The application of principles derived from recent ecosystems to paleoecosystems is an important tool for testing the universality of these principles, as well as identifying deviations that require further investigation. Here, we estimate the predator and prey biomass in nine Italian Plio-Pleistocene mammalian paleocommunities and compare their relationships with recent ecosystems. The predator-prey biomass relationship is shown to be identical in extant and fossil communities for prey species less than 450 kg body mass (BM), thus indicating that biomass fluctuations from prey to predators were similar in Plio-Pleistocene and recent ecosystems for this BM range. However, if herbivores of 450 kg up to 1000 kg were also accepted as prey, the predator-prey biomass relationship differs significantly between Plio-Pleistocene paleoecosystems and recent ecosystems. Herbivores within this BM range occurred after the early Pleistocene, whereas both smaller (<450 kg) and larger (>1000 kg) herbivores were present in all ecosystems studied. The results of this study suggest that fossil herbivores of this particular BM range were ecologically different from similar-sized extant populations and did not constitute regular prey species for the predators present. Their emergence was not accompanied by a corresponding increase in biomass of predators. Thus, middle-Late Pleistocene ecosystems were potentially controlled from the bottom up rather than from the top down. The results reported herein show how successive estimates of body mass, density, and biomass can be used to reconstruct paleoecosystems.