145 MILLION YEARS ago, the Jurassic Period transitioned into the final stage of dinosaur evolution, the Cretaceous Period. Sometimes the switch between geological periods happens with a flourish, as when the megavolcanoes closed out the Triassic. Other times, it’s barely noticeable, and more a matter of scientific bookkeeping, a way for geologists to break up long stretches of time without any major changes or catastrophes. The changeover between the Jurassic and Cretaceous is that type of boundary.
