During the cretaceous period, reptiles were the dominant species, and dinosaurs ruled earth. plants developed flowers and fruits. at the end of the cretaceous period, about 65 million years ago, 50% of the plants and animals on earth became extinct and dinosaurs were never to be seen again. what is thought to have caused the mass extinction at the end of the cretaceous period? a. global warming b. an asteroid hitting earth in mexico c. extensive volcanic activity world-wide d. the start of glaciation and climatic changes