Not only grassland animals grew in size to mega proportions, but there's also evidence to suggest that some jungle animals did as well. The grasslands can perhaps be explained by increased aridity, but that alone can't explain twice size pleistocene tree-top monkeys, can it? http://188.8.131.52/webdocs/anatomy/Brazil.htm The skull of Caipora bambuiorum, one of the two complete primate skeletons recovered from Toca da Boa Vista. It closely resembles the living spider monkey, but is more than twice the size, suggesting that South American monkeys participated fully in the mega-faunal phenomenon of the last Ice Age. Frontal view of the crania of Protopithecus (left) and Caipora (right), both from Toca da Boa Vista. They resemble living South American monkeys that inhabit the top levels of the tropical forest canopy, but they were significantly larger than any living species. Further exploration of Toca da Boa Vista hopefully will yield more primate species that also were quite large compared to modern monkeys. The trees must have grown much larger, surely? More sunshine from reduced cloud cover can't explain all of the growth, could it?