Dinosaurs became the masters of the Earth during the Jurassic Period, and their reign continued through the Cretaceous Period. No great extinction occurred as the Cretaceous Period began. Instead, dinosaurs became more diverse and specialized as the period moved forward. Mammals began rising up from the tiny mouse-like creatures and birds started to take wing. Oceans were filled with fish and reptile predators. The skies saw larger and stronger pterosaurs. Albertosaurus was a large predator related to Tyrannosaurus, but arriving several million years earlier. It looked much like T. rex, but it was lighter, indicating it could move swiftly when chasing prey. Bambiraptor was birdlike in appearance and likely bore feathers. The shape and size of its hind legs suggest it ran quickly, and a large brain compared to its body size might indicate high intelligence. Based on reptile growth rates and how they believed dinosaur bodies functioned, paleontologists once thought individual lifespans might be as much, as 200 years.