Arikareean

Arikareean
Name	Arikareean
Start	30.8
End	20.6
Unit	North American Land Mammal Age
Preceded by	Whitneyan
Followed by	Hemingfordian

Arikareean The Arikareean is a North American Land Mammal Age used in stratigraphy to characterize faunal assemblages of the late Oligocene to early Miocene. It is recognized in correlation with international stages and tied to stratigraphic units, formations, and fossil localities across North America, used by paleontologists, stratigraphers, and geologists for faunal correlation and paleoenvironmental reconstruction.

Definition and temporal range

The Arikareean is defined as a North American Land Mammal Age assigned to strata deposited roughly between the late Oligocene and early Miocene, with numeric ages often cited near the Oligocene–Miocene boundary and correlated to the Chattian, Aquitanian, and early Burdigalian intervals. Its temporal placement is constrained by magnetostratigraphy from sections tied to the Geological Time Scale, radiometric dates from volcanic ash beds such as those in the John Day Formation and the Arikaree Formation, and biostratigraphic ties to faunal turnovers known from sites like Agate Fossil Beds National Monument and the Harrison Formation. Key numeric age limits are commonly aligned with global chronologies used by the International Commission on Stratigraphy and regional frameworks developed by the Paleontological Society.

Geologic and geographic distribution

Arikareean-age deposits are widespread across the North American interior and western margin, recorded in formations including the Arikaree Formation, John Day Formation, Harrison Formation, Chadron Formation, Brule Formation, Gering Formation, Sharps Formation, Ogallala Formation, Seminole Formation, Cedar Mountain Formation, and Sage Creek Formation. Significant localities occur in states and provinces such as Nebraska, South Dakota, Wyoming, Montana, Colorado, Oregon, Idaho, Nevada, Texas, Kansas, North Dakota, and New Mexico. Correlative strata extend into the Great Plains, the Rocky Mountains foreland, the Great Basin, and patchily into southern Canada. Stratigraphic sections with Arikareean faunas are often intersected during mapping by agencies like the United States Geological Survey and studied in academic programs at institutions such as the University of Nebraska State Museum and the University of California Museum of Paleontology.

Fauna and flora associations

Arikareean faunas include diverse mammals and associated plants documented at paleontological sites such as Agate Fossil Beds National Monument, Ashfall Fossil Beds State Historical Park, and the John Day Fossil Beds National Monument. Mammalian groups represented include early Perissodactyla such as Equidae and brontothere relatives, Artiodactyla like early Camelidae and Ruminantia precursors, carnivorans including primitive Canidae and Mustelidae relatives, endemic Rodentia such as sciurids and geomyoids, and insectivores including Erinaceomorpha and early Lipotyphla-grade taxa. Notable genera found in Arikareean assemblages comprise Mesohippus-grade equids, Procamelus, Merychippus precursors, Daphoenus-like amphicyonids, Hesperocyon-grade canids, Palaeolagus lagomorphs, and Thomomys-grade rodents. Avian remains include taxa comparable to Gruidae and Anatidae representatives, while reptilian fossils include turtles and crocodilian traces comparable to Alligatoridae. Plant macrofossils and pollen from Arikareean deposits show associations with genera and families recognized by botanists at institutions like the Smithsonian Institution and the New York Botanical Garden, including elements related to Salix, Populus, Quercus, Pinus, Juniperus, Acer, and grasses tied to the expansion of the family Poaceae.

Paleoenvironment and climate

Paleoenvironmental reconstructions for Arikareean localities indicate a range from woodland and gallery forest to open grassland and savanna-like habitats in the Great Plains and more arid scrublands in the Great Basin and intermontane basins. Stable isotope studies tied to laboratories at the University of California, Berkeley and the University of Arizona suggest shifts in C3 to C4 vegetation and seasonality trends similar to climatic signals inferred from marine records such as the Mediterranean Miocene and the Eocene–Oligocene transition. Paleosol analyses, sedimentology by researchers affiliated with the Society of Vertebrate Paleontology, and associated freshwater mollusk and ostracod assemblages indicate variable precipitation regimes and episodic fluvial deposition in basins comparable to the White River and Fort Union paleoenvironments. These reconstructions are often integrated with global climate models used by the Intergovernmental Panel on Climate Change to contextualize regional responses to late Oligocene–early Miocene climatic changes.

Biostratigraphy and correlation

Biostratigraphic frameworks for the Arikareean rely on mammal first and last appearances, faunal turnover events, and correlation to marine chronologies. Index taxa used in correlation include particular equid lineages, camelids, and carnivorans recognized by paleontologists at museums such as the American Museum of Natural History and the Field Museum of Natural History. Correlations tie Arikareean assemblages to the European land mammal ages and Asian faunas through shared genera documented in faunal lists curated by entities like the Natural History Museum, London and the Paleobiology Database. Magnetostratigraphic ties to polarity chrons as published by the Geological Society of America and radiometric dates from tuffs processed at facilities such as the Argonne National Laboratory help align Arikareean zones with the Chattian and Aquitanian stages of the international timescale.

History of research and taxonomy

The concept of the Arikareean emerged from early 20th-century stratigraphic and paleontological work in the Great Plains and western United States, with foundational contributions by researchers associated with the United States Geological Survey, the University of Nebraska, and museums such as the American Museum of Natural History. Key stratigraphic names and revisions trace through publications by paleontologists and geologists from institutions including the Smithsonian Institution, the University of Kansas Natural History Museum, and the Museum of Comparative Zoology at Harvard University. Taxonomic work on Arikareean mammals has been refined by systematists publishing in journals like the Journal of Vertebrate Paleontology and the Palaeontology Journal, incorporating techniques from anatomical comparative studies at universities such as Yale University, Columbia University, Stanford University, University of Michigan, and University of Chicago. Ongoing research programs by teams at the University of California, University of Texas, University of Wyoming, and international collaborators continue to revise species-level taxonomy, stratigraphic boundaries, and paleoecological interpretations.

Category:North American Land Mammal Ages