Permian Extinction Event

Introduction

The Permian extinction event occurred ~250 million years ago forming the boundary between the geologic periods of the Permian and the Triassic (Barry 2002). This extinction event was the most severe of all five mass extinction events, with up to 96% of all marine species and 70% of terrestrial vertebrate becoming extinct, and is the only known mass extinction of insects (Benton 2005). As such a gargantuan amount of biodiversity was lost, recovery of life after the Permian extinction event took much longer than any other event (Benton 2005).

Many scientists believe that there were upto three distinct pulses of extinction, with several proposed mechanisms for the extinction. It is believed that the earlier phase(s) were due to gradual environmental change, while the latter phase(s) was likely due to a catastrophic event.

What was lost?

Marine organisms experienced the greatest losses during this extinction event. Among the losses were:

• 100% of Blastoids
• 100% of Trilobites (which were in decline since the Devonian, with only 2 genera living before the extinction).
• 100% of Eurypterids
• 99% of Radiolaria
• 98% of Gastropods
• 97% of Foraminifera 
• 59% of Ostracods

(Diagram of a Permian blastoid. From www.accessscience.com)

Statistical analyses of the marine losses during the Permian suggest that the decrease in diversity was due to a sharp increase in extinctions as opposed to a decrease in speciation (Bambach et al. 2004). Those organisms with calcium carbonate skeletons were primarily affected, especially those reliant on ambient Carbon Dioxide levels to produce their skeletons (Benton 2005). The groups with the highest survival rates generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification (Payne et al. 2004). 

The Permian had a great biodiversity of invertebrate species, including the largest insects to have ever existed (Barry 2002). During the Permian extinction event up to nine insect orders became extinct, and up to ten more greatly reduced in diversity. This is the only known extinction of insects.

A massive rearrangement of plant ecosystems occurred during the Permian extinction event, with many land plants entering an abrupt decline. Large gymnosperm woodlands were replaced by an increase in herbaceous plants such as lycopodiophyta (Looy et al. 2001). Gymnosperms subsequently recovered 4-5 million years later.

Of the terrestrial vertebrates even the groups that survived suffered extremely heavy losses of species, with some of the surviving groups not persisting long past this period. It is known that over 70% of terrestrial labyrinthodont amphibians, sauropsid (reptile) and therapsid (mammal-like reptile) families became extinct (Benton 2005). Large herbivores suffered the greatest losses, with all Permian anapsid reptiles dying out.

(Possibly what a Millerosaurus - an extinct genus of anapsid from the Permian period may have looked like. http://guidetoreptiles.blogspot.com)

Recovery

One disaster taxa such as the hardy Lystrosaurus recovered quickly after the Permian extinction event. Specialised animals that formed complex ecosystems with high biodiversity, complex food webs and a variety of niches took much longer to recover. This is likely due to the successive waves of extinction inhibiting recovery and prolonging the application of environmental stress to organisms, continuing into the early Triassic (Lehrmann et al. 2006). Most scientists believe that recovery did not begin for ~5 million years, and took ~30 million years to complete (Benton 2005).

Prior to the extinction ~67% of marine organisms were sessile and attached to the seafloor, but during the Mesozoid only ~50% were sessile, and the rest free-living. Also prior to the extinction ~50% of marine ecosystems were simple, but post-recovery complex communities outnumbered these simple communities by three to one (Wagner et al. 2006). Bivalves, and motile species also became much more prevalent.

Land vertebrates took an unusually long time to recover from the Permian extinction event, and is believed to have not been complete until the late Triassic. Lystrosaurus, a pig-sized herbivore constituted as much as 90% of the earliest Triassic land vertebrate fauna, smaller carnivorous therapsids also survived. By the late-Triassic dinosaurs, pterosaurs, crocodiles, archosaurs, amphibians, and ammaliforms were also abundant and diverse (Benton 2005).

(What a lystrosaurus was believed to look like. From www.planetdinosaur.com)

Postulated Causes

There are a number of proposed mechanisms for the extinction event, including catastrophic and gradualistic processes. Any hypothesis about the cause must explain: the selectivity of the event, which primarily affected organisms with calcium carbonate skeletons; the large period before recovery started; and the minimal extent of biological mineralization once the recovery began (Knoll 2004).

• Impact event - If an impact event was a major cause of the Permian extinction, it is likely that the crater would no longer exist. 70% of the Earth's surface is ocean, and the Earth has no ocean floor crust older than 200 million years due to sea-floor spreading and subduction. It has also been speculated that craters created by very large impacts on land may be masked by extensive lava flooding from below after the crust is punctured or weakened.
• Volcanism - The final stages of the Permian saw two flood basalt events, the Emeishan and the Siberian Traps (Wignall et al. 2009). These eruptions may have caused dust clouds and acid aerosols, which would have blocked out sunlight and thus disrupted photosynthesis both on land and in the photic zone of the ocean, leading to a collapse of food chains. This may have killed land plants, and organisms with calcium carbonate shells as was experienced in the Permian extinction. Carbon Dioxide would have also been released, leading to global warming. 
• Anoxia - There is evidence that the oceans became deficient in Oxygen towards the end of the Permian, with a notable and rapid onset of anoxic deposition near East Greenland (Wignall and Twitchett 2002). Uranium/Thorium ratios also confirm this. This would have resulted in widespread extinctions except for anaerobic bacteria living in the sea-bottom mud. This anoxia may have been due to global warming slowing down or stopping the thermohaline circulation, reducing the mixing of oxygen in the ocean. 
• Pangaea - Half-way through the Permian all the continents joined to form the supercontinent of Pangaea. This configurations severely decreased the extent of shallow aquatic environments, the most productive part of the seas, exposing formerly isolated organisms of the rich continental shelves to competition from invaders. Ocean circulation and atmospheric weather patterns were also both affected, creating seasonal monsoons near the coasts and an arid climate in the vast continental interior. The formation of Pangaea did not affect terrestrial organisms.

Conclusion

It is likely that a combination of causes, most likely a sequence of catastrophes, each one worst than the previous, led to the Permian extinction event. This led to the greatest loss of biodiversity the Earth has seen during the Phanerozoic, which took millions of years to recover.

References

Barry, P. L. (2002). The great dying. Science and Technology Directorate, NASA.
Benton, M. J. (2005). When life nearly died: The greatest mass extinction of all time. Thames and Hudson.
http://guidetoreptiles.blogspot.com
Knoll, A. H. (2004). Biomineralization and evolutionary history. In P. M. Dove, J. J. DeYoreo and S. Weiner (Eds.) Reviews in mineralogy and geochemistry.
Lehrmann, D. J. et al. (2006). Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from South China. Geology. 12: 1053-1056.
Looy, C. V., et al. (1999). The delayed resurgence of equatorial forests after the Permian-Triassic ecologic crisis. Proceedings of the national academy of sciences. 96: 13857-13862.
Payne, J. L., et al. (2004). Large perturbations of the carbon cycle during recovery from the end-Permian extinction. Science. 305:506-9.
Wagner P. J., et al. (2006). Abundance distributions imply elevated complexity of post-palaeozoic marine ecosystems. Science. 314: 1289-1292.
Wignall, P. B. and R. J. Twitchett, (2002). Permian-Triassic sedimentology of Jameson Land, East Greenland: incised submarine channels in an anoxic basin. Journal of the geological society. 159: 691-703.
Wignall, P. B. et al. (2009). Volcanism, mass extinction, and carbon isotope fluctuations in the middle Permian of China. Science. 324: 1179-1182.
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