Complex sentence: The Mastodon and Saber-Toothed Tiger are mammals that are post-Cretaceous, while Pterodactyls are pterosaurs, a separate group of archosaurs from dinosaurs.
Split into simpler sentences: The Mastodon and Saber-Toothed Tiger are mammals which are post-Cretaceous. Plus, pterodactyls are pterosaurs, a separate group of archosaurs, distinct from dinosaurs.