There is a book I read as a kid that I'm trying to track down. It was a children's sci-fi book where everyone other than the main character turns into a dinosaur. Or rather, the main character woke up in a mirror universe where everyone is a dinosaur, and they need to get back to the human universe. In this universe he has to hide, because humans are extinct and everyone freaks out. I seem to recall this was caused by some science experiment, and the main character needed to help some mad scientist complete the experiment to send him back.
I've tried to find this by doing google searches, but I mean, there's tons of children's books about dinosaurs. Plus this particular book is pretty old, and the results are mostly flooded with stuff that's fairly recent.