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.

Paleontology in Nevada refers to paleontological research occurring within or conducted by people from the U.S. state of Nevada. Nevada has a rich fossil record of plants and animal life spanning the past 650 million years of time. The earliest fossils from the state are from Esmeralda County, and are Late Proterozoic in age and represent stromatolite reefs of cyanobacteria, amongst these reefs were some of the oldest known shells in the fossil record, the Cloudina-fauna. Much of the Proterozoic and Paleozoic fossil story of Nevada is that of a warm, shallow, tropical sea, with a few exceptions towards the Late Paleozoic. As such many fossils across the state are those of marine animals, such as trilobites, brachiopods, bryozoans, honeycomb corals, archaeocyaths, and horn corals.