Trilobite Extinctions

Extinction Events

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 Trilobite Family Diversityextinction 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.

Trilobite Extinctions