|
The transition between the end of the Eocene and the beginning of the Oligocene, called the Grande Coupure (the "Great Break" in continuity) in Europe, is marked by large-scale extinction and floral and faunal turnover (although minor in comparison to the largest mass extinctions). Most of the affected organisms were marine or aquatic in nature. They included the last of the ancient cetaceans, the Archaeoceti. This was a time of major climatic change, especially cooling, not obviously linked with any single major impact or any catastrophic volcanic event. One cause of the extinction event is speculated to be extended volcanic activity. Another speculation is that the extinctions are related to several meteorite impacts that occurred about this time. One such event happened near present-day Chesapeake Bay, and another at the Popigai crater of central Siberia, scattering debris perhaps as far as Europe. New dating of the Popigai meteor supports the impact theory as the cause for the mass extinction.〔(【引用サイトリンク】url=http://news.yahoo.com/russias-popigai-meteor-crash-linked-mass-extinction-113359696.html )〕 The leading scientific theory on climate cooling at this time is decrease in atmospheric carbon dioxide, which slowly declined in the mid to late Eocene and possibly reached some threshold approximately 34 million years ago. This boundary is closely linked with the Oligocene Oi-1 event, an oxygen isotope excursion that marks the beginning of ice sheet coverage on Antarctica. ==Grande Coupure== The ''Grande Coupure'', or "great break" in continuity,〔also termed the MP 21 event.〕 with a major European turnover in mammalian fauna about 33.5 Ma, marks the end of the last phase of Eocene assemblages, the Priabonian, and the arrival in Europe of Asian immigrants. The Grande Coupure is characterized by widespread extinctions and allopatric speciation in small isolated relict populations.〔Called "dispersal-generated origination" in Hooker et al. 2004〕 It was given its name in 1910 by the Swiss palaeontologist Hans Georg Stehlin,〔H.G. Stehlen, 1910. "Remarques sur les faunules de Mammifères des couches eocenes et oligocenes du Bassin de Paris," in ''Bulletin de la Société Géologique de France,'' 4'.9, pp 488-520.〕 to characterise the dramatic turnover of European mammalian fauna, which he placed at the Eocene-Oligocene boundary. A comparable turnover in Asian fauna has since been called the "Mongolian Remodelling". The Grande Coupure marks a break between endemic European faunas before the break and mixed faunas with a strong Asian component afterwards. J. J. Hooker and his team summarized the break: :"Pre-Grande Coupure faunas are dominated by the perissodact family Palaeotheriidae (distant horse relatives), six families of artiodactyls (cloven-hoofed mammals) (Anoplotheriidae, Xiphodontidae, Choeropotamidae, Cebochoeridae, Dichobunidae and Amphimerycidae), the rodent family Pseudosciuridae, the primate families Omomyidae and Adapidae, and the archontan family Nyctitheriidae. :"Post-Grande Coupure faunas include the true rhinoceros (family Rhinocerotidae), three artiodactyl families (Entelodontidae, Anthracotheriidae and Gelocidae) related respectively to pigs, hippos and ruminants, the rodent families Eomyidae, Cricetidae (hamsters) and Castoridae (beavers), and the lipotyphlan family Erinaceidae (hedgehogs). The speciose genus Palaeotherium plus ''Anoplotherium'' and the families Xiphodontidae and Amphimerycidae were observed to disappear completely. :"Only the marsupial family Herpetotheriidae, the artiodactyl family Cainotheriidae, and the rodent families Theridomyidae and Gliridae (dormice) crossed the faunal divide undiminished." Whether this abrupt change was caused by climate change associated with the earliest polar glaciations〔A major cooling event preceded the Grande Coupure, based on pollen studies in the Paris Basin conducted by Chateauneuf (J.J. Chateauneuf, 1980. "Palynostratigraphie et paleoclimatologie de l'Éocene superieur et de l'Oligocene du Bassin de Paris (France)" in ''Mémoires du Bureau de Recherches Géologiques et Minières'', 116 1980).〕 and a major fall in sea levels, or by competition with taxa dispersing from Asia, few argue for an isolated single cause. More spectacular possible causes are related to the impact of one or more large bolides in northern hemisphere at Popigai, Toms Canyon and Chesapeake Bay. Improved correlation of northwest European successions to global events〔 confirms the Grande Coupure as occurring in the earliest Oligocene, with a hiatus of about 350 millennia prior to the first record of post-Grande Coupure Asian immigrant taxa. An element of the paradigm of the Grande Coupure was the apparent extinction of all European primates at the Coupure: the recent discovery of a mouse-sized early Oligocene omomyid, reflecting the better survival chances of small mammals, further undercut the Grand Coupure paradigm. Evidence in the world’s ocean current system indicates an abrupt cooling from 34.1 to 33.6 Ma across the Eocene–Oligocene boundary at 33.9 Ma. The remarkable cooling period in the ocean is correlated with pronounced mammalian faunal replacement within continental Asia as well. The Asian biotic reorganization events are comparable to the Grande Coupure in Europe and the Mongolian Remodeling of mammalian communities.〔Zhang, R., Kravchinsky, V.A., Yue, L. (2012). Link between Global Cooling and Mammalian Transformation across the Eocene-Oligocene Boundary in the Continental Interior of Asia]. International Journal of Earth Sciences, DOI 10.1007/s00531-012-0776-1〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Eocene–Oligocene extinction event」の詳細全文を読む スポンサード リンク
|