THE LATE DEVONIAN MASS EXTINCTION EVENT
THE LATE DEVONIAN MASS EXTINCTION EVENT
Sharon Goehring
GEOL 345: Paleontology
December 4, 2001
THE LATE DEVONIAN MASS EXTINCTION EVENT
As one of five major extinction events, the Late Devonian was a time when life was devastated on
Earth. Worldwide, all marine and terrestrial ecosystems felt its effects. Studies show flora and fauna
experienced major losses at all taxonomic levels from microscopic algae and invertebrates to the first
terrestrial 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 as
most believe a combination of events affected global conditions.
BACKGROUND
The Devonian period is divided into three epochs: Early, Middle, and Late. These epochs are
further subdivided into several stages. The mass extinction event occurred around the Frasnian and
Famennian stages of the Late Devonian. The Frasnian began about 377 million years ago and ended 367
million years ago with the start of the Famennian. The Devonian ends after the Famennian at its
boundary with the Tournaisian stage of the Early Carboniferous, about 362 million years ago.
Biostratigraphic zonations of the
type section provide detailed subdivisions Conodont Zonation
Famennian
ca 365.5 Ma Early or Lower crepida
of the Devonian. The Frasnian and
ca 366 Ma
Late or Upper triangularis
Famennian stages are zoned by
ca 366.5 Ma Middle triangularis
conodonts. Table 1 provides the details of
Frasnian
ca 367.5 Ma Linguiformis
the official zones. The boundary type
section is at an abandoned quarry in
Uppermost gigas
Coumiac of Montagne Noire, France and
ca 368 Ma
Late rhenana
is equivalent to the conodont linguiformis
Upper gigas & Lower gigas
and triangularis zones (Walliser, 1996a).
ca 368.5 Ma Early rhenana
The extinction at Frasnian-Famennian
(F-F) boundary is known as the
Lower gigas
Kellwasser event. The upper Famennian
Table 1: Conodont Zonation of the Devonian (McGhee,
extinction, at the boundary with the
1996)
Carboniferous, is known as the
Hangenberg 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 moved
from 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 and
dominated 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 family
level in less than fifteen million years. It must affect worldwide terrestrial and marine environments and
include both flora and fauna. Sepkoski (1982) believes mass extinctions occur when there is an abrupt
termination of many or most species at a single horizon or within a limited stratigraphic interval. It must
be widespread and independent of facies changes. The appearance of species above the event should not
be 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 late
rhenana zone through the middle triangularis, with the most devastating period during the linguiformis
zone. The second smaller event occurred in the early crepida zone. Specifically, the conodont extinction
seen in the Schmidt Quarry in Germany has five centimeters of shale lacking conodont fossils between
the Frasnian linguiformis zone and the triangularis zone. Sandberg estimated this gap ranged from
12,500 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 the
Frasnian (McGhee, 1982). Losses are estimated at 13% to 38% at the family level, compounding further
down 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 and
stromatoporoid sponges. These ecosystems were seriously hit during the Late Devonian allowing
hermatypic Scleractinas to become the major reef building organisms later. Terrestrial life was also
affected with 43% to 50% plant species lost.
LOSS OF LIFE
Algae and zooplankton extinction rates are often disputed as they are difficult to determine. The
fossil record of algae and phytoplankton is poor and does not include all species. Indirect evidence exists
for the extinction of nonpreservable phytoplankton. Over 90% of phytoplankton are affected by the
event (Rossbach, 1989). About 60% of Prasinophycean green algae and 81% of the Acritarch species die
by the Carboniferous (McGhee, 1996). The calcareous algae are better preserved than the
phytoplankton. The Receptaculitid chlorophytes were eliminated (McGhee, 1996). Many experts
disagree 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 Frasnian
extinctions include five families of the chitinozoans: the Ancyrochitinidae, Conochitinidae,
Hoegispharidae, and Lagenochitinidae. The family Desmochitinidae made it past the F-F boundary, but
were eliminated during the Tournaisian (McGhee, 1996).
Foraminiferal extinction rates seem related to the composition and structure of their tests. All
siliceous agglutinated test forams survive, while 45% of calcareous forams went extinct. The species
with primitive spherical, uniserial, and agglutinated tests survived, while species with advanced septate
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architectures were eliminated. All the pseudoseptate and quasi-septate test forams suffer significant
reductions. On the family level, Semitextulariidae, Multiseptidae, Nanicellidae, and Paratextulariidae,
and Eogeinitzinidae were eliminated. Thirty species of the Semitextulariina-Nodosarioid assemblage and
over 70% of the Tournayellidae species were lost (McGhee, 1996).
In the Devonian, almost all stromatoporoids disappeared worldwide, with fore-reef, reef and
lagoonal forms especially hard hit (McLaren, 1970). Studies show their decline began before the end of
the 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 species
during 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 corals
seen in the late Frasnian survive (McLaren, 1982). More species of tabulates and rugosans went extinct
in the Devonian than the surviving species that died at the Permian-Triassic event. 25 families of rugose
and tabulate corals were lost (Sepkoski, 1982). Tabulates lost 80% to 92% of their genera, with the
favosites becoming extinct. The branching form of coenenchymal imperforates were lost. The family
Phillipsastraiidae became extinct in the Frasnian with rare unverified exceptions making it into the
Famennian (McLaren, 1970). Rugosan extinction rates depended on whether the species was colonial or
solitary 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 generas
experienced 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 the
Devonian, 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 dominant
shellfish. Seven groups of brachiopods were lost (Copper, 1986). Articulate brachiopods lost seventeen
families (Sepkoski, 1982) and had only ten of 71 genera survive (Johnson, 1974). The loss was felt
harder 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 Stropheontidae
family and the Orthacea, Pentameracea, and Atrypacea orders were eliminated (McLaren, 1970). The
Orthida 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 World
Realm of eastern North America. The complete loss of the New World Realm species could be due to
the dominance of the Old World species and not necessarily caused by the extinction event itself as
exemplified by the loss of Mucrospirifer mucronatus and Orthospirifer mesastrialis (McGhee, 1996).
Many extinctions were experienced in the Mollusca. Cephalopods were severely affected, with
the loss of fourteen families during five periods of diversity crises (Sepkoski, 1982). Clymeniidae
became extinct, except the species Cymaclymenia evoluta (Walliser, 1996b). Only eight cephalopod
genera survived, with nautiloid cephalopods losing 29 genera during the Frasnian. The gephuroceratid,
beloceratid, and many tornoceratid and anarcestid ammonoids went extinct. All together, ammonoids
lost 88% of their species during this event (Walliser, 1996a). Bivalves, gastropods and other mollusks
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were also affected. Generally, gastropods survived the event, although it is argued that the family
Palaeotrochidae went extinct. Four families and eight genera of bivalves were lost including the families
of Antipleuridae and Ambonychiidae. In a phylum similar to the molluscans, Tentaculites went extinct
prior to the Famennian (Schindler, 1990).
Many different families of echinoderms were affected by the Late Devonian extinction. The
carpoids and rhombiferans were lost. Three families of fissiculate blastoids went extinct, although
generally Blastozoa and Echinozoa were not affected. 42% of Asterozoas were lost with five families
becoming 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 radiate
later. The trilobites experienced a steady decline from the Middle to Late Devonian in losses not
associated with the Devonian extinction. Eight Givetian families of trilobites were lost in the Frasnian
with 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), representing
about 42% of their genera (Rossbach and Hall, 1998). The Illaenacea, Harpina, Lichida, and
Odontopleurida trilobites went extinct (Rossbach, 1989). Malacostracan crustaceans lost 68% of their
species in the late Frasnian with only seven species surviving. They radiate in the mid-Famennian only
to experience a 88% species loss in the Late Famennian with only three species making it into the
Carboniferous (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-F
border. The fossil record of myriapods and primitive hexapods is too fragmentary to determine definite
extinction numbers. The eurypterids lost 27% of their genera at the F-F boundary and lost 63% more
during the Famennian. The conchostracans lost 33 species in the Early Frasnian, 29 more in the Late
Frasnian, and had only one surviving species reach the Famennian. Some experts believe these numbers
to be exaggerated due to the poor fossil records of these groups (McGhee, 1996).
Conodonts experienced severe losses during the Devonian. Ancyrodellid were eliminated and
almost 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 zoning
species, lingiformis experienced a drop in diversity from thirteen species to one during no more than
20,000 years (McGhee, 1996).
Both salt and fresh water fish experience the Devonian extinction event. Of the salt water
species, the last of the remaining agnathan fishes die. The groups heterostracans and thelodonts were
flourishing during the Frasnian, but died at the F-F boundary. The jawed fishes, gnathostomes, had many
extinctions. The placoderms lost 65% and the acanthodians lost 87% of their species respectively. Two
orders of the chondrichthyans and four families of the ostreichthyans went extinct. Fresh water fish also
experienced losses. As with their salt water relatives, the fresh water agnathans became extinct. 35% of
the placoderm and 30% of the acanthodian species died. These numbers reflect that the fresh water
species fared better than their salt water counterparts (McGhee, 1996).
The Devonian was the period when the amphibians began their terrestrial life, therefore, their
fossil 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
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their reappearance in the Famennian. This is assumed to be evidence of their losses from the extinction
event (McGhee, 1996).
Flora experienced extinction during the Devonian and land plants were among them. For fossil
taxonomy of plants, species are split into two types. The spore species, known from their reproductive
structures, are called microfloral species. The macrofloral species, or leaf species, are known from the
record of their leaves, roots, and bark. The microflorals experienced a decline from 57 to 51 species
from the Givetian to the Frasnian. During the F-F extinction, 43% of the spore species were lost and
later declined further with only 29 species remaining by the Carboniferous. Many experts doubt the
validity of these numbers as they could result from poor preservation of spore fossils. Macrofossils also
experienced 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 the
early 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 are
suggested from climatic changes or tectonics to a bolide impact. Regressive and transgressive cycles and
related anoxic conditions have been blamed for the extinctions. One popular idea for the Kellwasser
event is the possibility of a single or multiple asteroid impacts. Although many theories have been
offered, none are assumed to be solely responsible. Many believe the extinction was caused by a
combination 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 disruption
of 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. This
would disrupt the low-latitude circumequatorial flow of warm water. High-latitude colder water would
flow into equatorial areas on the western margins of the joined continents, creating restricted circulation
and anoxic conditions in warm-water basins on the eastern margin. This theory is supported by the
subtropical reef and perireef life experiencing higher extinction rates (Copper, 1986). Also,
hyalosponges, a cool-water preferring organism, shows evidence of expansion into shallow marine
habitats, an indicator of cooler oceanic temperatures during the F-F time (McGhee, 1982). The cooling
of oceanic waters might have been compounded by a glaciation event in South America (Rossbach and
Hall, 1998). Evidence of glaciation is found in the Amazonas Basin of South America. Glacial
diamictites of Upper Devonian age have been discovered (Caputo, 1985). However, other supporting
evidence for the collision of continents is lacking. Paleomagetic data and biogeographic data place the
collision 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 the
greenhouse 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 be
significant 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 an
explosion in reef growth which would eventually create a depletion of seawater bicarbonate. In turn, this
would cause the extinction of reef organisms and the creation of anoxic conditions as shallow marine
oxygen levels are affected by episodic nutrient pulses. Some experts believe the drop in carbon dioxide
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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 many
marine organisms live closer to their upper temperature limits of physiologic tolerances, small changes
could bring them into intolerable temperatures. Also, this provides a mechanism for anoxic waters as
warmer water holds less oxygen. This would bring lower organisms up into the shallower waters to
reach greater oxygen levels, leading to competition. Isotopic oxygen and carbon studies have supported
this theory, showing an increase of temperature to over 60EC at the F-F boundary. This differs from the
mean range of 36EC to 54EC over the earlier Devonian. However, the average lethal temperature for
organisms is 38EC. Thompson and Newton have altered their estimates to bring the boundary
temperature 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 world
should 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 with
phases of accelerated plate tectonic activity. With increased magma upwelling and spreading at
mid-oceanic ridges, ocean water would be displaced onto continents. An increase in global volcanism
would add carbon dioxide into the atmosphere. The slowing of plate tectonics would mean less carbon
dioxide enters the atmosphere, starting an icehouse cycle. Rapid changes in these conditions could cause
a global ecosystem collapse. Studies have shown that the Late Devonian experienced a change from
warm to colder conditions without rapid fluctuations. Another tectonic process which might have caused
the extinction event is pulsation tectonics, a theory closely related to the tectonic megacycles. Sheridan
proposed six cycles occurred involving tectonics and Earth’s magnetic field polarities. According to his
study, the Late Devonian falls into a period of warm, humid climate and high sea levels. This would lead
to 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 for
effects of biotic productivity changes and organic carbon distribution. With high productivity in shallow
waters and stagnation in the deeper ocean, conditions of anoxic water with major deposition of organic
carbon would occur. This would increase the removal of carbon dioxide from the atmosphere, cooling
the global climate. The reestablishment of deep-sea currents would start a greenhouse cycle and a related
marine transgression with the spread of anoxic bottom waters onto perched environments. A later
regressive event would lower these deeper waters. These oscillations would create difficult conditions
for 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 between
lowered sea levels and high extinction rates (McGhee, 1989). Glaciation would cause a regression and
loss 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 occur
during a regressive event (Johnson, 1974). In fact, the rock record provides evidence of a transgressive
event at the end of the Frasnian (McGhee, 1982) with the F-F boundary occurring during an interval of
global sea-level highs (McGhee, 1989). Regression might have taken place during the Famennian with
the start of glaciation in Gondwana, but it would be too late to cause the Kellwasser extinction. A Late
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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 and
re-emergence by fluctuating sea levels (Walliser, 1996a).
The Devonian extinction might also have been caused by a transgressive event. Transgression
could lead to the “drowning” of perched reef ecosystems along with the influx of anoxic deeper waters
into shallow areas killing sessile sea floor marine organisms (McGhee, 1989). The characteristic black
shale deposits of the Devonian are accepted as evidence of widespread anoxic waters. Frasnian strata in
Alberta 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 black
shale deposits (Rossbach and Hall, 1998). Studies have not shown how the entire global marine
ecosystem could become anoxic, but two theories have been proposed: oceanic overturn and an asteroid
impact. However, if a severe worldwide overturn occurred, high-latitude organisms should have been
harder 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 marine
waters. This increase in biotic productivity could not only cause extinctions, but also accounts for global
black shale deposition. Evidence was found in the Geneseo Formation in New York that Devonian seas
alternated in periods of thermocline establishment and periods of water column mixing, releasing
nutrients which would promote greater productivity. Analysis of carbon, nitrogen, and phosphorous in
the 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 flora
and fauna would create a corresponding increase in carbon deposition, accounting for the formation of
black shale (Gillette, 2000). Also, the increase in productivity of tropical and subtropical organisms
adapted to low nutrient, clear-water conditions would eventually harm their environment. Massive
phytoplankton populations would eclipse the water surface eliminating photosynthesis for planktic and
nektic organisms (Geldsetzer, et al., 1987).
Asteroid impacts have become a popular theory examined at extinction events. McLaren (1970) proposed
the first bolide-induced extinction for the Late Devonian. To be a single impact, studies have shown that
the asteroid would need a diameter greater than ten kilometers. If an asteroid of those proportions
impacted earth, it would kill life in the target area, generate earthquakes, tsunamis, wildfires and ballistic
molten debris. Tsunamis, especially, would affect shallow marine ecosystems. The blast would heat the
atmosphere 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 an
icehouse effect. Global dust clouds could block sunlight, making photosynthesis impossible.
Temperatures could drop below survivable ranges for many organisms. Evidence of a Devonian impact
crater has been found. The Siljan Ring in Sweden, dated to the F-F boundary, had the largest diameter of
52 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 be
accepted or dates too uncertain based on differing opinions. Larger craters might have been created in
the 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.
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McLaren (1982) stated the short
Dia. (km)
Age (Ma)
time interval involved and the
ecology of the organisms affected
would eliminate the single impact
theory. Raup and Sepkoski’s
(1982) study demonstrated the
biosphere could withstand a lethal
radii impact without the diversity After Grieve and Robertson (1987)
Siljan, Sweden
52
368 +- 1
losses seen in the Devonian.
46
360 +- 25
McGhee (1982) believes multiple Charlevoix, Quebec
Kaluga, USSR
15
380 +- 10
impacts would be more likely to
Lac La Moinerie, Quebec
8
400 +- 50
create a global dust cloud than a
Crooked Creek, MO
5.6
360 +- 80
single impact. If a single impact
3.8
360 +- 20
were oblique to the surface, rather Flynn Creek, TN
Table 2: Known Impact Craters that span F-F Boundary (367 Ma) –
than direct, longer-term climatic
(McGhee, 1996)
consequences could occur. This
would spread more debris into orbit around Earth, perhaps pieces as big as 100 to 1,000 meters in
diameter. 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 enough
effect to have created the extinctions, especially as physical evidence is lacking.
Crater
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 to
biological concentration by the cyanobacteria or from secondary chemical or diagenetic processes. A
study of the atomic ratios of the iridium in Australia shows it is not consistent with chondritic or iron
meteoroids (Playford et al., 1984). Paleomagnetic studies by Hurley and Van der Voo (1990) find five
magnetic reversals during the iridium anomaly and including sedimentation rates suggested a time span
of 250,000 years for the deposits, further supporting biologic origins. Nicoll and Playford (1993) believe
there was slow deposition during the period and without supporting evidence such as microtektites, they
doubt asteroids were the iridium source. Another discovery of iridium was made in Xiangtian, southern
China, in the Lower triangularis zone by Wang and colleagues. It was deposited into an environment
lacking phytoplankton activity. As the strata in Xiangtian lacks any fossils, biologic reasons for the
accumulation of iridium are impossible. However, there was no physical evidence of an impact, such as
shocked minerals or microtektites (Wang, et al., 1991). Some believe it not to be a spike in iridium
levels, but a normal period between two negative peaks levels caused by decreased oceanic
temperatures. Claeys and others (1992) reported the discovery of microtektite-like glassy spherules near
the F-F boundary at Senzeilles in Belgium in the Lower triangularis zone. As the composition of the
spherules are consistent with crystalline rocks of the Baltic Shield, these tektites probably are from the
Siljan Ring impact. In Nevada, the Alamo impact occurred during the Devonian when Nevada was
covered by an ancestral Pacific Ocean. Models show the impact would have created a 300 meter tsunami
wave demolishing carbonate platforms of the region. Central Europe shows breccia present in
Alamo-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 sufficient
length of time to be devastating to bottom dwelling filter-feeders and their larvae (McLaren, 1970).
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CONCLUSION
Although many theories for the Late Devonian extinction exist, none have been accepted as the
cause of the event. Many suggestions are interrelated, such as an extensive transgressive event and
anoxic marine waters or global cooling and glaciation with regression. Better physical evidence in
support of a bolide impact may be difficult to find as the culprit may have been in an oceanic area now
subducted and destroyed. Generally, experts agree that several of these theories could have created
intolerable 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 existing
taxa, every ecosystem was affected. Reef systems were forever changed with the massive deaths of
stromatoporoids and tabulate corals. Brachiopods lost their stronghold as the dominant shelled marine
invertebrate. Entire classes, such as the agnathan fishes, went extinct. From the loss of microscopic
plankton 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 Geologic
Institute. Online. Sept. 12, 2001.
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U.S. Geological Survey. Online.
http://www.usgs.gov/public/press/public_affairs/press_releases/pr553m.html. Sept. 12, 2001
Digitally signed by Sharon Goehring
cn=Sharon Goehring, c=US
Date: 2002.01.05 16:02:38 -05'00'
Reason: I am the author of this document
Elizabeth City, NC
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