Complex sentence: The Gomphotheres are a diverse group of extinct elephant-like animals (proboscideans) that were widespread in North America during the Miocene and Pliocene epochs, 12-1.6 million years ago.
Split into simpler sentences: The Gomphotheres are a diverse group of extinct elephant-like animals, (proboscideans). They appeared in the Miocene and Pliocene epochs, 12-1.6 million years ago.