mixed habitat. Cenograms and body-mass clumps confirm conclusions drawn 40 years ago that the Marmes PHT habitat was much like that of today but cooler and with more plant biomass and greater structural diversity than today.