Permo-Triassic boundary

The Permo-Triassic boundary is the time in Earth's history marked by the transition between the Permian extinction period and the beginning of the Triassic period. The Permian extinction was the largest in the Earth's history, resulting in the loss of more than 90 percent of species on the planet. Evidence for the Permian extinction includes geological sediment indicating a lack of oxygen and much paleontological data indicating the disappearance of large numbers of species from the fossil record. Reduced species concentrations and diversity in the wake of the Permian extinction allowed new species to flourish in both the terrestrial and marine environments, contributing to an overall increase in species diversity throughout the Mesozoic period.

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The geological discipline of stratigraphy involves studying the layers of Earth to discern patterns of deposition for clues to ancient geological events. Paleontologists have discovered patterns of the appearance and disappearance of certain types of fossils. These patterns can be used to mark the division between geologic time periods. The disappearance of a certain type of fossil may indicate the extinction of that species. The end of one geologic stage is usually marked by a significant change in the stratigraphy and associated fossil assemblages.

Paleontologists have noted portions of the geological record wherein many types of fossils seem to vanish from the sediment within a relatively short span of geologic time. Paleontologists believe that these major changes in the fossil record represent mass extinctions when environmental conditions changed with sufficient rapidity to drive extinction rates far higher than their normal level. Paleontologists have discovered five major extinction events in Earth's history: the Triassic-Jurassic, Permian-Triassic, Devonian-Carboniferous, Ordovician-Silurian, and Cretaceous-Tertiary. Each of these mass extinctions represents changes in the biota significant enough to warrant a division between geologic periods.

Evidence for the Late Permian Extinction

Paleontologists have found significant evidence of a mass extinction at the end of the Permian period (290 to 248 million years ago), marking the division between the Permian and the Triassic periods. The extinction occurred about 251 million years ago and led to the loss of more than 90 percent of species on Earth, including nearly 95 percent of marine species and more than 70 percent of terrestrial plants and animals. In addition, paleontologists have estimated that more than 50 percent of species families disappeared during the Permian extinction, representing a major reduction in biodiversity on a global scale.

Evidence of the Permian extinction has been growing since English paleontologist John Phillips published a series of papers in the 1840s describing evidence that the end of the Permian coincided with a major turnover in marine species. Since that time, paleontologists have found evidence of similar changes in species composition from strata uncovered around the globe. With the advent of radiometric dating in the 1950s and 1960s, paleontologists were able to estimate the absolute age of geological samples and, in turn, to refine the geologic timescale. Using radiometric dating, paleontologists created a more accurate timeline for the Permian extinction.

Fossil evidence of the Permian extinction is widespread and includes the disappearance of thousands of species worldwide. One recognizable fossil example is the chert gap, observed in global deposits from former marine habitats. Chert, a mineral formed from the bodies of millions of microscopic shelled organisms that accumulate on the ocean floor, is absent in the strata representing the shift between the Permian and the Triassic. Paleontologists believe that this gap was caused by a mass extinction that affected the microscopic shelled organisms whose bodies contribute to the buildup of chert on the sea floor. Similarly, paleontologists studying fossilized pollen have noted that many species common in the Permian period seem to have disappeared at the Permo-Triassic boundary. Fossilized pollen from early Triassic sediment is largely representative of smaller, weed-like species that were presumably better able to colonize nutrient-poor environments.

In addition to global shifts in fossil assemblages, geologists noted the presence of unusual mineralogical samples at the Permo-Triassic boundary. These geological indicators include a layer of dark, iron-rich sediment at the boundary layer, which indicates the presence of an anoxic environment wherein oxygen is absent or present in very low levels. Geologists have found similar evidence of shrinking oxygen levels from the ratios of oxygen and carbon isotopes found at the Permian boundary. Sediment from the early Triassic also contains evidence indicating that there was a lack of foliage in many terrestrial environments immediately following the Permian extinction. This lack of plant life may have been responsible for the observed anoxia in the local environment.

Geological evidence, therefore, indicates that, at the end of the Permian, most of the plant life on Earth disappeared and oxygen levels plummeted worldwide within a relatively short time. Paleontologists now believe there was no single cause for the Permian mass extinction; instead, they believe a combination of factors occurring through a few million years changed the environment in relatively sudden and drastic ways, leading to a global loss of species.

and the Permian Extinction

During the Permian and the early Triassic, the continents of the world were united in a single supercontinent known as Pangaea. Earth's continents change position gradually in response to forces generated within its core. Geologists are now evaluating a new theory that the continents unite into a single, unified landmass on a repeating cycle of about 500 million years. During the formation of Pangaea, both terrestrial and marine environments experienced major environmental changes that may have had significant effects on species in both environments.

The vast majority of marine organisms live in the shallow areas off the coast of continental landmasses, where nutrients are in much greater supply than in deeper waters. When the continents joined to form Pangaea, less of this continental shelf would have been available, possibly leading to an overall loss of species. As diversity fell in the shallow coastal areas, local food chains may have collapsed, eventually affecting species farther from the continental shelf and precipitating the collapse of the entire marine ecosystem.

Paleontologists also have noted evidence of seafloor regression occurring during the Permian extinction. Seafloor regression occurs when portions of the sea floor become exposed at sea level, leading to a reduction in habitat available to marine organisms. The causes of seafloor regression are poorly understood, but some geologists theorize that the phenomenon may be related to changes in the rates of seafloor spreading. Seafloor regression may have been another factor contributing to the loss of shallow marine environments, thus placing further pressure on the biota in these normally species-rich environments.

In terrestrial environments, the formation of Pangaea had major effects on habitat diversity and weather patterns. The central regions of Pangaea were isolated from the tempering effects of the ocean. They would, therefore, have developed into vast deserts with little vegetation and with violent shifts in temperature. Coastal areas were subject to flooding, and many formerly tropical and temperate zones would have developed into floodplains, contributing to lower levels of diversity.

In addition, the formation of Pangaea may have reduced species diversity because a single species, or group of closely related species, might have been able to colonize vast areas. When the continents were separated, different species would evolve to fill similar niches. Paleontologists studying Permian sediment have found that certain species were exceedingly widespread, appearing in vast areas across the globe. This lack of species diversity, coupled with a drop in habitat diversity, could have helped to make the biosphere vulnerable to environmental changes.

Extraterrestrial Causes

Some paleontologists have suggested that Earth may have suffered a meteor impact around the time of the Permo-Triassic extinction. Evidence suggests that a meteor impact was one of the contributing causes of the extinction at the end of the Cretaceous period, which resulted in the loss of all dinosaur species, with the exception of birds. This evidence suggests that extraterrestrial causes occasionally play an important role in mass-extinction events.

In 2001, geophysicist Luanne Becker and colleagues published an article in the journal Science, reporting on the presence of unusual carbon molecules, called fullerenes, in sediment from the Permo-Triassic boundary. These Permo-Triassic fullerene molecules contained trapped molecules of helium and argon with isotopic compositions similar to helium and argon isotopes uncovered from extraterrestrial sources, leading Becker and colleagues to theorize that an extraterrestrial impact occurred near the boundary layer. In 2004, Becker and colleagues published the results of a study indicating that the Bedout crater, off the northwest coast of Australia, appears to have been caused by a meteor impact that occurred at the end of the Permian with sufficient size and apparent composition to be the source of the unusual sediments found at the boundary layer.

The Bedout crater is roughly 120 miles (193 kilometers) across, indicating that it may have been formed by a meteor similar in size to that which impacted Earth at the end of the Cretaceous period. Paleontologists believe that an impact of this magnitude would have vaporized most living organisms within a wide area surrounding the impact site and would have further ejected particulate matter into the atmosphere, significantly altering temperature and weather patterns on a global scale. For hundreds of years following the impact, patterns of vegetative growth would have been significantly reduced, leading to a global collapse in food chains and to the extinction of thousands of species. Changes in weather patterns brought about by the Bedout impact would have led to the loss of sufficient vegetation to cause the global drop in oxygen levels observed in sediment from the Permo-Triassic boundary.

Volcanic Causes of Extinction

Another potential factor contributing to the Permo-Triassic extinction may have been a global increase in volcanic activity. Geologists have discovered a set of large flood basalts, known as the Siberian traps, in sediment from the Permo-Triassic boundary. Flood basalts are sediments created by liquid lava floes, and the Siberian traps indicate a continuous period of eruptions that lasted for a minimum of four million years and covered with lava an area larger than Europe.

The eruptions at the Siberian traps would have devastated the environment for hundreds of miles surrounding the eruption site, while gases and particulate matter ejected into the atmosphere from the plume of the active volcanoes might have led to worldwide devastation in the biosphere. Sulfur dioxide released from a volcanic eruption of this type would react with water to form sulfuric acid, which may have rained on the earth thousands of miles from the eruption site for millions of years. Sulfuric acid would devastate plant life across the planet and poison oceanic environments, accounting for a precipitous drop in global oxygen levels.

In addition, carbon dioxide released in volcanic plumes, especially over such a long period, could have had a major impact on global warming. Isotope levels measured from sediment at the Permo-Triassic boundary indicate a minimum of a 9-degree Celsius increase in temperature during the period. Geologists theorize that the volcanic eruptions at the Siberian traps may have been sufficient to account for the observed increase in temperature.

Methane Gas Increases

Paleontologist James Kennett has suggested that the Siberian traps may have destabilized global deposits of methane hydrate, or clathrate, which is a lattice of ice-like molecules that forms when water combines with gaseous methane. Methane hydrate exists in large quantities in the modern environment within the polar oceans and under layers of permafrost in terrestrial environments. Though the substance resembles sheets of frozen water, it is highly reactive to heat.

Benton has theorized that global warming may have raised the temperature of the oceans during the Permian extinction. As the temperature of the oceans increased, large deposits of methane hydrate may have destabilized and returned to gaseous form. As this occurred, the methane gas would have bubbled to the surface of the oceans in large quantities, contributing to the buildup of greenhouse gases. When methane released from the ocean reacted with oxygen in the atmosphere, it would form carbon dioxide, thereby further contributing to the warming trend.

Benton and others have suggested that the carbon dioxide effect could have led to the establishment of a feedback cycle, wherein global warming caused the release of large deposits of methane, which then further increased warming and led to the release of additional greenhouse gases. This warming cycle could have caused the extinction of most plant life in terrestrial environments, and it could, therefore, account for the anoxic conditions observed in Permo-Triassic sediment.

The Triassic Recovery

In the wake of the Permian extinction, both the terrestrial and marine environments were sparsely populated. Research indicates, however, that life was quick to respond to the empty environment. A 2011 study from the University of Rhode Island indicates that, in some environments, genetic diversity among the terrestrial biota returned to levels similar to those before the Permian mass extinction within only five million years. Paleontologists believe that Earth recovered quickly because those species that remained were forced to explore new habitats and resources, producing evolutionary pressures that led to a rapid explosion of species in a relatively short period of evolutionary time.

During the early Triassic, the few marine species that survived the extinction colonized the continental shelf around Pangaea. As the supercontinent began to break apart in the Jurassic, marine diversity increased exponentially. Whereas in the Permian many oceanic species were bottom-dwelling organisms, few of these species survived; the Triassic seas became home to a larger proportion of organisms that were able to swim and travel to obtain necessary resources. Overall, marine diversity increased globally because of the Permian extinction.

Some paleontologists have theorized that the radiation of species in the newly empty environment created a situation in which a large number of organisms were competing for a limited supply of materials. This competitive regime then fueled the evolution of new species that were able to capitalize on alternative food sources and types of habitats.

In terrestrial environments, the Permian extinction allowed for an explosive radiation of conifers as one of the dominant forms of terrestrial vegetation. The archosaurs, a group of reptiles that would eventually give rise to the first mammals, underwent an evolutionary explosion in the wake of the Permian extinction, spreading across the world and evolving into a variety of species, ranging from small herbivores to large predators. The reptile lineage, in general, experienced a significant advantage in the Triassic, including the evolution of the first dinosaur species.

Overall, the Permian mass extinction led to higher levels of diversity in all environments on Earth. Paleontologists have found similar changes in the biota accompanying other mass extinction events. As such, mass extinction has been a driving force in the evolution of life and biological diversity on the planet.

Principal Terms

anoxia: the conditions caused by an absence of oxygen

archosaur: a reptilian branch that colonized much of the terrestrial environment during the Permian and early Triassic periods and gave rise to the earliest mammalian ancestors

chert: a mineral sediment that often contains the pulverized remains of diatoms, shelled microbes that inhabit marine and aquatic ecosystems

flood basalt: the sediment of liquid rock; associated with ancient patterns of volcanic eruption

mass extinction: a period in Earth's history in which the species extinction rate rises sufficiently to cause large numbers of extinctions on a global scale and at a rate faster than the normal level of extinction for Earth as a whole

plate tectonics: a branch of geology concerned with studying the movement and formation of continents

radiometric dating: a scientific dating method that uses the decay of naturally occurring radioactive elements in the strata to provide an approximate date for the deposition of a mineral sample

stratigraphy: a branch of geology concerned with studying rock layers and the process involved in sedimentary layering

supercontinent: a combination of two or more of Earth's continents into a larger landmass

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