Mikko's Phylogeny Archive Craters Page   Paleogene Impacts [24-65 Ma]

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Seminar of Ecological Paleontology, Spring 1999
Department of Geology
Section of Geology and Paleontology
University of Helsinki

La Grande Coupure, A True Mass-Extinction?

Mikko Haaramo & Mikko Kangas

Abstract:

The dramatic difference between Eocene and Oligocene terrestrial biotas has been known for over a century. One of the early 20th century scientists was so impressed by these differences that he gave it a name "la Grande Coupure" (French: "The Great Cut"). This change separates early and middle Palaeogene-environments from late Palaeogene and Neogene-environments and is characterised by quite dramatic faunal and an equally dramatic climatic and floral change.

I. Eocene settings:

The Land-vertebrate fauna of Eocene is characterised by browsing mammals like primitive perissodactyls (palaeotheres, primitive equioids, primitive tapir-like animals). Among these animals were also several primitive herbivorous mammalian orders, like Brontotheria, Pantotheria, Tillodonta. Some of these animals were of considerable size, like the pantodont Coryphodon, which growed to 2.5 m long and was formed little like modern hippo (Hippopotamus). Within the tree-canopy lived already ancient multituberculates (Multituberculata), as well as frugi- or folivorous rodent-like plesiadapiform and lemur-like prosimian primates.

The best known floral assemblages of European and North American Eocene-environments appear to be similar to present day Southern Asian or Middle American environments, and not very different of Late Cretaceous or Palaeocene plant-assemblages. In general Eocene-environments were characterised by relatively high annual temperatures, relatively high wetness and generally tropic or sub-tropic climates with dry-wet cycle. This is proven by extensive limestone and lignite formation in North America and Middle Europe.

II. Changes to Oligocene

Transition from Eocene to Oligocene is characterised by major changes.The climate changes from generally wet and tropical to more seasonal, drier and subtropical. These chances shadow the first hints of the later Tertiary cooling trend, which really kicks in during following Miocene.

Unfortunately in North America and Europe, Oligocene was mainly erosional episode, after a major mountain-formation events of Eocene. In Asia, the Indian plate collides with Eurasian plate during Middle Oligocene and first mountain-formation events of Himalayan cycle began.

These events had probably a serious effect to Middle and East Asian Oligocene environments. But these events have also a reflective effect to world environments in general.

The Eocene-Oligocene boundary is characterised by large faunal changes from typically tropical, browser-type animals to more temperate grazer-type forms. This trend is evident in all continents, and appears as loss of medium to large, tapirid-like browsers and tree-living animals. Typical Eocene-faunas appear to have gradually replaced by less diverse Oligocene-fauna, composite mostly of small ground-living rodents, lagomorphs and primitive artiodactyls. Several mammal orders, which originated from Mesozoic and/or Early Paleogene go extinct, like multituberculates, primitive insectivorous mammals, most of dermopterans, mesonychids, arctocyonids, primitive condylarths, etc.

Marine mammals, like the primitive archaeocete-whales go extinct and are replaced by their modern relatives. The Planktonic and invertebrate life forms are effected as well, going through a major taxonic change.

III. The causes of Change:

What causes these large faunal and floral changes?

Current view appears to be that main causes to changes were probably endogenic events, like continental movements, mountain building, and their effects on environments. These effects cause the climate to deteriorate to a more fluctuating one, with marked seasonality, cooling and in certain continents, like Asia, more dry environments.

What makes these events so difficult to tackle, is the fact that there was no single defining event of extinction as the K/T event. These events were gradual, and expanded for a relatively long period of time from late Middle Eocene to Early Oligocene, that is some 13 - 15 million years. There appears to have been three separate events, first during late Middle Eocene, second during Late Eocene and last during Early Oligocene.

The late Middle Eocene event was probably caused by onset of glaciation at Antarctica. The heaviest extinction takes place during this phase, since animals and plants effected by this extinction were warm adapted. As typical to mass-extinction the victims were the most diversified and most highly adapted forms.

There is noticeable sharp regression in sea levels at Eocene-Oligocene boundary and a noticeable cooling of seawater as well. Evidence dictates that first large ice-sheets extended from Antarctica to sea during Early Oligocene, and this probably caused the cooling of Early Oligocene world as well as some of the regression.

A lesser scale extinction takes place also during Late Eocene and Early Oligocene.

What causes these extinctions is still unclear, but causes were probably similar to late Middle Eocene extinctions. During Late Eocene there is two large impacts; Chesapeake Bay, ~85 km diameter and Popigai ~100 km diameter. Both of these impact are about same age, circa 35.5 Ma. Despite being truly large impacts, either of these impacts appears to have caused a distinct extinction, although last of the archaeocete's die out about this time.

Reference(s):

       Abate, B., Koeberl, C., Kruger, F. J. & Underwood, Jr., J. R., 1999: BP and Oasis impact structures, Libya, and their relation to Libyan Desert Glass. 177-192
in Dressler, B. O. & Sharpton, V. L., (eds.) 1999: Large meteorite impacts and planetary evolution, II.
--The Geological Society of America, Boulder, 1999, viii-464

       Hodge, P., 1994: Meteorite craters and impact structures of the Earth.
--Cambridge University Press, London, 1994, 1-124

       Lehtinen, M., 2001: Personal correspondence.

       Masaitis, V. L., Naumov, M. V. & Mashchak, M. S., 1999: Anatomy of the Popigai impact crater, Russia. 1-17
in Dressler, B. O. & Sharpton, V. L., (eds.) 1999: Large meteorite impacts and planetary evolution II.
--The Geological Society of America, Boulder Special Paper 339, 1999, viii-464

       Pearson, P. N., Ditchfield, P. W., Singano, J., Harcourt-Brown, K. G., Nicholas, C. J., Olsson, R. K., Shackleton, N. J. & Hall, M. A., 2001: Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs.
--Nature: Vol. 413, pp. 481-487

       Pong, C. W., Hutchinson, D. R., Colman, S. M. & Lee, M. W., 1999: Seismic expressions of the Chesapeake Bay impact crater: Structural and morphological refinements based on seismic data. 149-164
in Dressler, B. O. & Sharpton, V. L., (eds.) 1999: Large meteorite impacts and planetary evolution, II.
--The Geological Society of America Special Paper 339, Boulder, 1999, viii-464

       Rampino, M. R., 1999: Impact crisis, mass extinctions, and galactic dynamics: The case for a unified theory. 241-248
in Dressler, B. O. & Sharpton, V. L., (eds.) 1999: Large meteorite impacts and planetary evolution, II.
--The Geological Society of America, Boulder, 1999, viii-464

       Vishnevsky, S. & Montanari, A., 1999: Popigai impact structure (Arctic Siberia, Russia): Geology, petrology, geochemistry, and geochronology of glass-bearing impactites. pp. 19-59
in Dressler, B. O. & Sharpton, V. L., (eds.) 1999: Large meteorite impacts and planetary evolution II.
--The Geological Society of America Special Paper 339, Boulder, 1999, viii-464

© Mikko Haaramo, / Last updated 2003-02-11 / http://www.fmnh.helsinki.fi/users/haaramo