Since birds were already common in the Lower Cretaceous, they would have competed vigorously with the smaller pterodactyloids. That may explain the extinction of the smaller pterosaur species, though the lack of fossils from the forested areas makes it difficult to judge. The huge Upper Cretaceous types clearly lived a different life-style from the smaller species, and one which was not yet accessible to birds. As the climate changed in the Upper Cretaceous, becoming colder and more seasonal, the number of pterosaurs became fewer. Like most of the larger species on Earth, the huge pterosaurs did not survive the K/T extinction event. At least some families of birds did. So ended the competition between the two types of flying reptile through the long 79 million years of the Cretaceous period.