The Hangenberg event, also known as the Hangenberg crisis or end-Devonian extinction, is a mass extinction that occurred at the end of the Famennian, the last stage in the Devonian Period (roughly 358.9 ± 0.4 million years ago). It is usually considered the second-largest extinction in the Devonian Period, taking place around 13 million years after the Late Devonian mass extinction (Kellwasser event) at the Frasnian-Famennian boundary. The event is named after the Hangenberg Shale, which is part of a sequence that straddles the Devonian-Carboniferous boundary in the Rhenish Massif of Germany.
Geological evidence The Hangenberg Event can be recognized by its unique multi-phase sequence of sedimentary layers, representing a relatively short interval of time with extreme fluctuations in the climate, sea level, and diversity of life. The entire event had an estimated duration of 100,000 to several hundred thousand years, occupying the upper third of the 'Strunian' (latest Famennian), and a small portion of the early Tournaisian. It is named after the Hangenberg Black Shale, a distinctive layer of anoxic sediment originally found along the northern edge of the Rhenish Massif in Germany. This layer and its surrounding geological units define the "classic" Rhenish succession, one of the most well-studied geological examples of the extinction. Sequences equivalent to the Rhenish succession have been found at over 30 other sites on every continent except Antarctica, confirming the global nature of the Hangenberg Event.
Prelude and extinction – the lower crisis interval Below the Hangenberg Event strata is the Wocklum Limestone, a pelagic unit rich in fossils (especially ammonoids). In some places the Wocklum Limestone grades into the Drewer Sandstone, a thin turbidite deposit which initiates the lower crisis interval. Increased erosion and siliciclastic input indicates that the Drewer Sandstone was deposited during a minor marine regression (sea level fall). This may have been caused by a small glacial phase, but other evidence suggests a warm and wet climate at the time. The uppermost part of the Wocklum Limestone and the Drewer Sandstone occupy the LE spore zone. They also belong to the praesulcata conodont zone (named after Siphonodella/Eosiphonodella praesulcata) and the DFZ7 foraminifera zone (characterized by Quasiendothyra kobeitusana). The last pre-extinction ammonoid faunas are dominated by wocklumeriids, forming the Wocklumeria genozone (also known as the UD VI-D zone). A very short subzone (UD VI-D2) diagnosed by Epiwocklumeria occurs in the first few layers of the lower crisis interval.
The main marine extinction pulse begins abruptly with the subsequent deposition of the Hangenberg Black Shale, a layer of organic material deposited in anoxic deep-water environments. This is correlated with the beginning of the LN spore zone, indicated by the first occurrence of Verrucosisporites nitidus. However, in some areas the boundary between the LE and LN zones is unclear and possibly based on geography more than chronology. The black shale was deposited during a large marine transgression (sea level rise), as indicated by flooding reducing the input of terrestrial spores and increasing eutrophication. The Hangenberg Black Shale corresponds to the Postclymenia zone (UD VI-E), an ammonoid genozone based on massive extinctions within the group, rather than new occurrences. This is also the case for the costatus–kockeli Interregnum (ckI) conodont zone. Foraminifera disappear from the fossil record during the black shale interval. Uranium-Lead dating of ash beds in Poland provide dates of 358.97 ± 0.11 Ma and 358.89 ± 0.20 Ma below and above the black shale. This constrains the main marine extinction pulse to a duration of 50,000 to 190,000 years.
Glaciation – the middle crisis interval
In the middle crisis interval, the black shale grades into a thicker deposit of more oxygenated shallow-water sediment. It may be represented by shale (Hangenberg Shale) or sandstone (Hangenberg Sandstone), and fossils are still rare. These layers are still within the ckI conodont zone and LN spore zone, and foraminifera are still absent. However, ammonoid fossils switch over to the lower Acutimitoceras (Stockumites) genozone (UD VI-F), indicating that post-Devonian ammonoids were beginning to diversify after the main extinction pulse. A major marine regression occurred during the middle crisis interval, as indicated by the increased amount of erosion and river-supplied siliciclastic material. Some areas even show deep incised valley fill deposits, where rivers have cut into their former floodplains. Strata in Morocco suggest that the sea level fell by more than 100 meters (328 feet) during the middle crisis interval.
This regression was caused by a cooling episode, and time-constrained glacial deposits have been found in Bolivia and Brazil (which would have been high-latitude areas), as well as the Appalachian Basin (which would have been a tropical alpine environment). These are known to have been deposited within the LE and/or LN spore zones, which are difficult to distinguish outside of Europe. Less well-constrained glacial deposits have also been found in Peru, Libya, South Africa, and central Africa. The Late Famennian glacial phase, along with other short glacial phases in the Tournaisian and Visean, acted as a prelude to the far larger and more prolonged Late Paleozoic Ice Age which stretched across much of the Late Carboniferous and Early Permian.
Aftershocks – the upper crisis interval The upper crisis interval begins with the return of prominent carbonate rocks: a marly unit, the Stockum Limestone, spans the Devonian–Carboniferous (D–C) boundary. Foraminifera reappear in the fossil record within the Stockum Limestone, forming the DFZ8 zone characterized by Tournayellina pseudobeata. The base of the Stockum Limestone also sees the beginning of the Protognathodus kockeli conodont zone and further ammonoid diversification within the upper Acutimitoceras (Stockumites) genozone (LC I-A1). A major extinction among land plants and palynomorphs indicates the beginning of the VI spore zone shortly before the D–C boundary. 'Survivor' faunas of marine invertebrates, such as the last cymaclymeniid ammonoids and phacopid trilobites, also die out at this time, making it the second largest extinction pulse of the Hangenberg Crisis. Conodont zones (usually characterized by Protognathodus kuehni or Siphonodella/Eosiphonodella sulcata) define the D–C boundary, but difficulty in finding reliable and universal index taxa has complicated study of the boundary in many areas. The sea level fluctuated during the upper crisis interval, as several minor regressions and transgressions continued to occur around the D–C boundary. Nevertheless, the general trend was sea level rise, with the melting of the glaciers which formed in the middle crisis interval. In the early Tournaisian, the crisis finally ends at the base of the Hangenberg Limestone, a fossiliferous limestone superficially similar to the pre-crisis Wocklum Limestone. The base of the Hangenberg Limestone is characterized by the first occurrence of gattendorfiine ammonoids (making up the Gattendorfia genozone, LC I-A2) and the MFZ1 foraminifera zone.
Extinction severity
Along with the Givetian and Frasnian stages, the Famennian was qualitatively acknowledged as having elevated extinction rates as early as Raup and Sepkoski's 1982 landmark paper on mass extinctions. However, late Famennian extinction rates were typically considered to be of lesser taxonomic severity than those in the Kellwasser Event, one of the "big five" mass extinctions. Depending on the method used, the Hangenberg Event typically falls between the fifth and tenth deadliest post-Cambrian mass extinctions, in terms of marine genera lost. Most estimates of proportional extinction have low resolution, only as fine as the stages in which the extinctions occur. This can lead to uncertainty in differentiating between the Hangenberg Event and other Famennian extinctions in broad-scale extinction trackers.
Benton (1995) estimated that 20–23.7% of all families went extinct in the Famennian, with marine families at a proportion of 1.2–20.4%. About 27.4–28.6% of continental families appear to have died out, but the early and low-diversity nature of Devonian continental life makes this estimate very imprecise.
Sepkoski (1996) plotted extinction rates for marine animal genera and families throughout the Phanerozoic. This study found that >45% of genera were lost during the Famennian, lowered to ~28% considering only "multiple interval" genera which appeared prior to the stage. The Famennian extinction(s) would be the eighth worst mass extinction by the latter metric. He also found that the percentage loss of "well-preserved" (hard tissue) marine genera in the last substage of the Famennian was around 21%, nearly as large as the rate in the last substage of the Frasnian. The Famennian-wide extinction rate for "multiple interval" marine animal families was around 16%. All of these estimates approached, but did not surpass, the end-Frasnian extinction, and the Givetian extinction(s) also surpassed the Famennian extinction(s) in the "multiple-interval" and "well-preserved" full-stage categories.
Using an updated biodiversity database, Bambach (2006) estimated that a total of 31% of marine genera died out in the last substage of the Famennian. By this metric, the Hangenberg Event was the joint seventh-worst post-Cambrian mass extinction, tied with the poorly-studied early Serpukhovian extinction in the Carboniferous.
McGhee et al. 2013 attempted to tackle extinction rates via a new resampling protocol designed to counter biases in biodiversity estimates, such as the Signor–Lipps effect and Pull of the Recent. They found a significantly higher extinction rate, with 50% of marine genera lost during the event. This estimate would rank the end-Famennian extinction as the fourth-deadliest mass extinction, ahead of the end-Frasnian extinction. They also ranked the end-Famennian mass extinction as the seventh most ecologically severe extinction, tied with the Hirnantian (end-Ordovician) mass extinction. This was justified by the fact that two whole communities within an ecological megaguild went extinct with no replacements. For the end-Famennian, these were chitinozoans within the pelagic filter-feeder megaguild, and stromatoporoids within the attached epifaunal (seabed-living) filter-feeder megaguild. Other taxa impacted by the extinction rediversified or their niches were filled rather quickly, but these communities were exceptions. By comparison, the end-Frasnian extinction was ranked as the fourth most ecologically-severe mass extinction, and the Givetian crisis was ranked as the eighth. Even in areas with oxygenated seabeds, such as parts of Morocco, the ecospace of Hangenberg event communities was restricted to only a few ecological groups, particularly slow-moving pelagic predators (i.e. ammonoids and acanthodians) and epifaunal sessile filter feeders (i.e. bivalves and bryozoans).
Impact on life
Reef builders Reef ecosystems disappeared from the fossil record during the Hangenberg Event, not returning until the late Tournaisian. Metazoan (coral and sponge) reefs had already been devastated by the Frasnian–Famennian event, and were still recovering during the Famennian. The end of the Famennian not only eliminated the metazoan reef community, but also many calcimicrobial reefs which were previously unscathed. Nevertheless, in the absence of pressures from metazoan communities, there was a brief resurgence of microbial carbonate in the early Tournaisian, a similar pattern to other mass extinctions.