Mammals and rainfall: paleoecology of the middle Miocene at La Venta (Colombia, South America).

A comparison of the species richness and macroniche composition of diet, locomotor and body-size classes among 16 nonvolant mammalian faunas in tropical South America reveals numerous significant positive correlations with rainfall. In particular, significant and strong positive correlations with rainfall are found in 18 attributes, including the number of nonvolant mammal species, number of primate species, number of frugivores, primary consumers, arborealists, and the number of species between 100 g to 10 kg in body weight. Estimates of annual rainfall derived from least-squares and polynomial regressions and principal components analysis yield a modal estimate of between 1500 and 2000 mm annual rainfall for the Monkey Beds assemblage at La Venta. This level of rainfall is associated today with the transition between savanna and forest environments in lowland equatorial South America. Paleontological evidence strongly suggests the presence of forest biotopes at La Venta. Paleontologic and sedimentologic evidence together indicate a dynamic and heterogeneous riparian mosaic associated with the shifting course of meandering rivers. Faunal evidence also suggests that habitat heterogeneity and canopy discontinuity extended into the interfluvial area. Seasonal rainfall was probably only of secondary importance in shaping the structural and spatial configuration of the dominantly forested mosaic habitat at La Venta. The fossil record is not consistent with the presence of extensive primary or undisturbed, continuous-canopy, evergreen tropical rainforest. The reconstructed middle Miocene environment at La Venta differs significantly from modern environments of similar geography on the piedmont east of the Andes at the same latitude. This in turn suggests that the extensive evergreen rainforests of the upper Amazonian piedmont that today receive more than 4000 mm of rainfall may post-date the initiation of Andean uplift.