Jack is an eight-and-a-half-year-old boy living in Frog Creek, Pennsylvania. He loves books, science, and learning through direct observation. As an older brother, he often feels responsible for keeping his sister out of trouble and relies heavily on logic and caution when faced with entirely new or impossible situations.

Annie is Jack's seven-year-old sister and possesses a highly adventurous spirit. She is quick to embrace the imaginary and the unknown, plunging into new environments with excitement rather than fear. Her empathy extends to the unusual creatures they meet, allowing her to easily connect with her surroundings.

This flying reptile from the Cretaceous Period is the first prehistoric creature the siblings encounter. It has surprisingly fuzzy skin and a calm, welcoming demeanor. Annie enthusiastically names it Henry because it reminds her of a friendly neighborhood dog back home.

A massive carnivorous dinosaur roaming the Cretaceous valley. It represents the physical danger of the prehistoric world, acting as a major obstacle that forces the siblings to use their wits and the environment to evade it.

This duck-billed dinosaur lives in a colony and acts as a fierce protector of its young. It reacts aggressively when perceived threats approach its nests but quickly calms down when offered food or non-threatening behavior.

A large, horned dinosaur that the siblings observe exploring the landscape. Despite its intimidating appearance, it is a slow-moving herbivore that pays very little attention to the human visitors.