page_title: Paleontology in Arizona
section_title: Prehistory
hierarchical_section_title: Paleontology in Arizona / Prehistory
caption_attribution_description: English: fossil of Rutiodon, an extinct archosaur
context_page_description: Paleontology in Arizona refers to paleontological research occurring within or conducted by people from the U.S. state of Arizona. The fossil record of Arizona dates to the Precambrian. During the Precambrian, Arizona was home to a shallow sea which was home to jellyfish and stromatolite-forming bacteria. This sea was still in place during the Cambrian period of the Paleozoic era and was home to brachiopods and trilobites, but it withdrew during the Ordovician and Silurian. The sea returned during the Devonian and was home to brachiopods, corals, and fishes. Sea levels began to rise and fall during the Carboniferous, leaving most of the state a richly vegetated coastal plain during the low spells. During the Permian, Arizona was richly vegetated but was submerged by seawater late in the period.
  During the Triassic, Arizona was home to a rich forest home to dinosaurs and early relatives of mammals. Jurassic Arizona had a drier climate and was covered by sand dunes where dinosaurs left behind footprints. During the Cretaceous, part of eastern portions of the state were covered by the Western Interior Seaway, home to marine reptiles, including plesiosaurs and turtles.
context_section_description: Arizona was covered by a shallow sea during the Precambrian. Stromatolites formed there. During the Proterozoic interval of Precambrian time, jellyfish lived in Arizona. Their fossils were preserved in what is now the Grand Canyon. Arizona was still covered by a shallow sea during the ensuing Cambrian period of the Paleozoic era. Brachiopods, trilobites and other contemporary marine life of Arizona left behind remains in the western region of the state. The sea withdrew from the state during the Ordovician and Silurian. Although some of the state's Ordovician sedimentary deposits have persisted to the present, all of the state's Silurian rocks have been eroded away. Accordingly, no contemporary fossils are known from the state. Deposition resumed during the Devonian when Arizona was once more submerged by the sea. Contemporary local marine life included brachiopods and corals. In the Devonian period, Arizona was home to primitive fishes. Local sea levels began to periodically rise and fall throughout the ensuing Carboniferous period. During periods of low water levels, much of the state was a coastal plain environment. Plants that grew in Arizona during the Mississippian left behind fossils. Near the end of the ensuing Permian period, Arizona was once more covered by sea water. Later plants from the Permian period did as well. A "great diversity" of plants composed the Permian flora of Arizona. They left fossils in the area now occupied by many of the trails leading down into the Grand Canyon.
  With the start of the Mesozoic era, geologic forces elevated the topography of the state's southwestern region. During the Late Triassic, this area was being drained by channels of water flowing toward Pangaea's western coast. At the time of the Dockum Group's deposition, about 220 million years ago, Arizona had a very similar environment to that of nearby Texas. Triassic plant fossils such as clubmosses and ferns were preserved in the Painted Desert and Petrified Forest. During the Triassic, Arizona was home to amphibians that left behind fossils near Meteor Crater. Triassic dicynodonts left behind remains near St. Johns. Arizona is also the best known source of Chindesaurus fossils. Chindesaurus is notable for providing an evolutionary link between the local fauna and the most primitive known dinosaurs such as Eoraptor and Herrerasaurus, which are found in South America. Prosauropods are present but rare in Arizona's Late Triassic deposits. The phytosaurid Rutiodon lived here as well. Fossils attributed to the possible dinosaur Tecovasaurus, known only from teeth, have also been found in the state.
  During the Jurassic, much of the state was covered in sand dunes. Local dinosaurs left behind their footprints. Among the larger fauna was the early sauropodomorph dinosaur Sarahsaurus.
  Into the Cretaceous, the uplift in western Arizona was forming mountains. Sea water once again intruded into the eastern part of the state. This sea was inhabited by marine reptiles such as turtles and plesiosaurs. On land, the brachiosaurid sauropod Sonorasaurus lived in what is now southern Arizona during the middle part of the Cretaceous.
  Most of Arizona was dry land during the Cenozoic era. There were a wide variety of different habitats where wildlife such as camels, horses, and mastodons lived. The Oligocene Mineta Formation of Arizona is one of only seven fossil footprint-bearing stratigraphic units of that age in the western United States. During the Miocene, Arizona was home to camels, which are preserved in what is now Yuma County. During the Pleistocene, the local animal life included camels, mastodons, rodents, and giant sloths. These left behind fossils in what is now Graham County. During the Quaternary, the local climate began to dry and many of the local bodies of water disappeared. At some point in the Pleistocene, mastodons were mired at the San Pedro Valley marsh site, not far from a salt lake. Their skeletons were actually preserved standing upright where they were trapped.
caption: Rutiodon
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