This has been addressed within the strip. A: Good news! I just came back from killing Krauss! B: Krauss? You were supposed to kill Hitler! A: How many of these people do I have to kill?!
I don’t think Stegosauruses and Tyrannosauruses were alive within ten million years from one another, but this is one of Calvin’s hallucinations, so I’ll give it a free pass.
Coming up on the cliffhanger ending…