The kids watch as they entered the Ice Age and finally end in the Cretaceous. Having gone back "67 million years, give or take a month or two", as Ms. Frizzle tells them, the kids are frightened away by the appearance of large, long necked dinosaurs called Alamosaurus (well, actually a titanosaur). They separated and scattered, Arnold running to Phoebe and Liz. He trips on a rock, and the egg went flying, Liz catching it in time. Arnold lets Phoebe see the egg, but an Ornithomimus steals it from her. Arnold runs after the dinosaur, yelling "That egg doesn't belong to you! Come to think of it...it doesn't belong to me either!". Phoebe runs after him, Liz quickly running to tell Ms. Frizzle. The rest of the class and Ms. Frizzle re-group, Liz explaining that Arnold and Phoebe ran off. Carlos, being paranoid about getting eaten by dinosaurs exclaims, "I knew this would happen! The dinos did them in!" The class hides out on a river bank, looking out for Arnold and Phoebe, and Ralphie spots them. They cross a herd of Parasaurolophus (where the Bus takes form of a Parasaurolophus) and land in an Edmontosaurus nest. The class continues on their way, only to find themselves with a Triceratops herd in a field. The Bus then takes form of a Triceratops. A pack of carnivorous Troodon chase a baby Triceratops, and the smaller Troodon are scared off by the larger Triceratops. While most of the dinosaurs encountered on the field trip were herbivores (plant eating dinosaurs) including Alamosaurus, Parasaurolophus, and Edmontosaurus, Troodon was a carnivore. At that, Dorothy Ann sees them and takes note that the Troodon eat meat indeed. Meanwhile, Arnold and Phoebe chase the Ornithomimus. It sniffs the air, and it drops the egg when it realizes a sleeping Tyrannosaurus rex is nearby. Like the Troodon, the Tyrannosaurus also eats meat. Arnold and Phoebe are unaware, and as they look, Arnold finds the egg near a large "rock". Phoebe helps him pull it free saying, "Something tells me we shouldn't be here". As Arnold asks, "Like what?", they wake the tyrant king and Phoebe exclaims, "Like that!" The two run as the Tyrannosaurus chases them, and they find themselves in the same field as the class. Arnold trips into a mud puddle, losing the egg, and he hides under the bus as Phoebe runs back on. Ms.