Ordovician Extinction Event

Introduction

The Ordovician extinction event was the first of two major extinctions that seriously affected the Palaeozoic evolutionary fauna (Brenchley 2001). The early-mid Ordovician was a time when fauna established the complex suspension-feeding communities, characterized by brachiopods, bryozoans, crinoids, and corals, which reached an equilibrium generic diversity by the start of the Late Ordovician.  (Sepkoski 1995). This equilibrium was disturbed after ~20 million years by the Ordovician extinction at ~439 million years ago. The extinction occurred in 2 main phases an estimated 0.5 to 1 million years apart.

(How the seas during the Ordovician period may have appeared from www.geology.wisc.edu)

The mass extinction seriously affected both benthic and planktonic faunas across all latitudes in both clastic and carbonate environments, and in deep and shallow marine regions (Brenchley 2001).

What was lost?

The Ordovician mass extinction event eliminated an estimated 85% of marine species, 55% of genera, 22% of families, however only a few orders, and no classes or phyla (Brenchley 2001). Both benthos and plankton were affected with high levels of generic extinction among the pelagic conodonts (76%), graptolites (85%), sessile benthic brachiopods (54%), rugose corals (69%), tabulate corals (72%) and benthic trilobites (58%) (Sepkoski 1995).

(Image showing changes in diversity across the 2 phases of the Ordovician mass extinction. Numbers in bold are the estimated generic diversity before and after the event, and the percentages are the amount of generic extinction at each phase of extinction. From Brenchley 2001.)

The first phase of extinction amongst the majority of groups was sharp. Graptolites suffered very significant losses losing ~75% of their genera, and trilobites and brachiopods lost ~50% of their genera, however, conodont losses were relatively small (Sheehan 1988). During this interval a new brachipod fauna, the Hirnantia fauna and the Mucrohaspis triblobite association appeared (Brenchley 2001). These eurytopic (can tolerate a wide range of environmental conditions), cool-adapted, opportunist species established themselves across a wide range of environments from high-latitudes to the marginal tropics. In the tropics the Edgewood brachiopod fauna became established alongside new low-diversity associations of eurytopic corals.

                                          (Trilobite fossil. From National Geographic 2011.)

The second stage of extinctions was also sharp, with the majority of groups excluding the graptolites further depleted, with the conodonts experiencing the greatest losses. Many long-established clades of trilobites that had survived the first phase of extinction disappeared, alongside the newly established Hirnantia and Edgewood brachiopod faunas and the Edgewood coral faunas (Brenchley 2001).

Immediately after the late-Ordovician mass extinction event most groups remained depleted, characterized by the residual members of the pre-extinction Ordovician faunas. There was also a rapid appearance of new genera among the conodonts.

Postulated Causes

There is a close correlation between the two phases of extinction and the growth and decay of the Gondwanan ice caps, suggesting that the rapid climatic changes that disrupted a long-established greenhouse climate may have played a major role in causing the Ordovician extinction event (Brenchley 2001). The first phase of extinction coincided with the initiation of major glaciation and the second phase with the decay of the ice caps. The growth of continental ice is reflected in an estimated global sea level fall of 50-100m and the waning of the ice caps by a similar magnitude. Oxygen isotope stratigraphy reflect the presence of a major ice cap, and a fall in shallow marine temperatures of up to 8°C, even in tropical regions (Brenchley et al. 1994). A synchronous positive shift of ~7‰ in ¹³C implies a major change in Carbon cycling such as may result from an increase in productivity and increased organic Carbon storage in sediments or deep-water (Brenchley et al. 1994).

(Environmental and biotic changes associated with the Ordovician extinction event, PDB is the international carbonate standard.. From Brenchley 2001).

Causes of the first phase: The decrease in marine temperatures which started abruptly during the first phase is likely to have eliminated those species living within a specific temperature range, particularly in tropical regions where the faunas were most likely adapted to greenhouse conditions. Sea level change was modest during this period, and is unlikely to have been a factor in the first phase, but may have subsequently played a role in the extinction of shallow marine faunas of tropical carbonate shelves (Brenchley 2001). Faunas inhabiting on or above slope areas may have been affected by the changes in ocean circulation associated with the development of thermohaline circulation in response to the cooling of high-latitude waters. This new circulation likely resulted in vigorous upwelling, which may have raised the thermocline and oxygen minimum zone and resulted in an overabundance of nutrients and substances toxic to the plankton inhabiting the near-surface mixed layer (Wilde et al. 1990). This may account  for a number of extinctions including gratolite assemblages and the planktonic and benthic trilobites. The conodonts mainly inhibited shelf waters which escaped the effects of upwelling, which explains why populations did not deplete significantly.

Causes of the second phase: The second phase was coincidental with a rise in sea level, temperature, a change in Carbon cycling (suggesting a return to warm stratified oceans), and widespread anoxia (Brenchley 2001). This rise in temperature is likely to have favoured the warm-adapted survivors of the first extinction, and played a key role in the elimination of the cool-adapted forms among the Hirnantiabenthic and nektobenthic faunas. Species sensitive to Oxygen deficiency would have been eliminated and high sea level may have restricted many shallow marine habitats.

Conclusion

Major biotic changes in the Late Ordovician appear to have been caused by a rapid change from a long history of stable greenhouse conditions preceding the extinction into and then out of an ice house climate with a number of associated changes. A mass extinction of species was experienced without eliminating any major groups, and without radically changing the ecological structure of communities. The holes in the structure of communities were progressively filled during the early Silurian with new taxa, but with little ecological innovation.

References

Brenchley, P. J. (2001). Extinction: Late Ordovician mass extinction. Encyclopedia of Life Sciences.
Brenchley, P. J, et al. (1994). Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period. Geology. 22: 295-298.
National Geographic (2011) at www.nationalgeographic.com
Sepkoski, J. J. Jr (1995). The Ordovician radiations: diversification and extinction shown by global genus-level taxonomic data. In Cooper, J. C. et al. (eds.) Ordovician Odyssey: Short papers of the Seventh International Symposium on the Ordovician system. pp. 393-396. Fullerton: Pacific Selection, Society of Sedimentary Geology.
Sheehan, P. M. (1988). Late Ordovician events and the terminal Ordovician extinction. New Mexico Bureau of Mines and Mineral Resources Memoir. 44: 405-415.
Wilde, P. et al. (1990). Vertical advections from oxic or anoxic waters from the main pycnocline as a cause of rapid extinctions or rapid radiations. In Kauffman, E. G. and O. H. Walliser (eds.) Extinction Events in Earth History, Lecture Notes in Earth Sciences 30. pp. 85-98. Springer-Verlag: Berlin.
www.geology.wisc.edu