page_title: Paleontology in Colorado
section_title: Prehistory
hierarchical_section_title: Paleontology in Colorado / Prehistory
caption_attribution_description: English: Allosaurus fragilis, an allosaurid theropod from the Late Jurassic of North America, pencil drawing
context_page_description: Paleontology in Colorado refers to paleontological research occurring within or conducted by people from the U.S. state of Colorado.
  The geologic column of Colorado spans about one third of Earth's history. Fossils can be found almost everywhere in the state but are not evenly distributed among all the ages of the state's rocks. During the early Paleozoic, Colorado was covered by a warm shallow sea that would come to be home to creatures like brachiopods, conodonts, ostracoderms, sharks and trilobites. This sea withdrew from the state between the Silurian and early Devonian leaving a gap in the local rock record. It returned during the Carboniferous. Areas of the state not submerged were richly vegetated and inhabited by amphibians that left behind footprints that would later fossilize. During the Permian, the sea withdrew and alluvial fans and sand dunes spread across the state. Many trace fossils are known from these deposits.
  The sea returned during the Triassic, while exposed areas were a richly vegetated coastal plain that was home to dinosaurs. Colorado was again submerged by a sea during the Cretaceous period that was home to plesiosaurs up to 70 feet long.
context_section_description: No Precambrian fossils are known from Colorado, so the state's fossil record does not begin until the Paleozoic. At the start of the Paleozoic, Colorado was located near the equator. The state was submerged under a warm shallow sea. At least part of Colorado was covered by shallow water during the Middle Ordovician. At the time, Colorado was home to invertebrates like articulated brachiopods, conodonts, gastropods, ostracods, pelecypods, sponges, trilobites, and worms (known from trace fossils). Contemporary vertebrates included armored jawless fish called ostracoderms. Sometime between the Silurian and early Devonian the sea withdrew from the state. While the sea was gone local sediments were eroded away rather than deposited. During the Carboniferous the absent sea returned, although some areas of the state remained dry land. Brachiopods, crinoids, sharks, and trilobites inhabited the sea. A rich variety of plants grew in Colorado's terrestrial environments. Examples include Calamites, conifers, and lycopods. Mountain ranges were being raised in the western part of the state by geologic forces. During the Carboniferous, footprints were laid down in Colorado by early tetrapods that would later fossilize. That being said, Colorado is not generally a good source of Carboniferous aged fossils. Western Colorado had a series of alluvial fans during the Permian when the Cutler Group was being deposited. Preserved in these sediments are tracks referred to the ichnospecies Limnopus cutlerensis, which may have been left by a temnospodyl amphibian. During the Permian the sea withdrew once more from the state. In its place were fields of sand dunes. Tracks left by ancient insects and reptiles were preserved in these dune deposits. Also like the Carboniferous, despite the presence of contemporary trace fossils, the fossil record of Permian life in Colorado is relatively poor compared to states like Kansas and Texas.
  Seawater returned to Colorado during the ensuing Triassic period, although it left significant areas of the state uncovered. These terrestrial areas included coastal floodplains vegetated by conifers and inhabited by creatures like amphibians and dinosaurs.  The Late Triassic also saw the formation of many footprints that would later fossilize. These are preserved in the sediments of the Chinle Formation of the northwestern part of the state. A chicken to turkey sized theropod dinosaur left behind footprints of the ichnogenus Agialopous. These tracks contain large number of the ichnogenus Rhynchosauroides, which resembles lizard footprints. The Chinle of Colorado also bears the greatest known abundance of the ichnogenus Gwynnedichnium. Both Triassic amphibians and reptiles left behind footprints near what is now the Fall Creek Post Office.
  During the Late Jurassic deposition of the sediments now known as the Morrison Formation, both sauropods and theropods left behind footprints. Only two large tracksites of fossil footprints are known from the Morrison Formation and both of them are located in Colorado. A tracksite called Rancho del Rio preserves both sauropod and theropod tracks. The Rancho del Rio site is located along the Colorado River in central Colorado. The other large tracksite is the Purgatoire Valley tracksite of the eponymous Purgatoire River in southeastern Colorado. The Purgatoire Valley tracksite is 400 meters at its widest and contains four track-bearing strata. One of the four track bearing strata bears more than 1,300 individual prints. A series of five parallel trackways left by young sauropods provides important evidence for dinosaur social behavior. The trackways of young sauropods found at the Purgatoire Valley site fill important gaps in the local body fossil record, as the vast majority of sauropods skeletal remains in the Morrison come from grown individuals.
  Much of Colorado was covered by an expanding sea during the ensuing Cretaceous period. This sea is known as the Western Interior Seaway. Algae, fish, molluscs, and marine reptiles inhabited its waters. The mosasaur Platecarpus was one such 
caption: Allosaurus.
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