"The Busasaurus" is the third episode of season two and the sixteenth episode overall of the animated children's series The Magic School Bus. It originally aired on September 23rd, 1995.
The class is visiting Ms. Frizzle's old friend, Dr. Carmina Skeledon at a dinosaur dig site. She lets a fascinated Arnold see a fossilized dinosaur egg that Ms. Frizzle finds as they are leaving, and he still has it when they suddenly go back in time.The class watches as they enter and exit the Ice Age and finally arrive in the late Cretaceous Period. Having gone back "67 million years, give or take a month or two," Arnold is startled to find that Dr. Skeledon's rock has "unfossilized back into an egg." The class is also frightened by the sudden appearance of large, long necked dinosaurs called Alamosaurus.
They separate and scatter, Arnold running after Phoebe and Liz. Arnold trips on a rock, and the egg goes flying, Liz barely managing to catch it in time. Arnold lets Phoebe see it, but an Ornithomimus suddenly appears out of nowhere and steals it from her. Arnold runs after it yelling, "Come back with my egg, you thief! It 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 regroup, Liz explaining in sign language to Ms. Frizzle that Arnold and Phoebe ran off after the Ornithomimus. Carlos, being paranoid about getting eaten by dinosaurs, misinterprets Liz's message and exclaims, "See!? I knew this would happen! The dinos did them in!" The class hides out on a riverbank, looking out for Arnold and Phoebe, and Ralphie spots them. They cross a river filled with a herd of Parasaurolophus and land in an Edmontosaurus nest. Carlos is grossed out, while the rest of the class is fascinated when the Edmontosaur mother vomits up pre-chewed berries for the babies.
Meanwhile, Arnold and Phoebe continue to chase the Ornithomimus, but they stop when it does. Both wonder if it eats plants or meat, but most likely, it only eats eggs. Phoebe tries to get Arnold to forget about it, "it's just an egg," but Arnold points out it belongs to Dr. Skeledon. He steps away from her and says, "And it's up to me to get it back to her!" He runs off, leaving a stunned Phoebe, who then smiles and says, "Gee, what a guy!" before she follows.
The class continues on their way, only to stumble upon a Triceratops herd in a field. Keesha handfeeds a baby Triceratops. A pack of carnivorous Troodon ambush the baby Triceratops, but the smaller Troodon are scared away by the larger Triceratops. When the Troodon turn on the class, Ms. Frizzle hands them "Tricera-shields" to scare them off.
Arnold and Phoebe are still chasing the Ornithomimus. It stops again and sniffs the air, dropping the egg before running away as it realizes there is a sleeping Tyrannosaurus Rex is nearby. Arnold and Phoebe are unaware, and as they look, Arnold finds the egg near the T-Rex, mistaking it for a large rock.Phoebe helps him pull it free saying, "Something tells me we shouldn't be here". As Arnold asks, "Like what?", they accidentally wake the T-Rex and Phoebe exclaims, "Like that!" They run for their lives as the T-Rex chases them, and the approaching T-Rex scares the Triceratops herd away. Arnold and Phoebe find themselves in the same field as the class. Arnold trips into a mud puddle, losing the egg. Phoebe cries out for him before she runs on the bus, and he runs to hide under the bus.
As the T-Rex viciously attacks the bus, Ms. Frizzle suggests the class study its digestive system, but the terrified class begs her to "DO SOMETHING!" and she says, "Okay, class! Think big!". Growing Arnold into a giant 20 foot tall human, she helps him intimidate the T-Rex away, making the class learn that the predator only wanted "a quick meal without getting hurt."
When Arnold recovers the egg from the mud puddle, Ms. Frizzle shrinks him back to normal size. Phoebe runs up to him and tells him, "It's a good thing you didn't stay home today, Arnold," to which he awkwardly replies, "Well, uh, I'll sure be glad when this is a rock again. It's a lot safer."
The class returns to the present. Once they're all back at the dig site, they find a fossilized footprint of the giant Arnold's sneaker. Carlos, seeing some mud on Arnold's sneakers, then says, "I'd say it was a sneakersaurus!"
- This episode is loosely based on the 1994 book The Magic School Bus in the Time of the Dinosaurs and there are a few differences. The episode focuses only on the Cretaceous Period (due to time constraints in order for the episode to be up to 30 minutes like other episodes) while the book shows all three Mesozoic periods. The class visits Ms. Frizzle's old friend in both, but in the episode, they visit Dr. Skeledon who is a female, and in the book, it was Jeff who is a male. The book has no central character focus while the episode's focus characters are Arnold, Phoebe, and Carlos.
- This episode shares some similarities with the book and movie versions of Jurassic Park such as the kids hand-feeding some of the plant-eating dinosaurs, and the T-Rex attacking the bus in a similar manner to the one in Jurassic Park.
- Goof: The Tyrannosaurus Rex's head changes colors from green to brown when it moves.
- This is the first time Arnold is seen without his glasses.
- It is heavily implied Phoebe develops a crush on Arnold in this episode.
- This is the first time Arnold and Phoebe chase a thieving animal. The second is "Goes Cellular."