The Calvinosaurus ("Calvin's Lizard") is a late Jurassic theropod dinosaur discovered by Calvin. Its first appearance was in Calvin's imagination. In it, it was said to be so large that it can "eat an Ultrasaurus (who is now considered to be simply a very large individual of Brachiosaurus) in a single bite". Based on how large it is shown relative to an "Ultrasaurus", it appears to be about 264 feet long, 70 feet high at the hips, and 1,410 tons in weight, with teeth up to 27.5 inches long, finger claws 4.5 feet long, toe claws 7 feet long, and a mouth 20.5 feet long.
Much later on, during show-and-tell, Calvin displayed a fragment of "jawbone fossil" he claimed to have found in his front yard (it was actually gravel from his driveway). He realized it belonged to a new dinosaur species. He also produced a dramatic illustration of the complete Calvinosaurus, which was very large, not too much unlike the version that appeared in his imagination. For reasons unknown, his demonstration had him sent to Principal Spittle. 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 driveway gravel to the ordinary layman.
- When Calvin was unearthing garbage he mistook for dinosaur bones, he intended to call the complete skeleton a Calvinosaur.
- Ultrasaurs can "weigh over 70 tons" and are bigger than Apatosaurs, so the Calvinosaurus is monumentally large.
- The Calvinosaurus resembles a Giganotosaurus. Interestingly, Giganotosaurus was discovered a couple years after the strip's publishing.
- In reality, no theropod similar to Calvinosaurus would grow to such an enormous size due to the Square-cube law.
- There was another Calvinosaurus appearing in the strip on December 9, 1990 as a show and tell item. Calvin brought a fragment of a Calvinosaurus jawbone and a drawing.