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In stratigraphy, paleontology, geology, and geobiology, an erathem is the total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale.

It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years – less than a geological eon, but greater than its successively smaller and more refined subdivisions (geologic periods, epochs, and geologic ages). By 3,500 million years ago (Mya) simple life had developed on earth (the oldest known microbial fossils in Australia are dated to this figure).^[1] The atmosphere was a mix of noxious and poisonous gases (methane, ammonia, sulfur compounds, etc.^[1] – a so-called reducing atmosphere^[2] lacking much free oxygen which was bound up in compounds).

These simple organisms, cyanobacteria ruled the still cooling earth for approximately a billion years^[1] and gradually transformed the atmosphere to one containing free oxygen. These changes, along with tectonic activity left chemical trails (red bed formation, etc.) and other physical clues (magnetic orientation, layer formation factors) in the rock record, and it is these changes along with the later richer fossil record which specialists use to demarcate times early in planet earth's history in various disciplines.

Erathems are not often used in practice. While they are subdivisions of eonothems and are themselves subdivided into systems, dating experts prefer the finer resolution of smaller spans of time when evaluating strata.

Erathems have the same names as their corresponding eras. The Phanerozoic eonothem can thus be divided into Cenozoic, Mesozoic, and Paleozoic erathems. Similarly, the Proterozoic eonothem is divided youngest to oldest into the Neoproterozoic, Mesoproterozoic and Paleoproterozoic erathems, and the Archean eon and eonothem are divided similarly into the Neoarchean, Mesoarchean, Paleoarchean and the Eoarchean, for which a lower (oldest) limit is undefined.^[3][4]

• Body form
• European Mammal Neogene
• Geologic time scale
  • New Zealand geologic time scale
• North American Land Mammal Age
• Fauna (animals)
• Type locality
• List of GSSPs

• Gehling, James; Jensen, Sören; Droser, Mary; Myrow, Paul; Narbonne, Guy (March 2001). "Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland". Geological Magazine. 138 (2): 213–218. Bibcode:2001GeoM..138..213G. doi:10.1017/S001675680100509X. hdl:10662/24314. S2CID 131211543. 1.
• Hedberg, H.D., (editor), International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure, New York, John Wiley and Sons, 1976 ^{[ISBN missing]}
• International Stratigraphic Chart from the International Commission on Stratigraphy
• US National Park Service description of geographic time
• Washington State University description of geographic time Archived 2011-07-26 at the Wayback Machine
• Web Geological Time Machine
• Eon or Aeon, Math Words – An alphabetical index

• The Global Boundary Stratotype Section and Point (GSSP): overview
• Chart of The Global Boundary Stratotype Sections and Points (GSSP): chart
• Geotime chart displaying geologic time periods compared to the fossil record – Deals with chronology and classifications for laymen (not GSSPs)