The Devonian extinction event was the second of two major extinctions that affected the evolutionary fauna of the Palaeozoic (Brenchley 2001). This extinction event occurred in two parts: Firstly the Kellwasser event which occurred at the beginning of the Devonian period ~374 million years ago (Racki 2005). The second part was the Hangenberg Event, which occurred at the end of the Devonian period ~359 million years ago (Caplan and Bustin 1999). During this mass extinction event 35% of all genera and 75% of all species became extinct, the lowest figures of all five mass extinction events (Barnosky et al. 2011).
It is evident that there was a significant loss of biodiversity during the Devonian period, however the extent of time during which these events took place is less certain, with estimates ranging from 500,000 to 25 million years (Stigall 2011). It is also not clear whether two periods of mass extinction occurred during the event, or a series of smaller ones. Some scientists believe that the Devonian extinction event may consist of as many as seven distinct events, over the period of ~25 million years.
The late-Devonian world was very different from that of today. The continents were arranged very differently with a supercontinent Gondwana covering the majority of the Southern continent. The continent of Siberia occupied the Northern hemisphere, while Laurussian existed on the equator and was drifting towards Gondwana. By the end of the Devonian the continents of Euramerica and Gondwana were beginning to converge to form Pangaea.
The biota of the Devonian was very different from that of the Ordovician. The plants which had been on land in forms similar to mosses, lichens and liverworts since the Ordovician had evolved to develop roots, seeds, and water transport systems, allowing them to live in areas that were not constantly wet, consequently forming wide-ranging forests on the highlands. The oceans had also undergone significant changes, and were now home to massive coral reefs, and the first tetrapods were beginning to evolve leg-like structures.
What was lost?
The Devonian mass extinction event primarily affected the marine community, most significantly affecting shallow warm-water organisms. The most important group affected by the Kellwasser event were the reef builders, including stromatoporoids and the rugose and tabulate corals. The collapse of the reef system was so severe that major reef-building did not recover until the Mesozoic era. Further taxa that were severely affected during the extinction event include the brachiopods, ammonites, acritarchs, trilobites, and conodonts. As with most extinction events specialist taxa occupying small niches were greater affected than those with wider tolerances (McGhee 1996).
The Hangberg event impacted both marine and freshwater communities, impacting ammonites and trilobites, as well as jawed vertebrates including our tetrapod ancestors (Sallan and Coates 2010). The Hangenberg is linked to the extinction of 44% of high-level vertebrate clades and the complete turnover of the vertebrate biota (Sallan and Coates 2010).
Postulated causes
The sedimentological record shows that the late Devonian was a time of environmental change, which directly affected organisms and caused extinction. During the middle and late Devonian there is evidence of widespread anoxia in oceanic bottom waters, the rate of Carbon burial rapidly increased, and benthic organisms were decimated especially in the tropics and reef communities (Algeo 1998). There is also strong evidence for high-frequency sea-level changes throughout the kellwasser event, the Hangenberg event has also been associated with sea-level rise followed rapidly by glaciation-related sea-level fall (Brezinksi et al. 2009).However, the cause of these changes is open to debate.
Possible triggers include:
1/ Bolide impact. However, there is no secure evidence of a specific impact during this event. Craters which are believed to be of this age often cannot be dated with sufficient precision to link them to the extinction event, and those which have been dated precisely have been found to be not contemporaneous with the extinction (Racki 2005).
2/ Plant evolution. During the Devonian land plants underwent an extremely significant phase of evolution, increasing their maximum height from 30 cm to 30 m. This increase in height was made possible due to the evolution of advance vascular systems allowing the growth of complex branching and rooting systems. Seeds also developed during this time allowing dispersal in areas previously inhospitable to plants such as upland and inland areas.
This affected weathering due to the plant root systems breaking up the upper layers of bedrock and stabilising a deep layer of soil. Soil promotes the chemical breakdown of rocks, releasing ions acting as nutrients to plants and algae. If these nutrients were input into a river eutrophication and subsequent anoxia may occur, which may have caused an extinction.
Increased weathering of silicate rocks would have likely drawn down Carbon Dioxide from the atmosphere, decreasing levels from ~15 times present levels to ~3 times. This reduction in concentrations would have likely led to global cooling.
The increase of plants on the continents during the Devonian would have also had a significant effect on Carbon Dioxide levels. An increase in photosynthesizing land plants is likely to have reduced Carbon Dioxide levels and produced a cooler climate. There is evidence of glacial deposits in northern Brazil (located in the South Pole during the Devonian) suggesting widespread glaciation at this time. This switch from a warm climate to a much cooler one may have led to extinctions of many species.
If these two reductions in Carbon Dioxide had acted alongside each other it is likely that significant environmental change would be experienced, with the Earth being pulled out of the 'greenhouse' state and into the 'icehouse' state that continued through the Carboniferous and Permian.
References
Major changes in the biota of the Devonian appear to have been caused by rapid environmental changes, yet the causes of these changes is still debated. Environmental changes may have occurred due to an extra-terrestrial impact, or due to the significant vegetation changes that occurred during the Devonian. This extinction event had the smallest magnitude of all five events, however ~75% of all species were still lost. These gaps in biota were subsequently filled by other species adapted to the new 'icehouse' environment of the Carboniferous.
References
Algeo, T. J. (1998). Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events. Philosophical Transactions of the Royal Society B: Biological Sciences. 353: 113-130.
Barnosky, A. D., et al. (2011). Has the Earth's sixth mass extinction already arrived? Nature. 471: 51-57.
Brenchley, P. J. (2001). Extinction: Late Ordovician mass extinction. Encyclopedia of Life Sciences.
Brezinski, D. K. et al. (2009). Evidence for long-term climate change in upper Devonian strata of the central Appalachians. Palaeogeography, Palaeoclimatology, Palaeoecology. 284: 315-325.
Caplan, M. L. and R. M. Bustin. (1999). High-resolution isotope stratigraphy of the Devonian-Carboniferous boundary in the Namur-Dinant Basin, Belgium.
McGhee, G. R. (1996) The late Devonian mass extinction: the Frasnian/Famennian crisis. Columbia University Press.
Racki, G. (2005). Toward understanding late Devonian global events: few answers, many questions. Understanding late Devonian and Permian-Triassic biotic and climatic events: Towards an integrated approach. 5-36. Elsevier.
Sallan, L. and M. Coates (2010). End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates. Proceedings of the National Academy of Sciences. 107: 10131-10135.
Stigall, A. L (2011). Speciation decline during the late Devonian biodiversity crisis related to species invasions. Public Library of Science.
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