Paleontology and geology
The Precambrian: Little is known about the Precambrian in Utah as most of these rocks are igneous and metamorphic and are buried beneath younger rocks. Some lightly metamorphosed Precambrian marine rocks exposed in the Uinta Mountains in the northeastern part of the state contain fossils of simple, single-celled organisms, indicating the presence of a marine environment.
The Paleozoic: The Paleozoic is well represented in Utah. Warm, shallow seas covered much of the state through most of this time interval. These seas were home to a diversity of organisms including clams, snails, trilobites, corals, brachiopods, algae, and sponges. Late Paleozoic (Carboniferous) sedimentary rocks are the most fossiliferous in the state. Fossils found in these rocks include nearly all groups of marine invertebrates, as well as rare occurrences of fossil fish.
The Mesozoic: Shallow seas retreated from the eastern part of Utah and broad, low floodplains developed during the Triassic. Some of the earliest dinosaurs roamed these floodplains, leaving behind their fossil footprints. Meanwhile, a shallow sea teeming with marine life still covered the western part of the state. Marine and non-marine conditions alternated throughout the Jurassic and Cretaceous. Great fields of sand dunes blanketed Utah, but were then covered with a shallow sea that once again flooded the region. Brachiopods, clams, snails, ammonoids, fish, and great marine reptiles lived in the seas, and a diverse and abundant dinosaur fauna roamed the land. During the Cretaceous, the seas retreated from Utah for the last time; Late Cretaceous deposits include fossils from numerous terrestrial vertebrates.
The Cenozoic: The Early Cenozoic (Tertiary) was a time of major uplift in Utah. Extensive fresh water lakes periodically covered the low areas of central Utah, leaving behind a diverse fossil record of the animals that lived in and around the lake system. These vast lakes were mostly gone by the end of the Tertiary. Uplift of the Colorado Plateau and the formation of the Basin and Range topography also began in the Late Tertiary. Utah developed its present topography during the Late Cenozoic (Quaternary). Regional uplift resulted in renewed erosion by rivers, forming the spectacular river canyons of the state. Lake Bonneville formed during a wetter interval in the Quaternary, only to shrink away through evaporation as the climate became more arid. The Great Salt Lake is a small remnant of ancient Lake Bonneville. Quaternary fossil vertebrates include saber-toothed cats, short-faced bears, giant ground sloths, bison, and musk ox, among many others.