Complex sentence: Grass started to expand in the Miocene, and the Miocene is where many modern day mammals like giant ungulates like "Paraceratherium" and "Deinotherium" evolved to rule the grasslands.
Split into simpler sentences: Grass started to expand in the Miocene, and the Miocene is where many modern day mammals first appeared. Giant ungulates like "Paraceratherium" and "Deinotherium" evolved to rule the grasslands.