North American Land Mammal Ages

North American Land Mammal Ages
Name	North American Land Mammal Ages

North American Land Mammal Ages The North American Land Mammal Ages are a regional biostratigraphic and chronostratigraphic scheme used to subdivide Cenozoic terrestrial deposits in North America, enabling correlation between fossiliferous strata and paleontological events. Developed through work by institutions and researchers such as the United States Geological Survey, the Paleontological Society, John C. Merriam, and later compilers at the University of California, the NALMA framework ties faunal turnovers to geological stages recognized by bodies like the International Commission on Stratigraphy and regional surveys.

Overview and definition

The NALMA scheme defines successive terrestrial faunal intervals based on diagnostic mammalian assemblages documented in type localities and reference sections across Wyoming, Montana, Colorado, Utah, and the Canadian Rockies, and is used alongside formats from the Geological Society of America, the American Association of Petroleum Geologists, and the Smithsonian Institution. Each NALMA interval is named for a characteristic locality or formation such as the Wasatch Formation, the Chadron Formation, the Hemingfordian, and the Clarendonian, and is tied to collections curated by museums including the American Museum of Natural History, the Smithsonian National Museum of Natural History, and the Natural History Museum of Los Angeles County. The scheme functions within stratigraphic protocols promulgated by organizations like the International Geological Congress and reflects practices used by field teams from universities such as Harvard University, Yale University, and the University of Chicago.

Chronostratigraphic framework and temporal range

NALMA intervals span from the late Paleocene through the Pleistocene and into the recent Quaternary, overlapping formal chronostratigraphic stages such as the Thanetian, Ypresian, Lutetian, Bartonian, Chattian, Aquitanian, Burdigalian, Langhian, Serravallian, Tortonian, Messinian, Zanclean, Piacenzian, and Holocene. Correlations align NALMA names like Puercan, Tiffanian, Clarkforkian, Wasatchian, Bridgerian, Uintan, Duchesnean, Chadronian, Orellan, Whitneyan, Arikareean, Hemingfordian, Barstovian, Clarendonian, Hemphillian, and Blancan with absolute ages provided by radiometric work from labs at California Institute of Technology, Massachusetts Institute of Technology, and the University of California, Berkeley. Chronostratigraphic calibration uses standards from the International Chronostratigraphic Chart and integrates magnetostratigraphic polarity chrons such as Brunhes Chron and Matuyama Chron.

Principal NALMA intervals and faunal stages

Major NALMA units include early Paleogene stages like the Puercan, mid‑Paleogene ones like the Bridgerian and Uintan, and Neogene–Quaternary units including the Hemingfordian, Barstovian, Clarendonian, Hemphillian, and Blancan. Each interval is characterized by first/last occurrences of key taxa documented in classic localities such as the Laramie Formation, Brule Formation, John Day Formation, Ashfall Fossil Beds, and the Cody Shale. Faunal stage definitions were refined through monographs and revisions by paleontologists affiliated with the Carnegie Institution, the Field Museum of Natural History, and the Royal Ontario Museum.

Methods of correlation and dating

Correlation among NALMA intervals employs vertebrate biostratigraphy using index mammals, lithostratigraphic relationships mapped by agencies like the United States Bureau of Land Management, and chronometric dating techniques including ^40Ar/^39Ar and U‑Pb analyses performed at facilities such as the Argonne National Laboratory and the Lamont–Doherty Earth Observatory. Magnetostratigraphy, cyclostratigraphy, and chemostratigraphy published in journals like Science, Nature, and the Journal of Paleontology support integration with the Geologic Time Scale. Correlative datasets from the Paleobiology Database and stratigraphic charts maintained by the United States Geological Survey are widely used to reconcile regional faunal stages with global events like the Paleocene–Eocene Thermal Maximum and the Eocene–Oligocene Transition.

Key fossil assemblages and representative taxa

Representative assemblages include early Paleocene mammals such as multituberculates and primitive placentals found in the San Juan Basin and Hell Creek Formation relatives; Paleogene assemblages dominated by artiodactyls, perissodactyls, and carnivorans in the Bridger Basin and Wasatch Formation; and Neogene faunas with proboscideans, equids, and camels in the Ogallala Formation, Hagerman Fossil Beds, and Arikaree Formation. Iconic genera documented in NALMA contexts include Mesohippus, Merychippus, Teleoceras, Oxydactylus, Procamelus, Daeodon, Smilodon (Late Pleistocene occurrences in the Rancho La Brea), and early primates cataloged by curators at the University of Michigan Museum of Paleontology and the Yale Peabody Museum.

Regional variations and provincialism

Provincialism in NALMA faunas reflects biogeographic partitions across areas such as the Great Plains, Rocky Mountains, Gulf Coast, Pacific Northwest, and Baja California, with distinct assemblages recorded in basins like the Williston Basin, Powder River Basin, and Colorado Plateau. Climatic shifts linked to events studied by researchers at the National Oceanic and Atmospheric Administration and the Scripps Institution of Oceanography influenced dispersal barriers related to the Interior Seaway remnants and orogenies including the Laramide Orogeny, producing faunal endemism evident in provincial schemes developed by paleontologists from the University of Kansas and Texas A&M University.

Historical development and usage in stratigraphy

The NALMA concept emerged from 19th–20th century fieldwork by figures such as Edward Drinker Cope, Othniel Charles Marsh, William Berryman Scott, and later systematization by George Gaylord Simpson and committees of the Paleontological Society and the American Geological Institute. Over decades, revisions arose from synthesis volumes, monographic work at institutions like the American Museum of Natural History and the Field Museum, and international meetings at the International Geological Congress. Today NALMA intervals are integral to continental vertebrate biostratigraphy, paleoclimatology, and basin analysis performed by academic groups at Stanford University, Princeton University, and federal surveys.

Category:Paleontology