Significance Closed-canopy rainforests are important for climate and ecology, yet identifying this ecosystem in the fossil record is challenging. An existing paradigm for identification of closed-canopy rainforests using fossil mammal carbon isotope data is the presence of highly negative δ ¹³ C _diet values (<−31‰) in the herbivore community, as observed in modern equatorial African rainforest ecosystems. Our data from western Amazonian mammals, however, show that the absence of these values is not evidence for absence of closed-canopy rainforests. Our results also document that the proposed relationship between carbon isotope spacing variables and traditional dietary ecological classifications is not straightforward, and that better characterizations of the mixture of nutrients in animal diets are necessary to fully understand diet-tissue isotopic fractionation patterns.