Calvinosaurus is a theropod dinosaur that first appeared in Calvin's imagination. It is so large that it can "eat an Ultrasaur in a single bite." One time, during show-and-tell, Calvin displayed a fragment of jawbone fossil he found in his front yard. He realized it belonged to a new dinosaur species. He also produced a dramatic illustration of the complete Calvinosaurus, which was very large, not unlike the version that appeared in his imagination. Curiously, Calvin claimed that he would be publishing his findings shortly, yet there is no proof that he followed through with this.
- Fragments of Calvinosaurus jawbone resemble ordinary driveway gravel.