The film opens with an Iguanodon mother forced to abandon her nest, with only one egg (D. B. Sweeney) surviving a carnivorous Carnotaur (Frank Welker) attack. The egg is taken by an Oviraptor, who drops it into a river while fighting another Oviraptor, and finally is taken by a Pteranodon to an island inhabited by lemurs, who see the egg hatch, name the baby dinosaur Aladar, and raise him as their own. When Aladar is an adult, the island is destroyed by a meteor, with only Aladar, his grandfather Yar (Ossie Davis), his mother Plio (Alfre Woodard), his best friend Zini (Max Casella), and his sister Suri (Hayden Panettiere) surviving the destruction of the island by swimming to the mainland.After fleeing a pack of Velociraptors (also Frank Welker), the family meets a multi-species herd of dinosaurs led by the Iguanodon Kron (Samuel E. Wright) and his lieutenant Bruton (Peter Siragusa), who are on a journey to reach the "Nesting Grounds", a valley believed to be untouched by the devastation of the meteor impact. Aladar and the lemurs befriend a trio of elderly dinosaurs: Baylene (Joan Plowright), a Brachiosaurus, Eema (Della Reese), a Styracosaurus, and Url (also Frank Welker), Eema's dog-like pet Ankylosaurus. Aladar also develops romantic feelings for Neera (Julianna Margulies), Kron's sister but she is uninterested. Meanwhile, the Velociraptors continue to follow them, but are scared off when a pair of Carnotaurus picks up the herd's trail too. The herd arrives at a lake theyve relied on for past trips, but it has dried up. Aladar saves the herd from dehydration when he and Baylene eventually dig up the trapped water beneath the ground. Aladar and Neera eventually fall in love after Neera sees Aladar helping the dinosaurs (especially the elderly ones) survive; however, Bruton, having been sent by Kron to find water upon arrival at the lake, returns injured by the Carnotaurus. Kron evacuates the herd from the lake bed in a rush, leaving Aladar, the lemurs, the elderly dinosaurs, and the injured Bruton behind.The small group recuperates in a cave during a rainstorm, with Bruton eventually befriending them. When the Carnotaurus pair attacks, Brutons loyalty is proven when he kills one of them as the others escape to the depths of the caves, being killed in the process. But one of the Carnotaurus escapes, enraged by the death of its partner but unable to follow the group. Upon reaching a dead end, Aladar begins to lose hope, after repeated failures and the loss of Bruton. His friends all join in breaking down the dead end, stabilizing his confidence. Finally, Baylene demolishes the wall. The dead end actually leads to the "Nesting Grounds", where Eema sees that the old entrance where the herd has gathered on the other side has been blocked by a landslide generated by the meteors.Aladar rushes to find the herd on the other side, accidentally alerting and being tracked by the Carnotaur. He finds the herd being directed by Kron to climb the rocks, which can't be passed without fatality, but when he suggests the alternate route he found, Kron battles him for leadership of the herd. Neera defends Aladar from her brother, and leaves with Aladar and the herd following. The Carnotaur then confronts them, but Aladar rallies the herd to drive it off by standing together and scaring it off with intimidation. The Carnotaur leaves the herd alone, but spots Kron stubbornly trying to climb the rocks alone, and is followed by Neera and Aladar as it charges to kill the former leader of the herd. In the ensuing fight, Kron is killed by the Carnotaur, but a brave Aladar is able to push the predator off a cliff to its death. Aladar and Neera lead the herd back to the "Nesting Grounds", where the two eventually mate and have children, and the lemurs find more of their own kind. Plio narrates the ending, wishing for their story to be remembered in history. She then says, "But one thing is for sure. Our journey is not over, we can only hope in some small way our time here will be remembered."
Output: Question: what causes an iguanodon to abandon her nest?, Answer: A Carnotaur attack.