Chadronian

Chadronian
Name	Chadronian
Time start	37.2
Time end	33.9
Unit	North American Land Mammal Age
Region	North America

Chadronian.

The Chadronian is a North American Land Mammal Age used to characterize late Eocene to earliest Oligocene terrestrial faunas in North America. It is commonly applied in mammalian biostratigraphy and regional correlation, appearing in stratigraphic sections, fossil lists, and paleoenvironmental reconstructions across sites linked to institutions and locales in the western United States and Canada.

Definition and temporal range

The interval is defined within Cenozoic chronostratigraphy and is typically placed between about 37.2 and 33.9 million years ago, overlapping with global stages such as the late Priabonian and the base of the Rupelian in the international geologic time scale as used by organizations like the International Commission on Stratigraphy and compared against boundaries established by the Geological Society of America and stratigraphers from the United States Geological Survey. Biostratigraphic limits are set by first and last occurrences of key taxa recorded in faunal lists from localities affiliated with museums such as the American Museum of Natural History, the Smithsonian Institution, the Denver Museum of Nature & Science, and the Yale Peabody Museum. Correlation with isotope records from cores studied by teams from Lamont–Doherty Earth Observatory, University of California, Berkeley, and University of Michigan helps to refine the numerical age.

Geologic context and stratigraphy

Chadronian deposits occur in formations exposed across basins like the Williston Basin, the Bighorn Basin, the Powder River Basin, and along outcrops of the White River Formation, the Brule Formation, and the Chadron Formation. Lithologies include fluvial sandstones, overbank mudstones, paleosols, and lacustrine shales documented by fieldwork from groups at University of Nebraska–Lincoln, University of Wyoming, University of Kansas, and the Field Museum of Natural History. Taphonomic and sedimentological analyses reference cores and sections described in bulletins from the Paleontological Society and reports by the Colorado School of Mines and the Ohio State University. Sequence stratigraphy has been integrated with regional tectonic syntheses involving the Laramide Orogeny and climatic signals tied to marine records curated by the National Oceanic and Atmospheric Administration.

Paleoenvironments and climate

Reconstructions for Chadronian landscapes draw on stable isotope studies and plant macrofossils housed in collections at the Royal Ontario Museum, the Canadian Museum of Nature, and the Illinois State Museum. Paleosols and vertebrate assemblages suggest a shift from warm, subtropical woodlands through seasonally drier woodlands to more open savanna-like habitats, paralleling global cooling events recognized by researchers at Princeton University, Massachusetts Institute of Technology, and Scripps Institution of Oceanography. Palynological records correlated with cores analyzed by teams from the University of British Columbia and the University of Texas at Austin indicate changes in angiosperm and gymnosperm dominance that mirror climatic trends interpreted in studies from Columbia University and Stanford University.

Fauna and flora

Chadronian faunas include diverse mammals documented in monographs and catalogs from the American Museum of Natural History, the Yale Peabody Museum, and the University of California Museum of Paleontology. Notable groups represented by species-level entries in collections and expedition reports include perissodactyls such as taxa comparable to members of Brontotheriidae and early Equidae, artiodactyls allied with precursors to Camelidae and Entelodontidae, and diverse multituberculate and rodentiform mammals recorded by researchers at the University of Michigan Museum of Paleontology and the Field Museum. Predators and carnivoramorphans appear alongside small insectivores and chiropterans cataloged through field programs run by the Carnegie Museum of Natural History and the Natural History Museum, London. Plant assemblages include deciduous and evergreen taxa represented in herbarium-linked studies at the New York Botanical Garden and the Royal Botanic Gardens, Kew, with fossil wood and leaf floras compared to taxa in databases maintained by the Smithsonian Institution and the Missouri Botanical Garden.

History of research and naming

The name traces to early stratigraphic and paleontological work in outcrops studied by expeditions sponsored by the United States Geological Survey and institutions such as the American Museum of Natural History and the University of Nebraska State Museum. Foundational descriptions and taxonomy were published in journals associated with the Paleontological Society, the Journal of Vertebrate Paleontology, and the Bulletin of the American Museum of Natural History by researchers affiliated with Harvard University, Princeton University, University of California, and University of Kansas. Subsequent revisions and correlations have involved collaboration among scholars at the Smithsonian Institution, Yale University, University of Chicago, and international colleagues at the Natural History Museum, London and the Muséum national d'Histoire naturelle.

Significance and correlation with other land mammal ages

The Chadronian serves as a key interval for correlating North American mammal evolution with faunal turnover events, mass extinction research, and the Eocene–Oligocene transition discussed by teams at Columbia University, University of Cambridge, and ETH Zurich. It correlates with European and Asian Land Mammal Ages and stages documented in work from the Natural History Museum, Vienna, National Museum of Natural History (France), and the Paleontological Institute of the Russian Academy of Sciences, allowing links to faunal lists of the Priabonian and Rupelian and to isotope stratigraphy from cores curated by the International Ocean Discovery Program. The stage is referenced in comparative studies addressing faunal provinciality and biogeographic dispersal involving researchers at Cornell University, Duke University, University of Florida, and McGill University.

Category:North American Land Mammal Ages