Complex sentence: Dinosaurs are a diverse group of animals that included the dominant terrestrial vertebrates of the Mesozoic era and whose descendants, the birds continue to represent the most abundant group of vertebrates on land.
Split into simpler sentences: Dinosaurs are a diverse group of animals that included the dominant terrestrial vertebrates of the Mesozoic era. The sole surivivng group, the birds, continue to represent the most abundant group of vertebrates on land.