Complex sentence: Previously, no grassland ecosystem had been identified prior to Miocene systems fifteen million years later than the Tinguiririca fauna, which spread as the earth's paleoclimate grew cooler and drier.
Split into simpler sentences: Previously, no grassland ecosystem had been identified prior to Miocene systems fifteen million years later than the Tinguiririca fauna. Grasslands spread as the earth's paleoclimate grew cooler and drier.