Dinosaurs are amazing extinct creatures that lived during the Mesozoic Era (from 252 to 66 million years ago).  Their fossils have been found on every continent.  Over their long history, many different species evolved in all different sizes and shapes. 

Paleontology is the field of science that studies the history of life on Earth that is preserved in fossils in rocks.  Sometimes paleontologists can find hard parts like teeth and bones, and other times, the only evidence left behind may be footprints, or impressions of a shell, leaf, or body.  Some fossils that are hundreds of millions of years old look very much like organisms that are alive today, like horseshoe crabs, cockroaches, and ginkgo trees.  Others, like fossilized dinosaurs, allow us to learn about creatures that became extinct long before there were any humans on Earth.