Capitanian mass extinction event

The Capitanian extinction event was an extinction event that occurred around 260 million years ago during a period of decreased species richness and increased extinction rates in the late Middle Permian during the Guadalupian epoch. It is also known as the end-Guadalupian extinction event because of its initial recognition between the Guadalupian and Lopingian series; however, more refined stratigraphic study suggests that extinction peaks in many taxonomic groups occurred within the Guadalupian, in the latter half of the Capitanian age.[2]

Marine extinction intensity during the Phanerozoic
%
Millions of years ago
Plot of extinction intensity (percentage of genera that are present in each interval of time but do not exist in the following interval) vs time in the past for marine genera.[1] Geological periods are annotated (by abbreviation and colour) above. The Capitanian extinction event occurred 260–259 million years ago, ~7 million years before the Permian–Triassic extinction event, with just over 35% (according to this source) failing to survive. (source and image info)

Magnitude

In the aftermath of Olson's Extinction, global diversity rose during the Capitanian. This was probably the result of disaster taxa replacing extinct guilds. The Capitanian mass extinction greatly reduced disparity (the range of different guilds); eight guilds were lost. It impacted the diversity within individual communities more severely than the Permian–Triassic extinction event.[3] Although faunas began rebuilding complex trophic structures and refilling guilds after the Capitanian extinction event,[3] diversity and disparity fell further until the Permian–Triassic boundary.[4]

Marine ecosystems

The impact of the Capitanian extinction event on marine ecosystems is still heavily debated by palaeontologists. Early estimates indicated a loss of marine invertebrate genera between 35–47%,[5][6] while an estimate published in 2016 suggested a loss of 33–35% of marine genera when corrected for background extinction, the Signor–Lipps effect and clustering of extinctions in certain taxa.[7] The loss of marine invertebrates during the Capitanian mass extinction was comparable in magnitude to the Cretaceous–Paleogene extinction event.[8] Some studies have considered it the third or fourth greatest mass extinction in terms of the proportion of marine invertebrate genera lost; a different study found the Capitanian extinction event to be only the ninth worst in terms of taxonomic severity (number of genera lost) but found it to be the fifth worst with regard to its ecological impact (i.e., the degree of taxonomic restructuring within ecosystems or the loss of ecological niches or even entire ecosystems themselves).[9]

Terrestrial ecosystems

Few published estimates for the impact on terrestrial ecosystems exist for the Capitanian mass extinction. Among vertebrates, Day and colleagues suggested a 74–80% loss of generic richness in tetrapods of the Karoo Basin in South Africa,[10] including the extinction of the dinocephalians.[11] In land plants, Stevens and colleagues found an extinction of 56% of plant species recorded in the mid-Upper Shihhotse Formation in North China,[12] which was approximately mid-Capitanian in age. 24% of plant species in South China went extinct.[13]

Timing

Although it is known that the Capitanian mass extinction occurred after Olson's Extinction and before the Permian–Triassic extinction event,[3] the exact age of the Capitanian mass extinction remains controversial. This is partly due to the somewhat circumstantial age of the Capitanian–Wuchiapingian boundary itself, which is currently estimated to be approximately 259.1 million years old,[10][11][14] but is subject to change by the Subcommission on Permian Stratigraphy of the International Commission on Stratigraphy. Additionally, there is a dispute regarding the severity of the extinction and whether the extinction in China happened at the same time as the extinction in Spitsbergen.[15]

The volcanics of the Emeishan Traps, which are interbedded with tropical carbonate platforms of the Maokou Formation, are unique for preserving a mass extinction and the cause of that mass extinction.[13] Large phreatomagmatic eruptions occurred when the Emeishan Traps first started to erupt, leading to the extinction of fusulinacean foraminifera and calcareous algae.[16]

In the absence of radiometric ages directly constraining the extinction horizons themselves in the marine sections, most recent studies refrain from placing a number its age but based on extrapolations from the Permian timescale an age of approximately 260–262 Ma has been estimated;[10][17] this fits broadly with radiometric ages from the terrestrial realm, assuming the two events are contemporaneous. Plant losses occurred either at the same time as the marine extinction or after it.[13]

Marine realm

The extinction of fusulinacean foraminifera in Southwest China was originally dated to the end of the Guadalupian, but studies published in 2009 and 2010 dated the extinction of these fusulinaceans to the mid-Capitanian.[18] Brachiopod and coral losses occurred in the middle of the Capitanian stage.[19] The extinction suffered by the ammonoids may have occurred in the early Wuchiapingian.[19]

Terrestrial realm

The existence of change in tetrapod faunas in the mid-Permian has long been known in South Africa and Russia. In Russia, it corresponded to the boundary between what became known as the Titanophoneus Superzone and the Scutosaurus Superzone[20] and later the Dinocephalian Superassemblage and the Theriodontian Superassemblage, respectively. In South Africa, this corresponded to the boundary between the variously named Pareiasaurus, Dinocephalian or Tapinocephalus Assemblage Zone and the overlying assemblages.[21][22][23][24] In both Russia and South Africa, this transition was associated with the extinction of the previously dominant group of therapsid amniotes, the dinocephalians, which led to its later designation as the dinocephalian extinction.[25] Post-extinction origination rates remained low through the Pristerognathus Assemblage Zone for at least 1 million years, which suggests that there was a delayed recovery of Karoo Basin ecosystems.[26]

After the recognition of a separate marine mass extinction at the end of the Guadalupian, the dinocephalian extinction was seen to represent its terrestrial correlate.[8] Though it was subsequently suggested that because the Russian Ischeevo fauna, which was considered the youngest dinocephalian fauna in that region, was constrained to below the Illawarra magnetic reversal and therefore had to have occurred in the Wordian stage, well before the end of the Guadalupian,[25] this constraint applied to the type locality only. The recognition of a younger dinocephalian fauna in Russia (the Sundyr Tetrapod Assemblage)[27] and the retrieval of biostratigraphically well-constrained radiometric ages via uranium–lead dating of a tuff from the Tapinocephalus Assemblage Zone of the Karoo Basin[10][28] demonstrated that the dinocephalian extinction did occur in the late Capitanian, around 260 million years ago.

Effects on life

Marine life

In the oceans, the Capitanian extinction event led to high extinction rates among ammonoids, corals and calcareous algal reef-building organisms, foraminiferans, bryozoans and brachiopods. It appears to have been particularly selective against shallow-water taxa that relied on photosynthesis or a photosymbiotic relationship; many species with poor physiological buffering also became extinct.[29]

The ammonoids, which had been in a long-term decline for a 30 million year period since the Roadian, suffered a selective extinction pulse at the end of the Capitanian.[4] 75.6% of coral families, 77.8% of coral genera and 82.2% of coral species that were in Permian China were lost during the Capitanian mass extinction.[30] The Verbeekinidae, a family of large fusuline foraminifera, went extinct.[31]

87% of brachiopod species found at the Kapp Starostin Formation on Spitsbergen disappeared over a period of tens of thousands of years; though new brachiopod and bivalve species emerged after the extinction, the dominant position of the brachiopods was taken over by the bivalves.[32] Approximately 70% of other species found at the Kapp Starostin Formation also vanished.[33] The fossil record of East Greenland is similar to that of Spitsbergen; the faunal losses in Canada's Sverdrup Basin are comparable to the extinctions in Spitsbergen and East Greenland, but the post-extinction recovery that happened in Spitsbergen and East Greenland did not occur in the Sverdrup Basin.[17] Whereas rhynchonelliform brachiopods made up 99.1% of the individuals found in tropical carbonates in the Western United States, South China and Greece prior to the extinction, molluscs made up 61.2% of the individuals found in similar environments after the extinction.[34] 87% of brachiopod species and 82% of fusulinacean foraminifer species in South China were lost.[17]

Most of the marine victims of the extinction were either endemic species of epicontinental seas around Pangaea that died when the seas closed, or were dominant species of the Paleotethys.[35]

After the Capitanian mass extinction, disaster taxa such as Earlandia and Diplosphaerina became abundant in what is now South China.[2]

Terrestrial life

Among terrestrial vertebrates, the main victims were dinocephalian therapsids, which were one of the most common elements of tetrapod fauna of the Guadalupian; only one dinocephalian genus survived the Capitanian extinction event.[8] The diversity of the anomodonts that lived during the late Guadalupian was cut in half by the Capitanian mass extinction.[36] Terrestrial survivors of the Capitanian extinction event were generally 20 kg (44 lb) to 50 kg (110 lb) and commonly found in burrows.[8]

Causes

'A combination of a second-order global drop of sea level and regional constraints, e.g., salinity fluctuations (Weidlich and Bernecker, in press) or collisions of micro-continents account for the destruction of many reef sites.'[37]

It is believed that the extinction was triggered by one or more eruptions of the Emeishan Traps, which released a large amount of carbon dioxide and sulfur dioxide into the stratosphere of the Northern and Southern Hemispheres due to the equatorial location of the Emeishan Traps, leading to sudden global cooling and long-term global warming; the eruptions also triggered ocean acidification, a depletion of seafloor oxygen and a severe disturbance of the carbon cycle.[12][15][16][17][29][32] The eruptions would have released high doses of toxic mercury.[38] Basalt piles from the Emeishan Traps currently cover an area of 250,000 to 500,000 km2, but the original volume of the basalts may have been anywhere from 500,000 km3 to over 1,000,000 km3.[29] Acid rain, global drying, plate tectonics, marine regression and biological competition may have also played a role in the extinction.[2][12][30] The extinction coincided with the beginning of a major negative δ13C excursion.[13]

References
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