Hemingfordian

Hemingfordian
Name	Hemingfordian
Time start	20.43
Time end	16.3
Time unit	Ma
Precedence	Arikareean (early)
Successor	Barstovian
Region	North America

Hemingfordian The Hemingfordian is a North American Land Mammal Age (NALMA) spanning part of the early Miocene, used in biostratigraphy to correlate terrestrial faunas across continental basins. It is defined by first and last appearance events of key mammals and is integral to chronostratigraphic frameworks employed in paleontology and geology. The interval is critical for understanding faunal turnover, paleoenvironmental shifts, and tectonic influences during the early Neogene.

Definition and temporal range

The Hemingfordian is commonly placed between about 20.43 Ma and 16.3 Ma within the early Miocene, bounded by radiometric and magnetostratigraphic tiepoints established in association with formations and sites such as the John Day Formation, Chandler Bridge Formation, San Gregorio Formation, and sections correlated to the Magnetostratigraphy of the Neogene. Its temporal limits have been calibrated against stages of the International Commission on Stratigraphy timescale including the Aquitanian and Burdigalian, and are cross-referenced with marine stages like the South American Land Mammal Ages and European mammalian zonations through faunal interchange events documented at localities such as Thomas Farm Site and Taunton River Basin.

Stratigraphic context and correlation

Hemingfordian strata are identified within continental sedimentary sequences including fluvial, lacustrine, and aeolian units of the Ogallala Formation, Arikaree Formation, John Day Formation, and the Gila Group. Stratigraphic correlation hinges on biostratigraphic markers—especially artiodactyls and perissodactyls—cross-validated with radiometric dates from volcanic tuffs analyzed by laboratories associated with United States Geological Survey and universities such as University of California, Berkeley and University of Michigan. Correlation with Eurasian and African sequences involves comparison to faunas from the Siwalik Group, Mesopotamian Basin, and the Afar Depression, aided by magnetostratigraphic records tied to the Geomagnetic Polarity Time Scale.

Paleoenvironments and climate

Hemingfordian environments reflect a mosaic of open grassland, woodland gallery systems, and riparian corridors preserved in deposits of the Great Plains, Basin and Range Province, and coastal basins like the Coast Ranges. Paleoclimatic reconstructions use stable isotope studies from sites including the John Day Fossil Beds National Monument and Pacific margin sections near the Monterey Formation, indicating regional trends toward increased seasonality and expansion of C3 and C4 plant communities. Paleoecological models integrate data from researchers at institutions such as the Smithsonian Institution, American Museum of Natural History, and the Field Museum to interpret shifts driven by tectonics associated with the Sevier Orogeny aftermath and evolving drainage patterns tied to the Colorado River evolution.

Fauna and floral assemblages

Hemingfordian faunas are characterized by diversification of perissodactyls (horses like genera recorded at Agate Fossil Beds National Monument), artiodactyls (camels and oreodonts documented at Hershey Ranch Locality and Runningwater Formation), proboscideans precursors in regions comparable to the Arapahoe Formation, and the notable radiation of rodents and carnivorans seen at Thomas Farm and Barstow Formation-equivalent deposits. Avian and squamate records from the Chesapeake Group and Florence Lake show range expansions linked to habitat heterogeneity. Floral assemblages preserved as macrofossils and palynological spectra from the John Day Formation, Florissant Fossil Beds, and Paskapoo Formation document temperate woodland taxa including relatives of Quercus, Acer, and Taxodium, alongside emerging grass-dominated assemblages traceable to taxa comparable with Poaceae lineages recognized in molecular studies housed at Kew Gardens and Royal Botanic Gardens, Edinburgh.

Geological occurrences and notable sites

Key Hemingfordian localities include Thomas Farm (Florida), Agate Fossil Beds National Monument, the John Day Fossil Beds National Monument, and the Runningwater Formation exposures in the High Plains. Internationally comparable occurrences are referenced against the Siwalik Hills sequences and the Moghra Formation where faunal affinities permit intercontinental correlation. Several museums and research centers—the Smithsonian Institution, American Museum of Natural History, University of California Museum of Paleontology—curate type specimens and stratigraphic collections from these sites, which have been the focus of major field campaigns led by researchers affiliated with United States Geological Survey and universities such as Harvard University and Yale University.

History of study and naming

The Hemingfordian concept emerged from early 20th-century North American vertebrate paleontology and biostratigraphy, building on foundational work by paleontologists associated with institutions like the United States Geological Survey, American Museum of Natural History, and universities including University of California, Berkeley. Its name derives from regional stratigraphic studies in locales associated with the town of Hemingford, Nebraska, and was formalized through syntheses published in outlets linked to organizations such as the Paleontological Society and the Geological Society of America. Subsequent refinements incorporated radiometric dating from volcanic ash beds analyzed at facilities like Lawrence Berkeley National Laboratory and magnetostratigraphic integration following advances by researchers at Lamont–Doherty Earth Observatory and the National Center for Atmospheric Research.

Category:Miocene North America