Extinctions at the Frasnian-Famennian and famennian-Tournaisian boundriesLate Devonian Mass Extinctions

One of the "Big Five"

Scientists recognize a number of mass extinctions (extinction events that far exceed background extinction rates and are not taxonomically restricted). Five of these were particularly severe: the terminal Ordovician, Late Devonian, terminal Permian, terminal Triassic, and terminal Cretaceous. There is general agreement that four of these "Big Five" events were relatively restricted in duration (i.e., <1-5 million years). The timing and duration of the Late Devonian mass extinction(s), however, are subject to considerable debate and a variety of interpretations.

For example, Thomas Algeo et. al. (2000) consider the Late Devonian mass extinction to be a prolonged marine biotic crisis extending for 20-25 million years (late Middle Devonian to the Devonian/Carboniferous boundary) and punctuated by 8-10 extinction events. They consider the two most extensive extinction events to be the Kellwasser Event (at the Frasnian-Famennian boundry) and the Hangeberg Event (at or near the Devonian-Carboniferous boundry). George McGhee (1996) recognizes several major extinction events during the Devonian, but regards the Kellwasser Event as the Late Devonian mass extinction. Maurice Streel et. al. (2000) conclude that there were two crises (intervals of prolonged biodiversity losses) followed by two episodic extinction events of much shorter duration. These are the Late Frasnian Crisis followed by the Kellwasser Event, and the end-Famennian Crisis followed by the Hangeberg Event.

The diversity declines leading up to and including the Kellwasser Event accounted for the extinction of about 20% of all animal families and 70-80% of all animal species. Major victims included ammonites, benthic foraminifera, brachiopods, conodonts, rugose and tablulate corals, jawless fishes, placoderms, stromatoporoid sponges and trilobites. The primary Devonian reef-builders (tabulate corals and stromatoporoids) never truly recovered from the extinctions and the changes in reef ecology were profound. Declines leading up to and including the Hangeberg event accounted for about 16% of all marine familes. Major victims include ammonites, ostracodes and placoderms.

The Devonian extinctions were particularly severe for benthic marine organisms that lived in shallow tropical seas. In fact, many of the taxa that thrived during and after the extinctions were typically deep-water or high-latitude relatives of the decimated forms. Significantly, the impacts—if any—on terrestrial plants and animals were considerably less than those in tropical marine habitats. In fact, terrestrial plants experienced major diversity declines in the early and middle Frasnian, but no significant decline is evident for the end of the Frasnian (i.e., near or including the Kellwasser Event). Moreover, the decline is attributable primarily to a reduction in the creation of new species rather than an accelerated extinction of existing species. A lesser decline (notable for the extinction of Archaeopteris) occurs near the Devonian-Carboniferous boundary, but occurs somewhat later than the Hangeberg event.

"The Devonian Plant Hypothesis"

A variety of causes have been proposed for the Devonian mass extinctions. These include asteroid impacts, global anoxia (widespread dissolved oxygen shortages), plate tectonics, sea level changes and climatic change. One of the more interesting of these is the "Devonian Plant Hypothesis". This theory, first proposed by Thomas Algeo, Robert Berner, J. Barry Manard and Stephen Scheckler in 1995, credits the expansion of terrestrial plants as the ultimate cause for mass extinctions in the tropical oceans.

Devonian marine deposits are notable in part for the widespread occurrence of black shales in the shallow inland seas of North America and Eurasia. These organic-rich sediments, which indicate anoxic (oxygen-deprived) bottomwaters, occur at about the same time as the multiple extinction events in the Middle and Late Devonian. Algeo et. al. argue that these deposits were the results on organic matter and nutrient imports from increasingly vegetated landscapes. In addition to causing widespread eutrophication in shallow seas, terrestrial plants —particularly with the spread of Archaeopteris forests—contributed to accelerated pedogenesis (soil formation) which in turn resulted in accelerated silicate weathering. This process, which creates calcium and magnesium carbonates, removes CO2 from the atmosphere. These carbonates enter the rivers and are exported to the oceans where they precipitate and become buried in marine sediments.

The marine burial of massive quantities of organic carbon and inorganic carbonates substantially reduced atmospheric CO2 levels. The loss of this greenhouse gas is believed to have contributed to global cooling. A short, but intense episode of glaciation occurred at the very end of the Devonian in parts of Gondwanaland and is associated with the Hangeberg extinctions. No direct evidence for glaciation has been found for the Kellwasser extinctions, but a rapid drop in sea level combined with an extended period of cooler temperatures in the first half of the Famennian suggest the presence of as yet undiscovered glaciers.

Ironically, the development and maturation of terrestrial environments fostered by the expansion of terrestrial plants may have wrecked havoc on the oceans from which life first arose.

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Thomas Algeo’s web page on the Devonian Mass Extinction:
Hallam A. and P.B. Wignall. 1997. Mass Extinctions and Their Aftermath. Oxford: Oxford University Press.
Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
McGhee GR Jr. 1996. The Late Devonian Mass Extinction. New York: Columbia University Press.
Scientific Papers:
Algeo, T.J., R.A. Berner, J.P. Maynard, and S.E. Scheckler. 1995. "Late Devonian oceanic anoxic events and biotic crises: "Rooted" in the evolution of vascular land plants?" GSA Today 5: 45, 64-66.
Algeo, T.J. and S.E. Scheckler. 1998. "Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events." Phil. Trans. R. Soc. Lond. B 353: 113-130.
Algeo, T.J., S.E. Scheckler and J. B. Maynard. 2000. "Effects of the Middle to Late Devonian spread of vascular land plants on weathering regimes, marine biota, and global climate." pp. 213-236. In: P.G. Gensel and D. Edwards (eds.). 2001 Plants Invade the Land: Evolutionary and Environmental Approaches. Columbia Univ. Press: New York.
Scheckler, S.E., 1986. "Floras of the Devonian-Missipian transition." pp. 81-96. In: T.W. Broadhead (ed.). Land Plants. Univ. Tenn. Dept. Geology. Studies in Geology 15. Knoxville.
Streel, M., M.V. Caputo. S. Lovboziak, and J.H.G. Melo. 2000. "Late Frasnian-Famennian climate: based on palynomorph analyses and the question of the Late Devonian glaciation." Earth-Science Reviews 52: 121-173.<
Image Credits:
The image at the top of the page is copyrighted © 2002, Dennis C. Murphy, (see Terms of Use).

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