Eocene–Oligocene extinction event

Eocene–Oligocene extinction event
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Name	Eocene–Oligocene extinction event
Time period	Late Eocene–Early Oligocene
Approx age	~33.9 million years ago
Major groups affected	Mammalia, Aves, Reptilia, Actinopterygii, Foraminifera
Proposed causes	Antarctic glaciation, climate cooling, sea-level change, volcanism, bolide impacts

Eocene–Oligocene extinction event The Eocene–Oligocene extinction event marks a profound turnover near the boundary between the Eocene and Oligocene epochs, producing faunal and floral reorganizations across marine and terrestrial realms. Many contemporaneous records from sites such as Florissant Fossil Beds National Monument, Quercy Phosphorites Formation, Bighorn Basin, Fur Formation, and Green River Formation document abrupt changes in community composition and diversity. Correlation of marine isotope records from the Vema Channel, Deep Sea Drilling Project, and Ocean Drilling Program tied this turnover to global shifts recorded in North Atlantic, South Pacific, and Southern Ocean archives.

Background and timing

High-resolution chronostratigraphy using biostratigraphy from nannofossil zones, magnetostratigraphy from the Brunhes–Matuyama reversal framework, and strontium isotope chemostratigraphy anchored timing to ~33.9 Ma. The event coincides with onset of major Antarctic glaciation inferred from sediment cores recovered by Glomar Challenger campaigns and later by the Integrated Ocean Drilling Program. Continental sequences in the Chad Basin, Siwalik Hills, and Tibetan Plateau show synchronous floral turnover, while vertebrate faunas in the Fayum Depression, Eurasian Basin, and Great Plains (North America) indicate near-contemporaneous shifts. Paleomagnetic tie points established by researchers associated with U.S. Geological Survey, British Antarctic Survey, and universities such as Harvard University and University of California, Berkeley refined the boundary horizon.

Causes and mechanisms

Hypotheses implicate combined drivers: initiation of permanent ice sheet growth on Antarctica, orbital forcing associated with the Milankovitch cycles, major reorganizations of ocean circulation tied to continental configuration including the opening of the Tasmanian Gateway and deepening of the Drake Passage, greenhouse gas drawdown via silicate weathering influenced by uplift of the Andes and Himalaya, and pulses of volcanism from provinces like the North Atlantic Igneous Province. Some paleontologists and geochemists invoked bolide scenarios compared with evidence from contemporaneous craters such as Popigai crater and Chesapeake Bay impact crater debates. Modeling efforts by groups at Max Planck Institute for Chemistry, Lamont–Doherty Earth Observatory, and National Center for Atmospheric Research emphasize feedbacks among albedo changes, carbon cycle perturbations, and ocean stratification as mechanistic pathways.

Extent and patterns of extinction

Marine microfauna, especially planktonic and benthic foraminifera and calcareous nannoplankton, show selective extinctions recorded in cores from the North Sea, Equatorial Pacific, and South Atlantic with turnover rates varying by latitude. Terrestrial faunal losses include contractions in archaic ungulate assemblages documented in the Balkan Peninsula, Central Asia, and Western Europe and faunal migrations across the Bering Land Bridge and North Atlantic Land Bridge. Avian and reptile lineages register regional extirpations evident in the Fossil Butte National Monument and Messel Pit deposits. Extinction patterns reveal taxon-specific susceptibility linked to habitat specialization, dietary ecology, and dispersal capacity; paleoecologists from institutions like Smithsonian Institution, Natural History Museum, London, and Muséum national d'Histoire naturelle have cataloged these disparities.

Paleoclimatic and environmental changes

Proxy data from stable isotopes (δ18O, δ13C), leaf physiognomy analyses from Paleobotany collections, and TEX86-derived SST reconstructions converge on a global cooling trend and increased seasonality. Expansion of ice on Antarctica produced eustatic sea-level fall evident in sequences along the Gulf Coast of the United States, North Sea Basin, and Patagonia. Changes in ocean circulation promoted development of modern mode and intermediate water masses analogous to present-day Antarctic Bottom Water and North Atlantic Deep Water signatures. Terrestrial records indicate shifts from greenhouse subtropical floras to more open, temperate communities across regions including the Iberian Peninsula, Central Europe, and North America with associated increases in wildfire signals reported by palynologists at University of Copenhagen and University of Vienna.

Biotic recovery and evolutionary consequences

Post-boundary adaptive radiations reshaped mammalian assemblages, with early representatives of modern orders appearing or diversifying in the Oligocene fossil record from localities such as Chad, Pakistan, and Europe. Grassland expansions facilitated evolution of hypsodont dentitions in ungulates, documented in South America and Africa lineages curated by museums including the Natural History Museum of Los Angeles County and Royal Belgian Institute of Natural Sciences. Marine ecosystems saw recovery and turnover with emergence of new foraminiferal assemblages and evolutionary innovations in cetaceans recorded in the Calvert Cliffs and Hoko River Formation. Biogeographic rearrangements—faunal exchanges via corridors like the Turgai Strait closure—were tracked by paleontologists affiliated with University of Oxford and University of Tokyo. The event set the stage for Oligocene climatic regimes that influenced subsequent Neogene evolution studied by researchers at Scripps Institution of Oceanography and Pennsylvania State University.

Category:Extinction events