PALEOZOIC ERA 510 Cambrian Period—First fossils deposited. Marine life. 430> Ordovician Period—Invertebrates increase greatly. 350> Silurian Period—Coral reefs formed. First evidence of land life. 310> Devonian Period—First forests. First amphibians. 250> Mississippian, Pennsylvanian, and Permian Periods.—Reptiles and insects appear. Continental uplift at end of this period. 180 MESOZOIC ERA Triassic Period—Small Dinosaurs. First mammals. 150> Jurassic Period—Dinosaurs and marine reptiles dominant. 125> Cretaceous Period—Dinosaurs reach zenith of development then disappear. Small mammals. Flowering plants and development of hardwood forests. 60 CENOZOIC ERA & PERIOD Paleocene Epoch—Archaic mammals. 50> Eocene Epoch—Modern mammals appear. 35> Oligocene Epoch—Great apes appear. 25> Miocene Epoch—Grazing types of mammals appear. 10> Pliocene Epoch—Man appears. 0 The next period of the earth’s age—the Ordovician period, which extended from 430,000,000 to 350,000,000 years ago—has left its mark just as visibly upon the Black Hills. It was during this period that the many species of invertebrate marine life reached a zenith of development, and that a bed of sediment was laid down and later compressed to a pinkish limestone. The fact that this Ordovician bed is less than forty feet thick indicates that the land mass from which the muds and sands were drawn was very low, and that the Cambrian sea was relatively shallow, entertaining only minor erosive currents along its shores.