Permian rocks do not occur in Kentucky. At that time, Kentucky was probably covered by silts and sands, which have since been completely eroded away. During the Permian, Kentucky was probably hot and arid. Large mammal-like reptiles, such as the sail-finned Dimetrodon, and others, probably lived in Kentucky. During the Permian, most of the continents were pushed together into one large land mass called Pangea. At the end of the Permian, something happened to cause the extinction of 90 percent of the animal and plant species on earth. This mass extinction was far greater in magnitude than the similar extinction of the dinosaurs at the end of the Cretaceous Period.