Speaking of Dinosaurs — Philip E. High

Beyond Evolution.

A certain gifted engineer accepted the Theory of Evolution until he wandered by chance into a museum. In that museum was the skeleton of a dinosaur, and the skeleton got him thinking and enquiring, with terrifying results.

Attempts are made on his life and then, in a horrifying time-shift, he finds himself naked and unarmed in the distant past, facing one of the very creatures which had aroused his interest - a dinosaur!