ERA Period Millions Distinctive fossils Events in Colorado of years ago CENOZOIC (Age of Mammals) Quaternary Modern types of Development of present animals and plants topography; glaciation in mountains 3 Tertiary Mammals, flowering Uplift and mountain plants building 70 MESOZOIC Dinosaurs and other (Age of Reptiles) reptiles Cretaceous Submergence, then uplift 135 Jurassic Desert, then submergence 180 Triassic Widespread floodplains and deserts 225 PALEOZOIC (Age of Fishes) Permian First reptiles Widespread floodplains and deserts 270 Pennsylvanian Swamp and forest “Ancestral Rocky plants Mountains” 310 Mississippian Reef corals, sharks Partial submergence 350 Devonian Armored fish, first Probable submergence insects 400 Silurian Corals and shellfish Probable submergence 440 Ordovician First fish Submergence 500 Cambrian First hard-shelled Gradual encroachment of animals sea from west 570 PRECAMBRIAN “Lipalian Interval” Erosion to almost flat surface or peneplain Primitive Alternate episodes of soft-bodied marine mountain building and organisms erosion 3,600 plus THE PRAIRIES Beneath the flat prairies of eastern Colorado, sedimentary rocks form a series of layers. Those near the surface are among the youngest rocks in Colorado. We know this from the fossils they bear, fossils of large mammals such as the hairy mammoth, which lived in early Quaternary time, the bison, and many smaller mammals living today.