Also I recall in the foreward of one of his big Calvin and Hobbes collection volumes, he wrote a bit about how his knowledge and portrayal of dinosaurs shifted over time as he wrote the comic. Watterson's early dinosaurs were very stereotypical reptilian monsters inspired by the dinosaurs he loved in his childhood:
As he continued work on the comic he researched further into modern dinosaur science of the time, which just so happened to be the height of the Dinosaur Renaissance in the 1980s and 90s. Dinosaurs were being reenvisioned as active, warm blooded, successful animals, and Watterson made an amazing effort to include accurate anatomy and scientific understanding where he absolutely could have just stuck to what he was familiar with. Instead, he created incredibly vivid and real-feeling depictions that absolutely belong alongside the works of other revolutionary palaeoartists of the 80s and 90s like Mark Hallett, Greg Paul and John Gurche.