We report the first radiocarbon datings and carbon (δ13C) and oxygen (δ18O) stable isotopes data to reconstruct the paleoecology of medium to large herbivorous mammals from late Quaternary of southwestern Amazon (Acre and Rondonia states, Brazil). AMS 14C dates for Neochoerus sp. (29,072 - 27,713 Cal yr BP), Notiomastodon platensis (25,454 - 24,884 Cal yr BP) and Eremotherium laurillardi (11,320 - 11,131 Cal yr BP) support the Lujanian ages. All fossils have low δ13C and δ18O isotopic values that suggest C3-dominated environments from closed canopy forests to wooded savannas, agreeing with paleovegetation reconstitution. Most species were browsers (piC3=100%; Niche breadth, BA=0), where the key species with the largest body mass, N. platensis (∼6,300 kg) and E. laurillardi (∼3,500 kg), possibly had a more generalized browser diet in closed-canopies to woodlands. Their diet distinguished from the C3/C4 generalist Trigodonops lopesi (∼1,900 kg), which foraged in wooded savannas (piC3=70%; BA=0.72), similarly with its relative Toxodon platensis (∼1,800 kg) that had a browse-dominated mixed feeder diet (piC3 ≥ 84%, BA ≤ 0.38) in Peruvian and Bolivian Amazon localities. Palaeolama major (∼280 kg) was possibly a strictly folivorous within forest canopies, whereas Tapirus sp. (∼250 kg) and Mazama sp. (∼40 kg) were browsers in closed-canopies to woodlands. Holmesina rondoniensis (∼120 kg) was a browser but not restricted, where could also feed on herbaceous from understories in woodlands, and Neochoerus sp. (∼200 kg) feeding predominantly herbaceous plants in wooded savannas (piC3=∼69%; BA=0.75). We estimate that the interspecific competition could have been avoid by different feeding strategies, although more investigations are still needed to better understand their ecological interactions in the habitats of the southwestern Amazon during the late Quaternary.
