Simplify the following text for a Grade 7 native English-speaking person.
Original: Dinosaurs are a diverse group of animals that were the dominant terrestrial vertebrates for over 160 million years , from the late Triassic period ( about 230 million years ago ) until the end of the Cretaceous ( about 65 million years ago ) .
Simplified: Dinosaurs appear in the fossil record in the Upper Triassic , 230 million years ago .