 While
the words extinction event conjures an image of an instantaneous,
cataclysmic conflagration of sorts, this is far from the truth.
Rather, the terminology should be understood as a rapid increase
in species extinction rate culminating in a remarkable decrease
in life on earth, though it is normally restricted to macroscopic,
eukaryotic life forms. Mass extinction is a synonym. Species
extinction and speciation are always ongoing, but during  extinction
events the rate of extinction increases compared to the rate
of new species appearing. Extinction is such a constant phenomenon
that some 98% of known species have met the fate. Extinction
events are accompanied by a concomitant decrease in biodiversity
(phenotype and genotype), but this is followed by an emptying
of environmental niches, that can be filled by new evolving species,
better adapted to the new selective pressures.
Geological time is punctuated by repeated extinction
events, some quite major, others less so. Trilobite diversity
was impacted accordingly. The trilobite fossil record shows how
trilobites were impacted by
extinction
events
throuout the Paleozoic. Some trilobite taxa perished, but other
survivors adapted or evolved to occupy alternate environmental
niches or lifestyles. As seen in the chart above and right,
trilobites sustained
great diversity in both the Cambrian and Ordovician periods.
Their diversity began a marked decline after the Ordovician
extinction event, the second worst in geological history,
a decline that continued unabated to the end of the Permian period
(P-T Event), when
the
final remaining
families
perished along with much of life on earth.
The figure above denotes seven mass
extinctions that notably affected trilobita. These were:
End-Botomian mass extinction (Middle Cambrian - 524 to 517 million
years ago)
With the cause unknown, the end of the Botomian
age of the Cambrian had a mass extinction that may well have
lost more species than the Permian extinction
even, with perhaps some 83% of genera of both hard and soft bodied
animals not surviving into the Middle Cambrian. However, the
Cambrian was a time of rapid steady state ongoing extinction,
compared to the Permian.
Agnostida suborder Eodiscina went extinct. Surviving
trilobites evolved
isopygous or macropygous pygidia
and thicker cuticles, providing better defense from predators (Nedin,
1999)
Dresbachian Extinction Event (Upper Cambrian -
501 to 497 million years ago)
The
Dresbachian extinction event during the Late
Cambrian was the second of two severe extinctions during the
first part of
the Paleozoic, following the prior End Botomian extinction during
the Middle Cambrian. According to data on extinction intensity
(see below), both extinction events slashed approximately 40
percent of marine genera. However, the two are poorly documented
due to a paucity of fossil evidence so early in the evolution
of life.
The
Cambrian–Ordovician extinction event (End of
Cambrian to Beginning
of Ordovician - 488 million years ago)
This extinction wiped out numerous eliminated many brachiopods
and conodonts, and severely affected trilobites, with many
species going extinct. The cause is not known, but hypotheses
associating
glaciation and depletion of marine oxygen have been put forth.
Almost all Redlichiid trilobites,
including those of superfamily Olenelloidea finally went
extinct.
Ordovician–Silurian extinction event (End of Ordovician – 450
to 440 Million Years Ago)
This
mass extinction is considered the second worst of the five
major extinction events (only the great
dying at the end of the Permian is considered worse).
Occurring at boundary of the Ordovician to Silurian periods, estimates are
that 27% of all families, 57% of all genera some 60% to 70%
of all species went
extinct. The event is actually considered to encompass two periods of rapid
extinction separated by about a million years (Sole et. al., 2002) where some
60% of marine invertebrates disappeared. The putative cause was the movement
of the Gondwana supercontinent toward the South Pole, leading to global cooling,
glaciation and decrease in sea levels.
The
major extinction event marking the end of the Ordovician Period
reduced the diversity of all trilobite orders
with most asaphid families disappearing,
and all of Order Ptychopariida. The only surviving asaphids were
members of superfamily Trinucleioidea, and they too disappeared
before the end of the Silurian Period. Trilobite Order
Agnostida also went extinct.
Ireviken
Extinction Event (Middle Silurian – 433
Million Years Ago)
This
was a relatively minor extinction event at the Llandovery/Wenlock
boundary corresponding to the Middle Silurian,
and is the namesake
of Ireviken, Gotland,
a Swedish province where more than 50% of trilobite species were determined
to have met extinction (Munnecke,
et. al., 2003). The event spanning some 200,000 years is considered
to have been a result
of anoxic conditions where marine environments became
completely depleted of below the surface levels (Wignall, et. al., 1996).
Pelagic (neither close to bottom or shore)and hemipelagic (continental
shelf) organisms
such as the graptolites, conodonts and trilobites were particularly impacted.
The only asaphid trilobites surviving the Ordovician were members
of superfamily Trinucleioidea, and they disappeared before
the end of
the Silurian.
Late
Devonian Mass Extinction (End of Devonian - 360 Million Years
Ago)
This
was another of the five major extinction events, and occurred
around 375–360 million years ago during the
Devonian to Carboniferous period’s transition. The late
Frasnian Age of the late Devonian had a series of extinctions
where
some 19% of all families, 50% of all genera, and 70% of all
species disappeared. The event spanned as much as 20 million
years. Trilobite Order Lichida went extinct during the Fransnian
stage of the late Devonian, and Trilobite
Order Phacopida during
the Famennian stage of the late Devonian, and Order
Corynexochida sometime in the late Devonian.
End Permian (P-T) Event (251 Million Years Ago)
This
was the mother-of-all mass extinctions, commonly called the
Great Dying, where some 57% of all families, 83% of all
genera and 90% to 96% of all species, with 53% of marine
families, 84%
of marine genera, and some 96% of all marine species and
an estimated 70% of land species, including insects) became
extinct. The long 275 million year run of the trilobite ended too.
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