- "The dominant species type were huge, brutish beasts a fantastic array of forms. Giant, slow-moving plant eaters and rapacious killers with tearing teeth and deadly talons. There was intelligence there, but no sentience, I could see so clearly."
- ―The Ellimist
Dinosaurs were a diverse group of reptiles that were once the dominant terrestrial predator of planet Earth. In In the Time of Dinosaurs, the Animorphs travel back in time, where they are hunted by dinosaurs and end up playing a pivotal role in their extinction.
- "I held tight with my talons. And I focused on the dinosaur. It may have been sixty-five or seventy or eighty million years B.C., but DNA was still DNA."
While being chased by a pack of Deinonychus, Tobias lands on one and acquires it in order to save Rachel, who is in grizzly bear morph, from being killed by the pack. After killing the pack's leader, Tobias is able to subdue one of the other Deinonychus so that Rachel can acquire a Deinonychus morph as well.
- "So I guess the Nesk don't mind dinosaurs. I mean, okay, if a Mercora flying saucer shows up at the Nesk home base, they blast it. But what if a whole different kind of army shows up there?"
"It's a miracle! Marco actually came up with a good idea. We can morph dinosaurs and stomp on in there, set off some big honkin' explosion and maybe undo this Sario Rip of Ax's!"
- ―Marco and Rachel
Tobias is the first of the Animorphs to acquire and use a dinosaur morph, using his Deinonychus morph to try and protect Rachel. He uses it to subdue another Deinonychus for Rachel to acquire, and the pair later use the morph when they meet a Spinosaurus, only to be confronted by the Nesk. Later that night, Jake is nearly eaten by a Tyrannosaurus Rex, which Jake escapes by acquiring and partially morphing into the dinosaur in order to break free. After meeting the Mercora and deciding to steal a nuclear explosive from a Nesk base, the Animorphs decide to use their dinosaur morphs as the Nesk do not see dinosaurs as a threat. During this mission, Cassie loses herself to the Tyrannosaurus's instincts, killing and eating a Triceratops.
Upon returning to their own time, Jake attempted to morph into the Tyrannosaurus Rex, although to no avail. He reported the incident to Cassie, who suggested that Ax might know the reason, although Jake refuses to bother, citing that he would not understand the reasoning anyways.
- The Invasion (mentioned)
- The Encounter (mentioned)
- The Message (mentioned)
- The Stranger (mentioned)
- The Android (mentioned)
- The Reaction (mentioned)
- The Unknown (mentioned)
- The Escape (mentioned)
- In the Time of Dinosaurs (Deinonychus, acquired and morphed by Rachel and Tobias; Tyrannosaurus Rex, acquired and morphed by Jake, Cassie, Marco and Ax)
- The Departure (mentioned; Cassie's nightmare)
- The Attack (mentioned)
- Elfangor's Secret (mentioned)
- The Weakness (mentioned)
- Back to Before (mentioned)
- The Ellimist Chronicles
- The Return (mentioned)
- The Sacrifice (mentioned)