The Middle Miocene vertebrate fossil sites from the Somosaguas Campus of the Complutensian University of Madrid (Pozuelo de Alarcon) have provided many fossils from 23 mammal species, dated in local zone E (MN5, Middle Aragonian). In this work we show new research results which allow recognizing a new species of rhinoceros for the Madrid Basin (Prosantorhinus douvillei) and defining a new species of hamster (Cricetodon soriae nov. sp.). The succession shows several episodes of debris-flow deposits transporting bones, pebbles and arkosic sands, discordant below and above lacustrine sediments. The deposits content an alteration sequence of micas, with neomorphic clays progressively more evolved and caliche lining on top, wich indicate increasing aridity. The richness of the Somosaguas fossil association allows us to relate this site with other assemblages of similar age, which gives a biogeographical pattern close to the palaearctic-palaeotropical transition associations. This is coincident with the palaeoclimatic data provided by the mammalian association, which indicates the presence of tropical conditions with strong hydric seasonality. A mosaic of environments integrated within a savanna biome is inferred in the area, combining open areas and small woodlands associated to a lacustrine environment. The isotopic analyses indicate a progressive cooling and drying trend, which correlates with the inferred global climatic changes in this period around 14 million years ago.