most of what Paleontologists focus on is the Mesozoic Era, which had the Triassic, Jurassic, and Cretaceous periods. Chances are the more complex a Dinosaur or organism seems, the more likely it is that they developed in a later period. We had a bunch of Dinosaurs that nobody understood for awhile, with the famous Brachiosaurus actually being some guy accidentally putting a Camarasaurus skull on a Apatosaurus body. From the 1% of creatures that fossilize, 1% of fossils have been discovered, that would mean if 1000 people died normally we would only have 1 skeleton. This shows that we do not know so much about the time before, with only 700 discovered and named, compared to our current 8.7 MILLION species.