Harper, David Alexander Taylor4; Hammarlund, Emma U.3; Rasmussen, Christian Mac Ørum5
1 Natural History Museum of Denmark, Faculty of Science, Københavns Universitet2 Natural History Museum of Denmark, Natural History Museum of Denmark, Faculty of Science, Københavns Universitet3 University of Southern Denmark4 Natural History Museum of Denmark, Faculty of Science, Københavns Universitet5 Natural History Museum of Denmark, Natural History Museum of Denmark, Faculty of Science, Københavns Universitet
a coincidence of causes
The end Ordovician (Hirnantian) extinction was the first of the five big Phanerozoic extinction events, and the first that involved metazoan-based communities. It comprised two discrete pulses, both linked in different ways to an intense but short-lived glaciation at the South Pole. The first, occurring at, or just below, the Normalograptus extraordinarius graptolite Biozone, mainly affected nektonic and planktonic species together with those living on the shallow shelf and in deeper water whereas the second, within the N. persculptus graptolite Biozone, was less focused, eradicating faunas across a range of water depths. In all about 85% of marine species were removed. Proposed kill mechanisms for the first phase have included glaciallyinduced cooling, falling sea level and chemical recycling in the oceans, but a general consensus is lacking. The second phase is more clearly linked to near-global anoxia associated with a marked transgression during the Late Hirnantian. Most recently, however, new drivers for the extinctions have been proposed, including widespread euxinia together with habitat destruction caused by plate tectonic movements, suggesting that the end Ordovician mass extinctions were a product of the coincidence of a number of contributing factors. Moreover, when the deteriorating climate intensified, causing widespread glaciation, a tipping point was reached resulting in catastrophe.
Journal review article
Gondwana Research, 2014, Vol 25, Issue 4, p. 1294-1307
End Ordovician; Extinction; Hirnantian; Isotopes; Palaeogeography