"the late devonian mass extinction event" by sharon goehring (2001)

THE LATE DEVONIAN MASS EXTINCTION EVENT

Sharon Goehring

GEOL 345: Paleontology

December 4, 2001


As one of five major extinction events, the Late Devonian was a time when life was devastated onEarth. Worldwide, all marine and terrestrial ecosystems felt its effects. Studies show flora and faunaexperienced major losses at all taxonomic levels from microscopic algae and invertebrates to the firstterrestrial amphibians. Many experts offer reasons for these extinctions including climatic changes,tectonics, sea level fluctuations, and asteroid impacts. However, no single theory has been accepted asmost believe a combination of events affected global conditions.

BACKGROUND

The Devonian period is divided into three epochs: Early, Middle, and Late. These epochs arefurther subdivided into several stages. The mass extinction event occurred around the Frasnian andFamennian stages of the Late Devonian. The Frasnian began about 377 million years ago and ended 367million years ago with the start of the Famennian. The Devonian ends after the Famennian at itsboundary with the Tournaisian stage of the Early Carboniferous, about 362 million years ago.

Conodont Zonation

Famennian ca 365.5 Ma Early or Lower crepidaca 366 Ma Late or Upper triangularisca 366.5 Ma Middle triangularis

Frasnian ca 367.5 Ma Linguiformis

Uppermost gigasca 368 Ma Late rhenana

Upper gigas & Lower gigasca 368.5 Ma Early rhenana

Lower gigasTable 1: Conodont Zonation of the Devonian (McGhee,1996)

Biostratigraphic zonations of thetype section provide detailed subdivisionsof the Devonian. The Frasnian andFamennian stages are zoned byconodonts. Table 1 provides the details ofthe official zones. The boundary typesection is at an abandoned quarry inCoumiac of Montagne Noire, France andis equivalent to the conodont linguiformisand triangularis zones (Walliser, 1996a). The extinction at Frasnian-Famennian(F-F) boundary is known as theKellwasser event. The upper Famennianextinction, at the boundary with theCarboniferous, is known as theHangenberg event. This later extinction is only 70% as severe as the Kellwasser (Sepkoski, 1996).

Life during the Early and Middle Devonian experienced great diversification. Animals movedfrom the oceans onto land, including the first amphibians, snails, oligochaete worms, nematodes,scorpions, millipedes and centipedes (Copper, 1986). It was the age of fishes. Insects began a


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radiational period. Previously, only primitive plants grew, but during the Devonian flora spread anddominated the land. The first forests grew in response to soil development (Copper, 1986).

EXTINCTION EVENT DEFINITION

To define an extinction as a major event, the main consideration must be its severity. McGhee(1996) defines a major extinction as one which affects a minimum of 15% of life diversity at the familylevel in less than fifteen million years. It must affect worldwide terrestrial and marine environments andinclude both flora and fauna. Sepkoski (1982) believes mass extinctions occur when there is an abrupttermination of many or most species at a single horizon or within a limited stratigraphic interval. It mustbe widespread and independent of facies changes. The appearance of species above the event should notbe closely related to those below it.

Many disagree whether the Devonian extinction occurred as one event or several smaller pulses. McGhee (1996) separates the mass extinction into two events. The first occurred between the laterhenana zone through the middle triangularis, with the most devastating period during the linguiformiszone. The second smaller event occurred in the early crepida zone. Specifically, the conodont extinctionseen in the Schmidt Quarry in Germany has five centimeters of shale lacking conodont fossils betweenthe Frasnian linguiformis zone and the triangularis zone. Sandberg estimated this gap ranged from12,500 years to days at minimum, while Schindler believes it took a few hundred thousand years(Walliser, 1996a).

Floral and fauna losses were severe. 60% of existing taxa experience extinction at the end of theFrasnian (McGhee, 1982). Losses are estimated at 13% to 38% at the family level, compounding furtherdown the taxonomic hierarchy with 55% to 60% losses at the genus level and 70% to 82% species loss(McGhee, 1996). Before the event, there were huge coral reefs comprised mostly of tabulate corals andstromatoporoid sponges. These ecosystems were seriously hit during the Late Devonian allowinghermatypic Scleractinas to become the major reef building organisms later. Terrestrial life was alsoaffected with 43% to 50% plant species lost.

LOSS OF LIFE

Algae and zooplankton extinction rates are often disputed as they are difficult to determine. Thefossil record of algae and phytoplankton is poor and does not include all species. Indirect evidence existsfor the extinction of nonpreservable phytoplankton. Over 90% of phytoplankton are affected by theevent (Rossbach, 1989). About 60% of Prasinophycean green algae and 81% of the Acritarch species dieby the Carboniferous (McGhee, 1996). The calcareous algae are better preserved than thephytoplankton. The Receptaculitid chlorophytes were eliminated (McGhee, 1996). Many expertsdisagree on the extinction of the Cricoconarida. Some argue one species of the family, Styliolinidae,survived while others believe them extinct in Famennian. Schindler (1990) places the extinction of Homoctenidae in the Famennian, while some place their loss in the Frasnian. Uncontested Frasnianextinctions include five families of the chitinozoans: the Ancyrochitinidae, Conochitinidae,Hoegispharidae, and Lagenochitinidae. The family Desmochitinidae made it past the F-F boundary, butwere eliminated during the Tournaisian (McGhee, 1996).

Foraminiferal extinction rates seem related to the composition and structure of their tests. Allsiliceous agglutinated test forams survive, while 45% of calcareous forams went extinct. The specieswith primitive spherical, uniserial, and agglutinated tests survived, while species with advanced septate



architectures were eliminated. All the pseudoseptate and quasi-septate test forams suffer significantreductions. On the family level, Semitextulariidae, Multiseptidae, Nanicellidae, and Paratextulariidae,and Eogeinitzinidae were eliminated. Thirty species of the Semitextulariina-Nodosarioid assemblage andover 70% of the Tournayellidae species were lost (McGhee, 1996).

In the Devonian, almost all stromatoporoids disappeared worldwide, with fore-reef, reef andlagoonal forms especially hard hit (McLaren, 1970). Studies show their decline began before the end ofthe Frasnian. They lost eleven families from the Givetian to the end of the Frasnian stage (Sepkoski,1982). 46% of genera went extinct at the F-F boundary with only the most primitive orders surviving: the clathrodictyids and labechiids. Studies in Alberta showed a decrease from thirteen to eight speciesduring the Early to Late Frasnian and similar losses in Belgium and Afghanistan (Farsan, 1986; Stearn,1987).

Corals experienced major losses in the Late Devonian extinction event. Only ten of 157 coralsseen in the late Frasnian survive (McLaren, 1982). More species of tabulates and rugosans went extinctin the Devonian than the surviving species that died at the Permian-Triassic event. 25 families of rugoseand tabulate corals were lost (Sepkoski, 1982). Tabulates lost 80% to 92% of their genera, with thefavosites becoming extinct. The branching form of coenenchymal imperforates were lost. The familyPhillipsastraiidae became extinct in the Frasnian with rare unverified exceptions making it into theFamennian (McLaren, 1970). Rugosan extinction rates depended on whether the species was colonial orsolitary and shallow verses deep water. About 29 of 45 rugosan genera went extinct in the Frasnian(Pedder, 1982). Overall, the solitary Rugosans were unaffected at the genus level while colonial generasexperienced a loss of 60%. Four of 148 shallow water rugosan species survive into the Famennian(Pedder, 1982) compared to a 30% to 40% survival rate of deep water species (Rossbach and Hall, 1998).

Solitary forms of bryozoans were devastated by the extinction. Bryozoans diversified during theDevonian, however their radiation ended at the F-F boundary which saw the loss of a group of bryozoans(Copper, 1986). In total, about 33% of bryozoan genera went extinct (Rossbach and Hall, 1998).

Until their devastation during the Devonian extinction, the brachiopods were the most dominantshellfish. Seven groups of brachiopods were lost (Copper, 1986). Articulate brachiopods lost seventeenfamilies (Sepkoski, 1982) and had only ten of 71 genera survive (Johnson, 1974). The loss was feltharder in the low-latitude tropical regions with a 91% family extinction rate compared to the 27%extinction of families with high-latitude, cool-water environments (McGhee,1989). The Stropheontidaefamily and the Orthacea, Pentameracea, and Atrypacea orders were eliminated (McLaren, 1970). TheOrthida and Strophomenida experienced severe losses (Rossbach and Hall, 1998). During the Devonian,the Old World Realm species of western North America emigrated and proliferated in the New WorldRealm of eastern North America. The complete loss of the New World Realm species could be due tothe dominance of the Old World species and not necessarily caused by the extinction event itself asexemplified by the loss of Mucrospirifer mucronatus and Orthospirifer mesastrialis (McGhee, 1996).

Many extinctions were experienced in the Mollusca. Cephalopods were severely affected, withthe loss of fourteen families during five periods of diversity crises (Sepkoski, 1982). Clymeniidaebecame extinct, except the species Cymaclymenia evoluta (Walliser, 1996b). Only eight cephalopodgenera survived, with nautiloid cephalopods losing 29 genera during the Frasnian. The gephuroceratid,beloceratid, and many tornoceratid and anarcestid ammonoids went extinct. All together, ammonoidslost 88% of their species during this event (Walliser, 1996a). Bivalves, gastropods and other mollusks



were also affected. Generally, gastropods survived the event, although it is argued that the familyPalaeotrochidae went extinct. Four families and eight genera of bivalves were lost including the familiesof Antipleuridae and Ambonychiidae. In a phylum similar to the molluscans, Tentaculites went extinctprior to the Famennian (Schindler, 1990).

Many different families of echinoderms were affected by the Late Devonian extinction. Thecarpoids and rhombiferans were lost. Three families of fissiculate blastoids went extinct, althoughgenerally Blastozoa and Echinozoa were not affected. 42% of Asterozoas were lost with five familiesbecoming extinct. The Crinozoa lost fifteen families or 32% of their family diversity (McGhee, 1996).

The arthropods suffered losses at the Devonian extinction, although many survived to radiatelater. The trilobites experienced a steady decline from the Middle to Late Devonian in losses notassociated with the Devonian extinction. Eight Givetian families of trilobites were lost in the Frasnianwith two families surviving into the Famennian (McLaren, 1982). The Scutellidae family were lost. Trilobites are almost unknown in the Early and Middle Famennian of North America (McLaren, 1982). Sixteen of 28 genera of trilobites became extinct during the Frasnian (McLaren, 1982), representingabout 42% of their genera (Rossbach and Hall, 1998). The Illaenacea, Harpina, Lichida, andOdontopleurida trilobites went extinct (Rossbach, 1989). Malacostracan crustaceans lost 68% of theirspecies in the late Frasnian with only seven species surviving. They radiate in the mid-Famennian onlyto experience a 88% species loss in the Late Famennian with only three species making it into theCarboniferous (McGhee, 1996). The ostracods lost four benthic families during the Givetian. Overall,the benthic ostracods were more affected than the planktonic varieties (Walliser, 1996a). In the Frasnian,three more benthic families were lost. All total, about 60% of ostracod species went extinct at the F-Fborder. The fossil record of myriapods and primitive hexapods is too fragmentary to determine definiteextinction numbers. The eurypterids lost 27% of their genera at the F-F boundary and lost 63% moreduring the Famennian. The conchostracans lost 33 species in the Early Frasnian, 29 more in the LateFrasnian, and had only one surviving species reach the Famennian. Some experts believe these numbersto be exaggerated due to the poor fossil records of these groups (McGhee, 1996).

Conodonts experienced severe losses during the Devonian. Ancyrodellid were eliminated andalmost all species of palmatolepids, polygnathids and ancyrognathids were lost by the Late Frasnian(Schindler, 1990). All together, 89% of conodont species were lost by the Famennian. The zoningspecies, lingiformis experienced a drop in diversity from thirteen species to one during no more than20,000 years (McGhee, 1996).

Both salt and fresh water fish experience the Devonian extinction event. Of the salt waterspecies, the last of the remaining agnathan fishes die. The groups heterostracans and thelodonts wereflourishing during the Frasnian, but died at the F-F boundary. The jawed fishes, gnathostomes, had manyextinctions. The placoderms lost 65% and the acanthodians lost 87% of their species respectively. Twoorders of the chondrichthyans and four families of the ostreichthyans went extinct. Fresh water fish alsoexperienced losses. As with their salt water relatives, the fresh water agnathans became extinct. 35% ofthe placoderm and 30% of the acanthodian species died. These numbers reflect that the fresh waterspecies fared better than their salt water counterparts (McGhee, 1996).

The Devonian was the period when the amphibians began their terrestrial life, therefore, theirfossil record is fragmentary. Generally, their losses are calculated by a gap in the Frasnian fossil record. Although present in the Frasnian, for about 4 to 5 million years there is no record of amphibians before



their reappearance in the Famennian. This is assumed to be evidence of their losses from the extinctionevent (McGhee, 1996).

Flora experienced extinction during the Devonian and land plants were among them. For fossiltaxonomy of plants, species are split into two types. The spore species, known from their reproductivestructures, are called microfloral species. The macrofloral species, or leaf species, are known from therecord of their leaves, roots, and bark. The microflorals experienced a decline from 57 to 51 speciesfrom the Givetian to the Frasnian. During the F-F extinction, 43% of the spore species were lost andlater declined further with only 29 species remaining by the Carboniferous. Many experts doubt thevalidity of these numbers as they could result from poor preservation of spore fossils. Macrofossils alsoexperienced a great decline from the Givetian to the Frasnian with a loss of about half of their species,but did not experience severe losses during the remaining Devonian. A few species became extinct in theearly Frasnian and a group was lost during the Famennian (McGhee, 1996).

SUGGESTED CAUSES

Experts can only speculate on the causes for the Late Devonian extinction. Many reasons aresuggested from climatic changes or tectonics to a bolide impact. Regressive and transgressive cycles andrelated anoxic conditions have been blamed for the extinctions. One popular idea for the Kellwasserevent is the possibility of a single or multiple asteroid impacts. Although many theories have beenoffered, none are assumed to be solely responsible. Many believe the extinction was caused by acombination of factors culminating in the loss of worldwide biota.

One theory offered for the Devonian extinction is global cooling, as it could lead to the disruptionof marine environments (www.owlnet, 2001). Cooling due to paleogeography has been suggested. Copper (1986) theorized the ocean between Laurussia and Gondwana closed at the F-F boundary. Thiswould disrupt the low-latitude circumequatorial flow of warm water. High-latitude colder water wouldflow into equatorial areas on the western margins of the joined continents, creating restricted circulationand anoxic conditions in warm-water basins on the eastern margin. This theory is supported by thesubtropical reef and perireef life experiencing higher extinction rates (Copper, 1986). Also,hyalosponges, a cool-water preferring organism, shows evidence of expansion into shallow marinehabitats, an indicator of cooler oceanic temperatures during the F-F time (McGhee, 1982). The coolingof oceanic waters might have been compounded by a glaciation event in South America (Rossbach andHall, 1998). Evidence of glaciation is found in the Amazonas Basin of South America. Glacialdiamictites of Upper Devonian age have been discovered (Caputo, 1985). However, other supportingevidence for the collision of continents is lacking. Paleomagetic data and biogeographic data place thecollision in the Carboniferous (McGhee, 1989). Also, anoxic waters were present throughout the world,not just on the eastern margin, which undermines the collision theory.

Another possible mechanism for cooling is the global icehouse effect, the opposite of thegreenhouse effect. Lowered levels of carbon dioxide would cause a worldwide loss of temperature. Floral diversification and the increase of plant biomass in fifteen to twenty million years would besignificant enough for the fixation of carbon to seriously deplete atmospheric carbon dioxide levels,leading to global cooling. A side effect would be increased calcium carbonate presence allowing anexplosion in reef growth which would eventually create a depletion of seawater bicarbonate. In turn, thiswould cause the extinction of reef organisms and the creation of anoxic conditions as shallow marineoxygen levels are affected by episodic nutrient pulses. Some experts believe the drop in carbon dioxide



to only be half-way through its decline until the Late Carboniferous when oxygen levels increased again. If so, extinctions should have continued until that point (McGhee, 1996).

Thompson and Newton proposed a theory involving a lethal temperature increase. As manymarine organisms live closer to their upper temperature limits of physiologic tolerances, small changescould bring them into intolerable temperatures. Also, this provides a mechanism for anoxic waters aswarmer water holds less oxygen. This would bring lower organisms up into the shallower waters toreach greater oxygen levels, leading to competition. Isotopic oxygen and carbon studies have supportedthis theory, showing an increase of temperature to over 60EC at the F-F boundary. This differs from themean range of 36EC to 54EC over the earlier Devonian. However, the average lethal temperature fororganisms is 38EC. Thompson and Newton have altered their estimates to bring the boundarytemperature down to 40EC, but because they changed the temperatures derived from their studies,experts doubt their findings. Conflicting evidence in the warmer waters theory is that a hotter worldshould allow reef ecosystems to flourish, but instead they were devastated (McGhee, 1996).

Several theories for the Devonian extinction have been derived from tectonic mechanisms. Fischer and Arthur created a climate model based on tectonic megacycles, changes associated withphases of accelerated plate tectonic activity. With increased magma upwelling and spreading atmid-oceanic ridges, ocean water would be displaced onto continents. An increase in global volcanismwould add carbon dioxide into the atmosphere. The slowing of plate tectonics would mean less carbondioxide enters the atmosphere, starting an icehouse cycle. Rapid changes in these conditions could causea global ecosystem collapse. Studies have shown that the Late Devonian experienced a change fromwarm to colder conditions without rapid fluctuations. Another tectonic process which might have causedthe extinction event is pulsation tectonics, a theory closely related to the tectonic megacycles. Sheridanproposed six cycles occurred involving tectonics and Earth’s magnetic field polarities. According to hisstudy, the Late Devonian falls into a period of warm, humid climate and high sea levels. This would leadto greater reef growth and increased land plant cover, conditions which do not apply in the Famennian. A third theory of tectonics affecting Devonian life is productivity autocycles. This proposal allows foreffects of biotic productivity changes and organic carbon distribution. With high productivity in shallowwaters and stagnation in the deeper ocean, conditions of anoxic water with major deposition of organiccarbon would occur. This would increase the removal of carbon dioxide from the atmosphere, coolingthe global climate. The reestablishment of deep-sea currents would start a greenhouse cycle and a relatedmarine transgression with the spread of anoxic bottom waters onto perched environments. A laterregressive event would lower these deeper waters. These oscillations would create difficult conditionsfor some organisms, but does not necessarily account for all global extinctions at the F-F boundary(McGhee, 1996).

Sea level fluctuations have been suggested as contributors to the Devonian extinction. However,other periods experienced transgressions and regressions without an associated extinction event(McGhee, 1982). One theory involves global regression, although no link has been established betweenlowered sea levels and high extinction rates (McGhee, 1989). Glaciation would cause a regression andloss of habitat for sessile shallow marine organisms or deadly hypersaline conditions in shallow waters. Weakening this theory is that there is no evidence in the United States of the erosion which would occurduring a regressive event (Johnson, 1974). In fact, the rock record provides evidence of a transgressiveevent at the end of the Frasnian (McGhee, 1982) with the F-F boundary occurring during an interval ofglobal sea-level highs (McGhee, 1989). Regression might have taken place during the Famennian withthe start of glaciation in Gondwana, but it would be too late to cause the Kellwasser extinction. A Late



Devonian rapid regressive-transgressive pulse could contribute to the decimation of perched faunas(McGhee, 1989). The disappearance of biohermal reef systems could be due to the drowning andre-emergence by fluctuating sea levels (Walliser, 1996a).

The Devonian extinction might also have been caused by a transgressive event. Transgressioncould lead to the “drowning” of perched reef ecosystems along with the influx of anoxic deeper watersinto shallow areas killing sessile sea floor marine organisms (McGhee, 1989). The characteristic blackshale deposits of the Devonian are accepted as evidence of widespread anoxic waters. Frasnian strata inAlberta containing pyrite and the loss of bioturbation are indicative of anoxic water conditions(Geldsetzer, et al., 1987). McGhee correlated the disappearance of brachiopod species with the blackshale deposits (Rossbach and Hall, 1998). Studies have not shown how the entire global marineecosystem could become anoxic, but two theories have been proposed: oceanic overturn and an asteroidimpact. However, if a severe worldwide overturn occurred, high-latitude organisms should have beenharder hit and evidence shows they were less affected than their low-latitude counterparts (McGhee,1996).

A newer theory offered for the Devonian extinction is widespread eutrophication of marinewaters. This increase in biotic productivity could not only cause extinctions, but also accounts for globalblack shale deposition. Evidence was found in the Geneseo Formation in New York that Devonian seasalternated in periods of thermocline establishment and periods of water column mixing, releasingnutrients which would promote greater productivity. Analysis of carbon, nitrogen, and phosphorous inthe Kellwasser horizons found an anomalous increase of phosphorous and nitrogen (Gillette, 2000). These high levels would promote eutrophication in shallow-water environments. The increase in floraand fauna would create a corresponding increase in carbon deposition, accounting for the formation ofblack shale (Gillette, 2000). Also, the increase in productivity of tropical and subtropical organismsadapted to low nutrient, clear-water conditions would eventually harm their environment. Massivephytoplankton populations would eclipse the water surface eliminating photosynthesis for planktic andnektic organisms (Geldsetzer, et al., 1987).

Asteroid impacts have become a popular theory examined at extinction events. McLaren (1970) proposedthe first bolide-induced extinction for the Late Devonian. To be a single impact, studies have shown thatthe asteroid would need a diameter greater than ten kilometers. If an asteroid of those proportionsimpacted earth, it would kill life in the target area, generate earthquakes, tsunamis, wildfires and ballisticmolten debris. Tsunamis, especially, would affect shallow marine ecosystems. The blast would heat theatmosphere sufficiently so that nitrogen could combine with oxygen to create nitric oxide and nitric acid. Rain falling in high concentrations could poison upper surface waters and destroy phytoplanktonic life. Calcareous shells would dissolve. Wildfires would produce dioxins and aromatic hydrocarbons,poisoning the environment. Significant addition of carbon dioxide into the atmosphere would create anicehouse effect. Global dust clouds could block sunlight, making photosynthesis impossible. Temperatures could drop below survivable ranges for many organisms. Evidence of a Devonian impactcrater has been found. The Siljan Ring in Sweden, dated to the F-F boundary, had the largest diameter of52 kilometers. Many craters have been studied for the

Late Devonian extinction event (table 2), however the dates of many craters are either too wide to beaccepted or dates too uncertain based on differing opinions. Larger craters might have been created inthe ocean floor, but would now be destroyed by tectonics (McGhee, 1996).

Others doubt a single asteroid impact could be responsible for such a severe extinction event.



Crater Dia. (km) Age (Ma)

After Grieve and Robertson (1987)Siljan, Sweden 52 368 +- 1Charlevoix, Quebec 46 360 +- 25Kaluga, USSR 15 380 +- 10Lac La Moinerie, Quebec 8 400 +- 50Crooked Creek, MO 5.6 360 +- 80Flynn Creek, TN 3.8 360 +- 20Table 2: Known Impact Craters that span F-F Boundary (367 Ma) –(McGhee, 1996)

McLaren (1982) stated the shorttime interval involved and theecology of the organisms affectedwould eliminate the single impacttheory. Raup and Sepkoski’s(1982) study demonstrated thebiosphere could withstand a lethalradii impact without the diversitylosses seen in the Devonian. McGhee (1982) believes multipleimpacts would be more likely tocreate a global dust cloud than asingle impact. If a single impactwere oblique to the surface, ratherthan direct, longer-term climaticconsequences could occur. Thiswould spread more debris into orbit around Earth, perhaps pieces as big as 100 to 1,000 meters indiameter. This would lower light levels reaching the Earth’s surface, adding to the cooling effect. Eventually, the orbital debris would return to Earth causing additional impact events. Walliser (1996a)cannot exclude multiple impacts as a cause of the extinction, but doubts it would have a large enougheffect to have created the extinctions, especially as physical evidence is lacking.

The problem with impact theories is the lack of physical evidence, such as high levels of iridium.An iridium anomaly has been found at the F-F boundary in Australia, but has been attributed tobiological concentration by the cyanobacteria or from secondary chemical or diagenetic processes. Astudy of the atomic ratios of the iridium in Australia shows it is not consistent with chondritic or ironmeteoroids (Playford et al., 1984). Paleomagnetic studies by Hurley and Van der Voo (1990) find fivemagnetic reversals during the iridium anomaly and including sedimentation rates suggested a time spanof 250,000 years for the deposits, further supporting biologic origins. Nicoll and Playford (1993) believethere was slow deposition during the period and without supporting evidence such as microtektites, theydoubt asteroids were the iridium source. Another discovery of iridium was made in Xiangtian, southernChina, in the Lower triangularis zone by Wang and colleagues. It was deposited into an environmentlacking phytoplankton activity. As the strata in Xiangtian lacks any fossils, biologic reasons for theaccumulation of iridium are impossible. However, there was no physical evidence of an impact, such asshocked minerals or microtektites (Wang, et al., 1991). Some believe it not to be a spike in iridiumlevels, but a normal period between two negative peaks levels caused by decreased oceanictemperatures. Claeys and others (1992) reported the discovery of microtektite-like glassy spherules nearthe F-F boundary at Senzeilles in Belgium in the Lower triangularis zone. As the composition of thespherules are consistent with crystalline rocks of the Baltic Shield, these tektites probably are from theSiljan Ring impact. In Nevada, the Alamo impact occurred during the Devonian when Nevada wascovered by an ancestral Pacific Ocean. Models show the impact would have created a 300 meter tsunamiwave demolishing carbonate platforms of the region. Central Europe shows breccia present inAlamo-age strata which could have resulted from the tsunami wave action (Sandberg and Morrow,1998). The turbulence and runoff from the land could create environmental conditions for a sufficientlength of time to be devastating to bottom dwelling filter-feeders and their larvae (McLaren, 1970).



CONCLUSION

Although many theories for the Late Devonian extinction exist, none have been accepted as thecause of the event. Many suggestions are interrelated, such as an extensive transgressive event andanoxic marine waters or global cooling and glaciation with regression. Better physical evidence insupport of a bolide impact may be difficult to find as the culprit may have been in an oceanic area nowsubducted and destroyed. Generally, experts agree that several of these theories could have createdintolerable conditions for flora and fauna, leading to the mass extinction of life.

The Devonian extinction had severe global effects. With a worldwide loss of 60% of existingtaxa, every ecosystem was affected. Reef systems were forever changed with the massive deaths ofstromatoporoids and tabulate corals. Brachiopods lost their stronghold as the dominant shelled marineinvertebrate. Entire classes, such as the agnathan fishes, went extinct. From the loss of microscopicplankton to terrestrial plants, all life on Earth was affected by this major extinction event.

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INTERNET

Devonian. Online. http://www.owlnet.rice.edu/~sehh/Dino/Mass_Extinction/extinct_history.htm. Sept.12, 2001

Gilette, Felix. Aug. 2000. New Suspect For Late Devonian Die-Off. Geotimes. American GeologicInstitute. Online. Sept. 12, 2001.

Sandberg, Charles A., and Morrow, Jared R. Jun. 1998. Tiny Teeth Forecase Ancient Comet Showers. U.S. Geological Survey. Online. http://www.usgs.gov/public/press/public_affairs/press_releases/pr553m.html. Sept. 12, 2001



"the late devonian mass extinction event" by sharon goehring (2001)

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