The Permian-Triassic Extinction

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    The greatest mass extinction of life on Earth occurred 252.28 million years ago, at the dividing line between the Permian and Triassic geological periods. Informally known as the Great Dying, it also marked the boundary between the Paleozoic and Mesozoic Eras. It was Earth's most severe extinction event, with about 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct. It was also unique in manifesting the only known mass extinction of insects: roughly 57 percent of all families and 83 percent of all genera in Class Insecta died out.

Time Division

Began

Ended

Permian Period

299.0±0.8 Mya

251.0±0.4 Mya

Upper/Late Permian Epoch

260.4±0.7 Mya

251.0±0.4 Mya

Changshingian Age

253.8±0.7 Mya

251.0±0.4 Mya

Wuchiapingian Age

260.4±0.7 Mya

253.8±0.7 Mya

Middle Permian/Guadalupian Epoch

270.6±0.7 Mya

260.4±0.7 Mya

Capitanian Age

265.8±0.7 Mya

260.4±0.7 Mya

Wordian Age

268.0±0.7 Mya

265.8±0.7 Mya

Roadian Age

270.6±0.7 Mya

268.0±0.7 Mya

Lower/Early Permian/Cisuralian Epoch

299.0±0.8 Mya

270.6±0.7 Mya

Kungurian Age

275.6±0.7 Mya

270.6±0.7 Mya

Artinskian Age

284.4±0.7 Mya

275.6±0.7 Mya

Sakmarian Age

294.6±0.8 Mya

284.4±0.7 Mya

Asselian Age

299.0±0.8 Mya

294.6±0.8 Mya

Table One: Temporal Divisions of the Permian Period of the Paleozoic Era

    Researchers have suggested that the Permian-Triassic extinction consisted of two extinctions some nine million years apart. Several mechanisms have been proposed to explain the extinctions; the first extinction was likely due to gradual environmental change, while the second extinction was, on the evidence, likely due to a catastrophic event. Among the mechanisms suggested to explain the latter we find large or multiple meteoric impacts, increased vulcanism, coal/gas fires, pyroclastic explosions from the Siberian Traps, and sudden release of methane from clathrates on the sea floor; gradual changes include a change in sea level, anoxia in the oceans, increasing aridity, and a shift in ocean circulation driven by climate change.

The Supercontinent Pangaea

    In the Kungurian age of the Permian's Cisuralian epoch (about half way through the Permian) all of Earth’s land-masses joined to form the super-continent Pangaea and the super-ocean Panthalassa. This configuration, by shortening the total length of coastline, radically decreased the extent of shallow aquatic environments, the most productive part of the seas. It also exposed organisms formerly isolated from each other on the rich continental shelves to increased competition from each other. As a consequence marine life suffered very high but not catastrophic rates of extinction after the formation of Pangaea -- almost as high as in some of the "Big Five" mass extinctions.

    Physically, Pangaea's formation altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons at and near the coasts and an arid climate in the vast continental interior. In spite of the increased aridity, the formation of Pangaea seems not to have caused a significant rise in extinction levels on land. In fact, most of the advance of mammal-like reptiles (therapsids) and the increase in their diversity seems to have occurred after the formation of Pangaea.

    So it seems likely that Pangaea initiated a long period of severe marine extinctions but was not directly responsible for the Great Dying at the end of the Permian. It simply helped prepare the way for the catastrophe.

Permian–Triassic extinction

    Until AD 2000, geologists thought that too few rock sequences spanned the Permian–Triassic boundary and that they contained too many gaps for scientists to determine reliably the details of that boundary. But since then a study of uranium/lead ratios of zircons found in rock sequences taken from multiple locations in southern China puts the date of the extinction at 252.28±0.08 million years ago; further, an earlier study of rock sequences found near Meishan in Changxing County of Zhejiang Province, China puts the date of the extinction at 251.4±0.3 million years ago, at the same time as massive flood vulcanism occurred in Siberia. The latter study also showed that an ongoing elevated extinction rate occurred for some time afterward.

    A large and abrupt global decrease in the ratio of the stable isotope C-13 to that of C-12 (by about 0.9%), coincides with this extinction and geologists sometimes use it to identify the Permian–Triassic boundary in rocks that are unsuitable for radiometric dating. Further evidence for environmental change around the Permian–Triassic boundary suggests that temperatures rose by 8 °C (14.4 °F) and that carbon dioxide levels increased by 2,000 ppm (parts per million; for comparison consider the fact that the concentration of carbon dioxide in the atmosphere immediately before the industrial revolution was 280 ppm and that in October 2010 the concentration was 388 ppm, some 108 ppm higher). Geologists also have evidence of increased ultraviolet radiation reaching Earth’s surface and causing the mutation of plant spores.

    That latter fact is important because it indicates that something destroyed the ozone layer in Earth’s atmosphere. Thus it ties together several of the other postulated causes of the extinction.

Extinction patterns

Marine organisms

    In the Permian-Triassic extinction marine invertebrates suffered the greatest loss of species. For example, in Southern China the intensively sampled layers of rock at the Permian–Triassic boundary show that 286 out of 329 marine invertebrate genera went extinct and did so within the final two sedimentary layers containing conodonts from the Permian period. Statistical analysis of the losses implies that the decrease in diversity came from a sharp increase in extinctions rather than from a decrease in speciation combined with a normal rate of extinction.

    Among benthic organisms, those that live on the sea bottom, the extinction event multiplied background extinction rates. Thus it caused the most damage to taxa that already had a high background extinction rate (by implication, taxa with a high turnover). The extinction rate of marine organisms was catastrophic, primarily affecting organisms with calcium carbonate skeletons, especially those reliant on ambient carbon dioxide levels to produce their skeletons.

    Surviving marine invertebrate groups include: articulate brachiopods (those with a hinge), which have suffered a slow decline in numbers since the Permian–Triassic extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which very nearly became extinct but later became abundant and diverse.

    A look at the causes of the extinction shows the double nature of the event. For example, the ammonoids (subclass Ammonoidea of class Cephalopoda, phylum Mollusca; essentially squid with shells), which had been in decline for about thirty million years, suffered a selective extinction at the end of the Guadalupian Epoch. The fact that this extinction significantly reduced disparity (lack of similarity), reducing the number of races and breeds within each species, suggests that environmental changes, such as those accompanying the formation of Pangaea, caused the extinction. Diversity (of a different kind, as in species among genera, genera among families, etc.) and disparity continued to diminish up to the Permian-Triassic boundary.

    The end-Permian extinction was non-selective, which implies that it was caused by some catastrophic event. Even so, environmental factors still played a role in determining which species died out and which survived. Those organisms most vulnerable to extinction were those that were heavily calcified (think of corals) and had low metabolic rates and correspondingly weak respiratory systems, while those most resistant to extinction were more lightly calcified and had a more active control of blood circulation and more elaborate gas exchange mechanisms in their gills.

    Then during the Triassic Period diversity increased rapidly while disparity remained low.

Terrestrial invertebrates

    The Permian-Triassic is the only know mass extinction of insects. The Permian period could rightly be called part of an Age of Insects: it had great diversity in insect and other invertebrate species, including the largest insects ever to have existed, that dominated the land. At the end of the Permian eight or nine insect orders became extinct and ten more were greatly reduced in diversity. Palaeodictyopteroids (insects with piercing and sucking mouthparts) began to decline during the mid-Permian, but those extinctions have been linked to a change in flora, likely due to the formation of Pangaea. The greatest decline occurred in the Late Permian and was likely caused by the Great Dying and not directly caused by weather-related floral transitions.

    In the fossil record, most insect groups found above the Permian–Triassic boundary differ significantly from those found in rocks below those of the Permian–Triassic extinction. In well-documented deposits from the Late Triassic, fossils consist almost entirely of modern insect groups.

Plant ecosystem response

    The geological record of terrestrial plants in the Permian is sparse, and based mostly on pollen and spore studies. Permian forests consisted largely of ferns, some standing as high as twenty feet with roots that climbed 2/3 of the way to the crown. Most plants reproduced through spores, though some seed-bearing plants were evolving. Plants are relatively immune to mass extinction: the impact of all the major mass extinctions is "negligible" at the family level. But much of the reduction observed in species diversity (of 50%) may be mostly due to taphonomic processes, processes that determine which organisms get fossilized and which don’t. However, a massive rearrangement of ecosystems did occur, with the abundances and distributions of plants changing profoundly; the Palaeozoic flora barely survived this extinction.

    At the Permian–Triassic boundary, the floral groups that had dominated the world changed. Many groups of land plants, such as Cordaites (gymnosperms) and Glossopteris (seed ferns), went into an abrupt decline. After the main event of the extinction occurred, lycophytes (clubmosses, spikemosses, and quillworts, for example) replaced the dominant gymnosperm genera. Botanists know extant lycophytes as recolonizers of disturbed areas, so we believe that they likely played the same role in the Permian-Triassic extinction.

    Palynological (pollen) studies of sedimentary rock strata laid down during the extinction period in what is now East Greenland indicate that dense gymnosperm woodlands existed before the event. Those large forests died out at the same time that marine invertebrate macrofauna were in decline. The die-offs were followed by a rise in the diversity of smaller herbaceous plants, such as the lycophytes. Later other groups of gymnosperms recolonized the old forest areas and dominated the vegetation only to suffer further major die-offs. Such cyclical shifts in the makeup of the flora occur several times over the period of the extinction and continue for some time afterwards. These fluctuations of the dominant flora between woody and herbaceous taxa indicate the occurrence of chronic environmental stress that resulted in a loss of most large woodland plant species. The pattern of the fluctuations implies the occurrence of pulses of hard ultraviolet radiation reaching the ground and destroying the plants by burning their leaves. Once the Great Dying ended the ultimate recovery of gymnosperm forests took four to five million years, perhaps a thousand times longer than it should have taken.

The Coal Gap

    No one knows of any coal deposits from the Early Triassic. Those found in Middle Triassic sediments are thin and low-grade. This deficiency represents a pause in the continuous creation of coal over geological time and researchers call it the coal gap. To explain it some have suggested that new, more aggressive fungi, insects and vertebrates evolved at the end of the Permian and killed vast numbers of trees. But these decomposers themselves suffered heavy losses of species during the extinction, so they could not serve as a likely cause of the coal gap. A more likely scenario has all coal forming plants being rendered extinct by the Permian–Triassic extinction. After that extinction it took 10 million years for a new suite of plants to adapt to the moist, acid conditions of peat bogs, where biomass begins its transformation into coal.

    To that point we may note that a cold climate had been the norm in eastern Australia for a long time in the Late Permian. As a result a peat bog ecosystem adapted to those conditions existed as well. But then about 95 percent of the peat-producing plants went extinct at the Permian-Triassic boundary. Oddly, though, coal deposits in Antarctica and Australia (which were merged at the time) underwent a fade-out a significant elapse of time prior to the Permian-Triassic extinction.

    However, we know of very few sediments of any type from the Early Triassic. The lack of coal from that period may simply reflect this incomplete picture. That fact leaves open the possibility that coal-producing ecosystems responded to the changed conditions by re-establishing themselves in areas where we have no sedimentary record for the Early Triassic.

Terrestrial vertebrates

    Class Reptilia emerged from Class Amphibia 340-335 million years ago, during the Carboniferous Period (just prior to the Permian) and Class Mammalia emerged from Class Reptilia in the early Mesozoic, 225 million years ago.

    Paleontologists have enough evidence to indicate that over two-thirds of terrestrial labyrinthodont (maze-toothed) amphibians, sauropsid ("reptile") and therapsid ("mammal-like reptile" aka synapsid) families became extinct. Large herbivores suffered the heaviest losses, reflecting the mass destruction of vegetation during the extinction event. All Permian anapsid reptiles died out except the procolophonids, lizard-like creatures that finally went extinct at the end of the Triassic. Pelycosaurs, such as the sail-backed Dimetrodon, died out before the end of the Permian. Too few Permian diapsid fossils have been found to support any conclusion about the effect of the Permian extinction on diapsids (subclass Diapsida, the reptile group from which lizards, snakes, crocodilians, and dinosaurs [including birds] evolved). Even the groups that survived the Great Dying suffered extremely heavy losses of species, with some terrestrial vertebrate groups very nearly becoming extinct at the Permian-Triassic boundary. Some of the surviving groups did not persist for long past this period, while others that barely survived later throve and went on to produce diverse and long-lasting lineages.

Possible explanations of these patterns

    Analysis of marine fossils from the Permian's final stage, the Changshingian Age, found that marine organisms with low tolerance for hypercapnia (high concentration of carbon dioxide) had high extinction rates, while those organisms with the most tolerance for excess carbon dioxide had very slight losses. The most vulnerable marine organisms were those that produced their hard parts from calcium carbonate and had both low metabolic rates and weak respiratory systems. Notable examples of such creatures include calcareous sponges, rugose and tabulate corals, calciate brachiopods, bryozoans, and echinoderms; approximately 81% of such genera became extinct. Closely related creatures without calcareous hard parts suffered only minor losses. We have, for example, sea anemones, soft creatures from which modern corals evolved. With the exception of conodonts (in which 33% of the genera went extinct), animals with high metabolic rates, well-developed respiratory systems, and non-calcareous hard parts suffered negligible losses.

    Hypercapnia comes about relatively easily because carbon dioxide is 28 times more soluble in water than is oxygen. Moreover, marine organisms are more sensitive to changes in the concentration of carbon dioxide because they normally function with lower concentrations of carbon dioxide in their bodies than land animals do. Thus modest increases in the concentration of carbon dioxide cause narcosis (intoxication) and then become toxic by reducing the ability of the respiratory system to oxygenate tissues and by making body fluids more acidic. The toxicity hampers the synthesis of proteins, reduces fertilization rates, and interferes with the production of calcareous hard parts (such as shells).

    The observed pattern of marine extinctions is also consistent with the effects of hypoxia, a shortage but not a total absence of oxygen. Although insufficient to account for the whole extinction event, hypoxia certainly played a role in making the disaster as devastating as it was.

    Detailed analysis of the extinction rates and survival rates of land organisms is difficult because few terrestrial fossil beds span the Permian–Triassic boundary. We do know that Triassic insects differ greatly from those of the Permian, but a gap in the insect fossil record spans about fifteen million years, from the late Permian into the early Triassic period. The best-known record of vertebrate changes across the Permian–Triassic boundary occurs in the Karoo Supergroup, a regime consisting mostly of shales and sandstones covering almost two-thirds of Southern Africa, but statistical analyses have so far not produced clear conclusions as to what happened there in the Great Dying. However, analysis of the fossil river deposits of the floodplains indicate a shift from meandering rivers to braided river patterns, which indicates a sudden lack of vegetation to stabilize the river banks and, thus, an abrupt drying of the climate. The climate change may have taken as little as 100,000 years, bringing about the extinction of the unique Glossopteris flora and the herbivores that fed on it and then the carnivores that fed on the herbivores.

    During the early Triassic (4 to 6 million years after the Permian–Triassic extinction), the land carried insufficient plant biomass to form coal deposits, a fact that implies the existence of a limited food mass for herbivores. Further, river patterns in the Karoo changed from meandering to the braided pattern found in deserts, indicating that there was insufficient vegetation to stabilize the river banks. These clues indicate that vegetation on land was very sparse for a long time.

Causes of the Extinction Event

    Because the Permian–Triassic extinction event occurred over 250 million years ago, much of the evidence that would have revealed the causes has either been destroyed or is concealed under many thick layers of rock. Further, plate tectonics, with its process of seafloor spreading and subduction, completely recycles the sea floor every 200 million years, so we won’t find any evidence beneath the ocean. But the fairly significant evidence that scientists have managed to accumulate seems to support several mechanisms that have been proposed to explain the Great Dying, which consisted of two extinctions about nine million years apart. The first involved gradualistic processes attending the formation of Pangaea and the second involved catastrophic events. The former includes a change in sea level, a combination of anoxia, euxinia (presence of H2S), and hypercapnia (excess CO2), and increasing aridity on Pangaea. The latter includes large or multiple meteoric impact events, increased volcanism, and sudden releases of methane from hydrates on the sea floor. In order to become a theory any hypothesis about the causes of the Great Dying must weave the evidence into a pattern that explains the selectivity of the event (which primarily affected organisms with calcium carbonate skeletons and shells), the long (four- to six-million-year) period before recovery started, and the minimal extent of biological mineralization (despite inorganic carbonates being deposited) once the recovery began. We have our evidence as follows.

Meteoric Impact

    Evidence that an impact event, involving an asteroid a few kilometers in diameter, caused the Cretaceous–Tertiary extinction event has led to speculation that similar impacts may have caused other mass extinction events, including the Permian–Triassic extinction. That speculation has led geologists to search for evidence of impacts at the times of other extinctions, primarily evidence in the form of large impact craters of the appropriate age.

    Geologists have proposed two impact craters as possible causes of the Permian–Triassic extinction. The Bedout structure off the northwest coast of Australia is one and the hypothesized Wilkes Land crater of East Antarctica is the other. In each of these cases, the idea that an impact was responsible has not been proven to full verification. However, the sizes of the two craters indicates that the impacts that created them caused a major disaster on Earth, one consistent with the Great Dying.

    In June of 2006, Dr. Ralph von Frese announced the discovery, in the Wilkes Land region of East Antarctica, of a crater that may mark the site of one of the impacts that caused the Permian-Triassic extinction. The crater lies about two kilometers beneath the East Antarctic Ice Sheet, so it could only be found indirectly by using a combination of gravity fluctuation measurements made by NASA’s GRACE satellites and images of the ground beneath the ice made with airborne radar. The resulting analysis revealed a plug of mantle material 320 kilometers (200 miles) wide lying in Earth’s crust and perfectly centered inside a circular ridge, the crater’s rim, 500 kilometers (300 miles) wide. Evidence gained so far [as of March 2014] indicates that the structure originated between 100 million and 500 million years ago – a broad time span that brackets the specific age of the Permian-Triassic extinction.

    The size of the structure implies that it caused a disaster of a magnitude consistent with that of the Great Dying. The Wilkes Land crater spans more than twice the width of the Chicxulub crater in the Yucatan peninsula, which crater marks the site of the impact whose effects extinguished the dinosaurs 65 million years ago. Geologists have estimated that the Chicxulub impactor (most likely an asteroid) was 10 kilometers (6 miles) wide, while the Wilkes Land impactor (either an asteroid or the nucleus of a comet) spanned up to 50 kilometers (30 miles). On the basis of mass alone the Wilkes Land impactor could have carried as much as 25 times as much energy as did the Chicxulub impactor and it all got turned into shock and heat in an elapse of mere seconds.

    The gravity measurements also suggest that the Wilkes Land impact set the stage for the breakup of Gondwana (the southern part of Pangaea) by creating the tectonic rift that separated Australia from East Antarctica and later pushed Australia northward. Approximately 100 million years ago, Australia split from Gondwana and began drifting north, pushed away from Antarctica by the expansion of a rift valley that extended into the eastern Indian Ocean.

    When large asteroids or comets hit Earth, the aftermath of the impact weakens or kills much of the life that throve prior to the impact. Release of carbon dioxide and other material into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Analysis of the ratios of carbon and boron isotopes in the fossil record provides evidence of increased levels of atmospheric carbon dioxide at the Permian-Triassic boundary. Material from the Earth's mantle released during volcanic eruption has also been shown to contain relatively high amounts of iridium, an element associated with meteorites. For now only limited and disputed evidence exists of iridium and shocked quartz occurring with the end-Permian event. In contrast, such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event.

    Adrian Jones, at University College London, has modeled the effects of impacts on Earth's crust. The model suggests that after an impact the crust rebounds to form a large shallow crater. A truly massive, high-energy impact provides enough heat, both in the impact and in the rebound, to melt the crust. Lava then floods through the floor of the crater and covers the crater with new crust. If Jones is right, the Permian meteorite crater can't be found because it’s covered over with lava flows.

    But in the past geologist John Gorter of Agip (General Italian Oil Company) found evidence of a circular structure 200 kilometers (125 mi) in diameter, called the Bedout, in currently submerged continental crust off the northwestern coast of Australia, and geologist Luann Becker, of the University of California, confirmed its nature as an impact crater, finding shocked quartz and brecciated mudstones associated with it. The geology of the area of continental shelf associated with the Bedout dates to the end of the Permian. The Bedout impact crater is also associated in time with extreme volcanism and the eventual break-up of Pangaea. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Dr. Becker explains. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout, I don't think we can call such catastrophes occurring together a coincidence anymore," Dr. Becker added in a news release.

    If a multiple meteoric impact is a major cause of the Permian–Triassic extinction, some of the craters it created may no longer exist. Some seventy percent of Earth's surface is ocean, so an asteroid or comet fragment is more than twice as likely to hit ocean as it is to hit land. But because the continuous process of seafloor spreading and subduction destroys it, Earth has no ocean-floor crust more than 200 million years old. However, subduction should not be accepted by itself as an explanation of why no firm evidence can be found: as with the Cretaceous-Tertiary event, we expect to see in rock formations from the time an ejecta blanket stratum rich in siderophilic elements (e.g., iridium).

    The hypothesis of a large meteoric impact attracts our assent because the force of the impact could induce the occurrence of other causes that would contribute to the extinction. We see one such secondary cause in the Siberian Traps eruptions and another in the rapid release of methane from undersea hydrates. The abruptness of an impact and of its associated secondary causes also gives us an explanation of why species did not evolve adaptations to the change as they would have done in response to a slower change or to one less than global in scope.

Vulcanism

    At the beginning and the end of the Late Permian Epoch two flood basalt events occurred. The smaller of the two, at the Emeishan Traps in China, centered in Sichuan Province, occurred at the same time as did the end-Guadalupian extinction, in an area that lay close to the equator at the time, about nine million years before the bigger extinction. So far [as of March 2014] no evidence of a meteoric impact that may have caused the event has been found. The flood basalt eruptions that produced the Siberian Traps at the end of the Late Permian Epoch were one of the largest known volcanic events to occur on Earth: they covered over 2,000,000 square kilometers (770,000 sq mi, about the size of the European Union) with lava. Scientists formerly believed that the Siberian Traps eruptions had lasted for millions of years, but recent research dates them to a brief time around 251.2 ± 0.3 million years ago, which puts them right before the end of the Permian Period.

    The Siberian Traps occupy the land between the Ural Mountains and the Lena River in the part of Siberia north of the East Siberian Mountains and Lake Baikal. The area covered lies between 50° and 75° north latitude and 60° to 120° east longitude.

    Vast volumes of basaltic lava paved over a large expanse of primeval Siberia in a flood basalt event. Today the area of the Siberian Traps covers about 2 million km2—roughly equal to western Europe in land area—and estimates of the original coverage go as high as 7 million km2. The original volume of lava is estimated to range from 1 million to 4 million km3.

    Some researchers have hypothesized the source of the Siberian Traps basalt as a mantle plume that impacted the base of the earth's crust and erupted through the Siberian Craton or to some other process related to plate tectonics. However, the more likely explanation, as with the Deccan Traps, consists of hypothesizing that a meteoric impact more or less on the opposite side of the planet generated shock waves that cracked the Earth’s crust and allowed magma to erupt onto the surface.

    The direct effects of the Siberian Traps eruptions would have been:

    I: Dust clouds and sulfuric acid aerosols which would have blocked sunlight and thereby stopped photosynthesis both on land and in the upper layers of the seas, causing food chains to collapse. Both the Emeishan and Siberian Traps eruptions may have caused those dust clouds and acid aerosols. Sulfur dioxide droplets in the stratosphere also diminished the ozone layer, allowing high-energy ultraviolet light to reach Earth’s surface to kill plants and animals directly.

    II: Immediate severe global warming, because the eruptions occurred in coal beds. This is an additional hazard which was apparently unique to the Siberian Traps eruptions. Massive volcanism usually causes short-term cooling because dust clouds and aerosols block a significant portion of the sun’s light. But these eruptions took place in an area which was rich in coal; the heating and burning of this coal released vast amounts of carbon dioxide and methane into the air, causing severe global warming. Other evidence confirms that a massive increase in atmospheric carbon dioxide occurred immediately before the "Great Dying".

    The eruptions themselves also emitted carbon dioxide, causing further global warming. When all of the dust clouds and aerosols washed out of the atmosphere, the excess carbon dioxide remained and the warming proceeded without any mitigating effects.

    Severe global warming can cause anoxic events in the oceans. The heat would disrupt the thermohaline circulation and cause convective overturn of the oceans, which would bring anoxic deep-sea water to the surface. Evidence shows that this happened at the end of the Permian.

    III: These eruptions also caused acid rain when the sulfuric aerosols washed out of the atmosphere. This may have killed land plants and molluscs and planktonic organisms which had calcium carbonate shells.

    The Siberian Traps had unusual features that made their eruptions even more destructive. Pure flood basalts produce a lot of runny lava and do not hurl debris into the atmosphere. But it appears that 20% of the output of the Siberian Traps eruptions was pyroclastic; that is, it consisted of ash and other debris thrown high into the atmosphere, increasing the short-term cooling effect. The basaltic lava erupted through or intruded into carbonate rocks and into sediments that were in the process of forming large coal beds, both of which would have emitted large amounts of carbon dioxide.

    In January 2011, a team led by Stephen Grasby of the Geological Survey of Canada—Calgary, reported evidence that the Siberian Traps eruptions caused massive coal beds to ignite, possibly releasing more than three trillion tons of carbon into the atmosphere. The team found deposits of coal ash in deep rock layers near what is now Buchanan Lake and reported in their article, "... coal ash dispersed by the explosive Siberian Trap eruption would be expected to have an associated release of toxic elements in impacted water bodies where fly ash slurries developed ...", and "Mafic (refers to a silicate material rich in magnesium and iron producing low viscosity lavas) megascale eruptions are long-lived events that would allow significant build-up of global ash clouds". In an additional statement, Grasby said, "In addition to these volcanoes causing fires through coal, the ash it spewed was highly toxic and was released in the land and water, potentially contributing to the worst extinction event in earth history."

    By themselves these eruptions were insufficient to cause a mass extinction as severe as the Permian-Triassic extinction. Equatorial eruptions are necessary to distribute dust and aerosols sufficiently to affect life worldwide, whereas the Siberian Traps eruptions, large as they were, occurred inside or near the Arctic Circle. Furthermore, if the Siberian Traps eruptions occurred within a period of 200,000 years, the consequent burning of carbonaceous material would have merely doubled the atmosphere's carbon dioxide content. Recent climate models imply that such a rise in carbon dioxide raised global temperatures by 1.5 to 4.5°C (2.7 to 8.1°F). Such an increase in temperature is likely insufficient to cause a catastrophe as great as the Permian–Triassic extinction.

Ozone Layer Depletion

    Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet radiation coming from the sun. Extremely short or vacuum ultraviolet (0.01–0.1 micron), the really hard stuff, is screened out by the atmosphere’s nitrogen. Ultraviolet radiation capable of penetrating nitrogen is divided into three categories, based on wavelength. Scientists refer to these as UV-A (0.4–0.315 micron), UV-B (0.315–0.28 micron), and UV-C (0.28–0.1 micron).

    UV-C, which would be very harmful to all living things, is entirely screened out by a combination of normal two-atom oxygen molecules (taking out wavelengths less than 0.2 micron) and ozone (taking out wavelengths greater than about 0.2 micron) by the time it has descended to an altitude of around 35 kilometers (115,000 feet). If this component of ultraviolet radiation reached Earth’s surface, life could not exist except in the oceans unless it were well armored or entirely nocturnal.

    UV-B radiation is the main cause of sunburn. Clearly harmful to the skin, it can, with excessive exposure, cause genetic damage, which results in problems such as skin cancer. The ozone layer (which absorbs the radiation with wavelengths from about 0.20 micron to 0.31 micron, with maximum absorption at about 0.25 micron) is more effective at screening out UV-B that most people know; for radiation with a wavelength of 0.29 micron, the intensity at the top of the atmosphere is 350 million times stronger than it is at Earth's surface. Nonetheless, some UV-B, particularly at its longest wavelengths, reaches the surface.

    Ozone is transparent to most UV-A, so most of this type of longer wavelength radiation reaches the surface and it constitutes most of the ultraviolet radiation reaching Earth’s surface. This softer type of ultraviolet radiation is significantly less harmful to DNA, though it still has the potential to cause physical damage, indirect genetic damage, and skin cancer.

    Sulfur dioxide droplets from volcanic eruptions can deplete ozone, but it would take a truly massive eruption to heave enough sulfur dioxide into the stratosphere to destroy the ozone layer. The pyroclastic flood vulcanism of the Siberian Traps may have been massive enough.

Methane Hydrate Gasification

    Permian strata in Greenland include rock layers tens of meters thick devoid of marine life. Using those layers, the geologist Paul Wignall determined that the entire extinction lasted a mere 80,000 years. The extinction appeared to kill land and marine life in three distinct phases: a brief period of almost complete extinction of marine life separated two periods of extinctions of terrestrial life. The whole process seemed to take too much time to be the result of a meteorite strike. But Wignall noticed that the carbon isotope balance in the rock showed an increase in carbon-12 relative to carbon-13 over time.

    Scientists have found evidence around the world of a swift decrease of about 1% in the ratio of C-13 to C-12 in carbonate rocks from the end of the Permian period. That change was the first, the largest, and the most rapid of a series of decreases and increases in the ratio of C-13 to C-12, a series that continued until the isotope ratio suddenly stabilized in the middle of the Triassic period. The recovery of organisms that use calcium carbonate to build hard parts, such as shells, followed soon after that stabilization. How can we account for that pattern?

    Gases from volcanic eruptions have a C-13 to C-12 ratio about 0.5 to 0.8% below standard, so volcanic eruptions by themselves will not produce a reduction of 1.0% worldwide. Of course, they will contribute their share of the change, especially the volcanic activity of the Siberian Traps.

    A reduction in organic activity would extract C-12 more slowly from the environment and leave more of it to be dissolved and incorporated into sediments, thus reducing the C-13/C-12 ratio. Because chemical reactions are driven by the electromagnetic forces between atoms and lighter isotopes respond more readily to those forces, biochemical processes use the lighter isotopes in preference over the heavier ones. But a study of a smaller drop of 0.3 to 0.4% in the C-13/C-12 ratio at the Paleocene-Eocene Thermal Maximum (PETM; 55 million years ago) concluded that even transferring all the organic carbon (that found in organisms, in soil, and dissolved in the ocean) into sediments would be insufficient: even such a large burial of material rich in C-12 would not have produced the smaller drop in the C-13/C-12 ratio of the rocks around the PETM. The standard explanation for such a spike at the Permian–Triassic boundary – rotting vegetation – thus seems insufficient.

    Buried sedimentary organic matter has a C-13/C-12 ratio 2.0 to 2.5% below normal. In theory, then, if the sea level fell sharply, shallow marine sediments would be exposed to oxidization. But 6,500–8,400 gigatonnes (1 gigatonne = 109 metric tons) of organic carbon would have to be oxidized and returned to the ocean-atmosphere system within less than a few hundred thousand years to reduce the C-13/C-12 ratio by 1.0%. That works out to 100 megatonnes per year or less. Scientists studying the problem do not believe this to be a realistic possibility.

    Geologist Gerry Dickens has suggested that the increased carbon-12 could have been produced rapidly by the release of methane from frozen methane hydrate in the seabeds. Experiments have suggested that a rise of deep sea temperature by 5°C (9 F) would be sufficient to sublimate methane hydrate. Released from the pressures of the ocean depths, methane expands to create huge volumes of one of the most powerful of the greenhouse gases. The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth. Strong evidence suggests the global temperatures increased by about 6°C (10.8°F) near the equator and by more at higher latitudes: a sharp decrease in oxygen isotope ratios (the ratio of O-18 to O-16) and the extinction of Glossopteris flora (Glossopteris and plants that grew in the same areas), which needed a cold climate, and its replacement by plants typical of lower paleo-latitudes.

    Carbon-cycle models confirm that the release of methane from methane clathrates would have been the one sufficiently powerful cause of the global 1.0% reduction in the C-13/C-12 ratio. Methane clathrates, also known as methane hydrates or methane ice, consist of methane molecules trapped in cages of water molecules forming a solid only slightly less dense than water. The methane, produced by methanogenic bacteria, has a C-13/C-12 ratio about 6.0% below normal. At the right combination of high pressure and low temperature, it gets trapped in clathrates fairly close to the surface of permafrost and in much larger quantities at continental margins (continental shelves and the deeper seabed close to them). Oceanic methane hydrates are usually found buried in sediments where the seawater is at least 300 meters (980 ft) deep. They can be found up to about 2,000 meters (6,600 ft) below the sea floor, but usually only about 1,100 meters (3,600 ft) below the sea floor. Much of that shallow sea in north Asia was covered by lava from the Siberian Traps eruptions, which lava provided the heat to release the methane from its hydrates.

    Some scientists, though, assert that the pattern of isotope shifts expected to result from a massive release of methane does not match the patterns seen in evidence obtained from rocks that span the early Triassic Period. But intermittent periods of ocean bottom hyperoxia and hypoxia shading into anoxia (high-oxygen and low- to zero-oxygen conditions) offer an explanation for the C-13/C-12 ratio fluctuations in the early Triassic. Indeed, global ocean anoxia may have been responsible for the fluctuation at the end of the Permian Period. Important fluctuations in oxygen levels would have occurred because the continents of the end-Permian and early Triassic were more clustered in the tropics than they are now. That fact implies that large tropical rivers would have dumped sediment into smaller, partially enclosed ocean basins at low latitudes. Such conditions favor oxic and anoxic episodes, increases and decreases in oxygen levels, which result in rapid releases and burials, respectively, of large amounts of organic carbon. That process may have been responsible, at least in large part, for the inferred pattern of fluctuating C-13/C-12 ratios.

Sea Level Fluctuations

    A lowering of sea level reduces or displaces shallow marine habitats, leading to biotic turnover. Shallow marine habitats are productive areas for organisms at the bottom of the food chain, so their loss increases competition for food sources. Researchers have found some correlation between incidents of pronounced sea level lowering and mass extinctions, but other evidence indicates no such relationship and the lowering of sea level may itself create new habitats. Some have also suggested that sea-level changes result in changes in sediment deposition rates and affect water temperature and salinity, factors that can result in a decline in marine diversity. This is a small factor in the mass extinction, but it is a factor nonetheless.

Anoxic Oceans

    Scientist have good evidence that the oceans became anoxic at the very end of the Permian. The uranium/thorium ratios of late Permian sediments found around East Greenland indicate that the oceans were severely anoxic around the time of the extinction. Marine life, except for anerobic bacteria in the sea-bottom mud, would have been devastated. Evidence also exists indicating that anoxic events can and did cause catastrophic hydrogen sulfide emissions for the sea floor.

    The sequence of events leading to the anoxic oceans would have been:

    I: Global warming reduced the temperature gradient between the equator and the poles.

    In this scenario, warming from the enhanced greenhouse effect would reduce the solubility of oxygen in seawater, causing the concentration of oxygen to decline. Increased weathering of the continents due to warming and the acceleration of the water cycle would increase the riverine flux of phosphate to the ocean. This phosphate would have supported greater primary productivity in the surface oceans. This increase in organic matter production would have caused more organic matter to sink into the deep ocean, where its respiration would further decrease oxygen concentrations. Once anoxia became established, it would have been sustained by a positive feedback loop because deep water anoxia tends to increase the recycling efficiency of phosphate, leading to even higher productivity.

    II: The reduction in the temperature gradient slowed or perhaps stopped the thermohaline circulation. Also known as the ocean conveyor, it comes from the changes in water density due to evaporation and cooling (such as we see in the Gulf Stream).

    III: The slow-down or stoppage of the thermohaline circulation prevented the dispersal of nutrients washed from the land to the sea, causing eutrophication (excessive growth of algae), which reduced the oxygen level in the sea.

    IV: The slow-down or stoppage of the thermohaline circulation also caused oceanic overturn. Surface water sank (it is more saline than deep water because of evaporation caused by the sun) and was replaced by anoxic deep water.

    The global warming which drove the anoxic event was likely caused by the meteoric impact, both directly through the release of carbon from the impact area and by causing the Siberian Traps eruptions, which happened in a coal-rich area. The combination of direct release of carbon and the burning of coal brought about a catastrophic greenhouse effect.

    Evidence for widespread ocean anoxia (severe deficiency in oxygen) and euxinia (presence of hydrogen sulfide) is found in sediments from the Late Permian to the Early Triassic. Throughout most of the Tethys and Panthalassic Oceans, evidence for anoxia, including fine laminations in sediments, small pyrite framboids (structures that resemble tiny raspberries), high uranium/thorium ratios, and biomarkers for green sulfur bacteria, appear at the extinction event.

    However, in some sites, including Meishan, China, and eastern Greenland, evidence for anoxia precedes the extinction. Biomarkers for green sulfur bacteria, such as isorenieratane, the diagenetic (from the process of converting sediment into rock at temperatures and pressures less than those required for metamorphosis) product of isorenieratene, are widely used as indicators of euxinia occurring in the photic zone, because green sulfur bacteria require both sunlight and hydrogen sulfide to survive. Their abundance in sediments from the Permian-Triassic boundary indicates hydrogen sulfide was present even in shallow waters.

    This wide spread of toxic, oxygen-depleted water would have devastated marine life and brought about extinctions of many species. Models of ocean chemistry show that high levels of carbon dioxide would have accompanied anoxia and euxinia. Thus the poisons hydrogen sulfide and carbon dioxide acted together with anoxia as a killing mechanism. Hypercapnia best explains the selectivity of the extinction, but anoxia and euxinia likely determined the high mortality of the event.

    The persistence of anoxia through the Early Triassic may explain the slow recovery of marine life after the extinction. One study found that during the Great Dying surface temperatures on the oceans went as high as 40 C (104 F). That fact gives us a good explanation for why the recovery from the mass extinction took so long: it was simply too hot for life to survive. The higher temperatures also kept oxygen levels low in the oceans. Models also show that anoxic events can cause catastrophic hydrogen sulfide emissions into the atmosphere.

Hydrogen Sulfide Emissions

    Severe anoxia in the oceans at the end of the Permian would have resulted in a flourishing of sulfate-reducing bacteria, making them the dominant force in oceanic ecosystems. As a by-product of their activity those bacteria produced large amounts of hydrogen sulfide, which would have been released into the atmosphere by upwelling of the water. The world would have reeked of rotten eggs. Evidence for such a release comes from biomarkers indicating anaerobic photosynthesis by Chlorobiaceae (green sulfur bacteria), which use sulfide ions as electron donors. Restricted to that zone of the ocean into which sunlight could penetrate, those bacteria lived close enough to the surface of the water that their hydrogen sulfide would have reached the atmosphere readily. This part of the event occurred from the late Permian into the Early Triassic. The fact that this part of the event persisted into the early part of the Triassic gives us one indicator that the recovery of the biosphere from the Permian-Triassic extinction occurred slowly.

    High levels of hydrogen sulfide in the atmosphere would have poisoned terrestrial plants and animals and also exposed them to lethal levels of ultraviolet radiation (by diminishing the ozone layer). This part of the hypothesis explains the mass extinction of plants, which should have flourished in an atmosphere rich in carbon dioxide. Further, fossil spores from the end of the Permian show deformities associated with exposure to ultraviolet radiation.

Combination of the Causes

    None of the causes described above was sufficient by itself to bring about the Great Dying. Strong evidence sketches out a sequence of catastrophes that seem to have worked together to create an even greater catastrophe.

    Continental drift brought the continents together to create the supercontinent Pangaea, thereby creating a precariously balanced global environment, one in which life was not robust enough to survive the later blows directed at it. The aridity of the continental interior and the rise of sea levels diminished the niches available to life.

    A giant meteoric impact filled the atmosphere with dust and gases and caused tsunamis that would have scrubbed the coasts around the world, just as the tsunami of December 2004 scrubbed coasts around the Indian Ocean. The shock wave from the impact, propagating through the planet cracked the crust on the antipode and caused the Siberian Traps eruptions. Because they occurred near coal beds and the continental shelf, those eruptions also triggered very large releases of carbon dioxide and methane, as well as lofting sulfur dioxide into the stratosphere. The resultant global warming caused perhaps the most severe anoxic event in the oceans' history, making the oceans so anoxic that anaerobic sulfur-reducing organisms dominated the chemistry of the oceans and caused massive emissions of toxic hydrogen sulfide. At the same time the sulfur dioxide diminished the ozone layer, allowing a blast of ultraviolet radiation to sweep the land and the upper layers of the oceans.

    Of course there are weak links in this described chain of events: the changes in the C-13/C-12 ratio expected to result from a massive release of methane do not match the patterns seen throughout the early Triassic; and the types of oceanic thermohaline circulation that may have existed at the end of the Permian are not likely to have supported anoxia in the deep sea. But other factors may have mitigated those criticisms. The overall hypothesis looks good nonetheless.

*****

Duration of the Event

    At one time paleontologists and geologists assumed that the Great Dying involved a gradual reduction in taxa over the span of several million years. But evidence indicates that organisms throughout the world, regardless of habitat, suffered similar rates of extinction over the same relatively short period: that evidence implies that the extinction occurred abruptly and world-wide, that it was neither gradual nor local. Now further information has led scientists to accept the idea that the main event lasted less than a million years, from 252.3 to 251.4 million years ago (both numbers ±300,000 years). A detailed study of plutonium-to-lead decay in zircons found in ash beds in China dates the extinction to 252.6 ± 0.2 million years ago, occurring at the same time as the Siberian flood volcanism.

    New evidence taken from strata in Greenland implies the occurrence of a double extinction, with a separate, less dramatic extinction occurring at the end of the Guadalupian epoch, about 9.4 million years before the Permian-Triassic boundary. Confusion of these two events likely influenced the early view that the extinction was a single extended event.

    In the picture we have now, the double extinction began with a mass extinction at the end of the Guadalupian Epoch of the Permian Period, roughly 260 million years ago. Likely caused by the climate changes attending the formation of Pangaea, the first pulse of extinctions sparsened the biosphere. For example, only one genus of dinocephalians of Order Therapsida survived the first extinction. After that extinction peaked, extinction rates remained higher than normal until the occurrence of the main extinction event on the Permian-Triassic boundary.

    Reanalysis of highly fossiliferous rocks from Meishan, China (reported in early 2014) indicate that the end-Permian extinction may have taken as little as 60,000 years to extinguish most of the species living at the beginning of the event. The reanalysis found that carbon levels in the oceans increased sharply about 10,000 years prior to the extinction, likely causing severe acidification of the seawater and a temperature increase of at least 18 F (10 C). This conforms to information inferred from a well-preserved sequence of rocks in East Greenland, which shows that the presence of animals declined over a period of 10,000 to 60,000 years, with plants taking several hundred more millenia to show the full effect of the event.

After the extinction event

    Earlier analyses indicated that life on Earth recovered quickly after the Permian extinctions, but this was mostly in the form of disaster taxa, opportunist organisms that took advantage of the devastated ecosystem, enjoying a temporary population boom and increase in their territory. A small number of genera dominated each major segment of the early Triassic ecosystem—plants and animals in both marine and terrestrial environments. Those genera appeared virtually worldwide in a rather bleak ecosystem. We have, for example, the herbivorous therapsid Lystrosaurus (which accounted for about 90% of early Triassic land vertebrates) and the bivalves Claraia, Eumorphotis, Unionites and Promylina.

    A healthy ecosystem has a much larger number of genera, each living in a few preferred types of habitat. Greater diversity protects an ecosystem from extinctions caused by relatively minor changes in the environment. Relatively few species go extinct and re-radiation of similar species refills the altered biological niches and restores the ecosystem to equilibrium. Such was not the case in the early Triassic.

    Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than recovery took after any other mass extinction. Research, including papers published in 2006, indicates that complex ecosystems with high biodiversity took as long as four to six million years to begin a proper recovery from the disaster. Ecosystems in which specialized animals occupied a wide variety of niches and interacted through complex food webs did not achieve full recovery until about thirty million years after the end-Permian event. In large measure that was true because life kept getting knocked down by successive waves of extinctions, aftershocks of the Great Dying caused by successive catastrophic releases of greenhouse gases. Pangaea was a bleak place in the early Triassic.

Changes in marine ecosystems

    In the oceans surrounding Pangaea complex ecosystems, in which species interacted with each other in new ways, became more common after the end-Permian event. For example, prior to the extinction about 67% of marine animals were sessile, but during the Mesozoic Era only about 50% of the marine animals were sessile while the rest were free-moving. Analysis of a survey of marine fossils from the relevant periods indicated a decrease in the abundance of sessile epifaunal suspension feeders (animals anchored to the sea floor) such a brachiopods and sea lilies and an increase in more complex mobile species such as snails, sea urchins, and crabs. Because any mutation that enables the bearer to escape from a predator will be promoted by natural selection, increases in the effectiveness of predators led to motility becoming far more prevalent.

    Before the Permian-Triassic extinction both complex and simple marine ecosystems were equally common, but after the recovery from the disaster the complex communities outnumbered the simple communities by nearly three to one. That change reflects the beginning of the Mesozoic Marine Revolution, in which the evolution of shell-crushing cephalopods and sea-going reptiles (such as ichthyosaurs) increased the predation pressure on sessile organisms. For example, crinoids (sea lilies) suffered a major decrease in the variety of their forms, but then a brisk adaptive radiation created new forms possessing flexible arms.

Changes in Land vertebrates

    Before the extinction, mammal-like reptiles were the dominant terrestrial vertebrates. Lystrosaurus (an herbivorous mammal-like reptile) was the only large land animal to survive the event, becoming the most populous land animal on the planet for a time. Then early in the Triassic, archosaurs (Superorder Archosauria) became the dominant terrestrial vertebrates, until they were overtaken by their descendants the dinosaurs. Archosaurs quickly took over all the ecological niches previously occupied by mammal-like reptiles (including the lystrosaurs' vegetarian niche), and mammal-like reptiles could only survive as small insectivores.

    Lystrosaurus, a pig-sized herbivorous dicynodont therapsid, constituted as much as 90% of some earliest Triassic land vertebrate faunas, making it by far the most abundant early Triassic land vertebrate. Smaller carnivorous cynodont (dog-toothed) therapsids, including the ancestors of mammals, also survived. In the Karoo region of southern Africa evidence shows that the therocephalians Tetracynodon, Moschorhinus and Ictidosuchoides survived, but they do not appear to have been abundant in the Triassic.

    Like other life forms, land vertebrates took an unusually long time to recover to normal levels of biodiversity following the end-Permian event. Recovery of life was extremely slow for the first five million years of the Triassic. The rate at which diversity evolved picked up after that period ended, but even then the full recovery took an additional 25 million years. Only then, 30 million years after the Great Dying, were dinosaurs, pterosaurs, crocodiles, archosaurs, amphibians, and mammaliforms abundant and diverse on the land.

The Fungal Spike

    For some time after the Permian-Triassic extinction, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10% of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100% of the available fossil record. We expect to see such a pattern around a mass extinction because fungi flourish where there are large amounts of dead organic matter.

    However, some researchers argue that, because their remains have only been found in shallow marine deposits, fungal species did not dominate terrestrial life. Alternatively, other researchers argue that fungal hyphae are simply better suited for preservation in the environment and thus create an inaccurate representation of fungi in the fossil record.

    Some researchers have suggested that the Permian–Triassic boundary is associated with a sharp increase in the abundance of marine and terrestrial fungi. Such an increase would be caused by the sharp increase in the amount of dead plants and animals that the fungi fed upon. For a while some paleontologists used this "fungal spike" to identify the Permian–Triassic boundary in rocks that are otherwise unsuitable for radiometric dating or which lack suitable index fossils. But even those researchers who favor the fungal spike hypothesis point out that "fungal spikes" may have been a repeating phenomenon created by the post-extinction ecosystem in the earliest Triassic. However, the very idea of a fungal spike has been criticized on several grounds. One criticism claims that Reduviasporonites, the most common supposed fungal spore, was actually a fossilized alga. Another criticism claims that the spike did not appear worldwide and that in many places it did not fall on the Permian–Triassic boundary. The algae, which were misidentified as fungal spores, may even represent a transition to a Triassic world dominated by lake-based ecosystems rather than an earliest Triassic zone of death and decay in some terrestrial fossil beds. Newer chemical evidence dilutes these critiques by agreeing better with a fungal origin for Reduviasporonites.

And At Last

    In the seas after the Great Dying there occurred a significant shift in simple plant life. Red phytoplankton evolved alongside the pre-existing green phytoplankton. From about 250 families after the Permian-Triassic extinction to over one thousand families today, marine life has diversified more less continuously alongside the evolution of the various forms of red phytoplankton (diatoms, dinoflagellates, coccolithophorids, etc.), which are more prolific and carry more nutrients per kilogram.. Phytoplankton (algae) feed zooplankton, which feed creatures higher up the food chain. In the Late Paleozoic and early Mesozoic forests spread on land and humidity increased, leading to increased weathering of rock. That weathering sent more nutrients into the sea and enabled the red algae to flourish. Thus we see an increasing diversity of sea animals, especially predators (with more to prey upon).

    And on the land, as much as 243 million years ago (8 million years after the end-Permian event), strangely mutated forms of archosaur began to appear and to fill in blank biological niches. On the prairies of Pangaea they walked with their legs under their bodies instead of sprawled out to the sides. Some 12 million years later (231.4 million years ago) the land was occupies by meter-long, bipedal omnivores called Eoraptor, the first known true dinosaur and possibly the common ancestor of all dinosaurs. Thus the first species, the first genus, of Order Dinosauria emerged from the Archosaurs as a branch of Class Reptilia. Adaptive radiation created new varieties of dinosaurs, which displaced the archosaurs to become the dominant lifeforms on the land.

    Slowly, then, the pale light of the Age of Dinosaurs dawned over the ruined planet. And under the feet of the dinosaurs scampered tiny, hairy insect-eaters, insignificant creatures of Class Mammalia that no reasonable person would guess had a future dominating life on Earth.

Appendix: The Erect-Limb Stance

    With your feet spread apart, crouch down until your hips are level with your knees and see how long you can hold that position. Next stand up straight and see how long you can hold that position. Clearly the erect stance of the mammal uses less energy than does the sprawled-out stance of the lizard. And the same holds true for walking and running: the erect stance uses less energy for a given distance covered.

    Now get down into the position used for push-ups, the posture in which your arms are splayed out. How much of your surroundings can you see? Now stand up and make the same observation.

    The advantages of the erect-limb stance seem clear. So why did evolution take so long to discover them? After all, it’s only in the mid- to late-Triassic, some 130 million years after vertebrates emerged onto the land, that we see animals with the erect-limb stance.

    The easy answer tells us that the sprawled-limb swagger of the lizard was sufficient for all land vertebrates, as it is for reptiles and amphibians today. Natural selection only promotes mutations when they offer an organism a net advantage in the struggle for survival and reproduction. Evolution is satisfied with good enough, so until the middle of the Triassic Period animals moved with the kind of swaggering gait that we see in crocodiles and alligators.

    But for crossing long distances crocodiles use the "high walk", in which the animal lifts its body as far off the ground as it can and then walks with an almost complete erect-limb posture. The high walk minimizes sideways flexing of the animal’s body, thereby freeing the animal from Carrier’s constraint.

    Described by D.R. Carrier in 1987, Carrier’s constraint denotes a difficulty in moving and breathing at the same time, which difficulty occurs in animals with paired lungs and a sideways flexing of their bodies as they walk or run. It’s the reason that lizards run in short bursts with frequent pauses to catch their breath. The constraint comes about because as an animal’s body flexes sideways it makes one lung expand while it compresses the other: as a consequence stale air from one lung gets pulled into the other instead of being expelled from the body, thereby interfering with proper oxygenation of the blood.

    On the vast savannahs and deserts of Pangaea the high walk gave animals an advantage over those animals that stuck with the sprawled-limb stance. They could breath easier as they walked and they could walk farther on a given amount of chemical energy. In arid and semi-arid conditions, with their scarcer sources of food and water, natural selection would promote mutations that enhanced those advantages. Slowly, generation by generation, bones changed shape and muscles shifted until Pangaea was populated by creatures whose legs were straight, vertical, and aligned alongside the body. The sprawled-out stance of the lizard had become the erect-limb stance of the dinosaur and the mammal.

    That stance then promoted the evolution of two other traits common to dinosaurs and mammals – endothermy and fluffy body coverings (feathers for dinosaurs and hair for mammals). Here again we see that evolution doesn’t do only one thing. The evolution of one trait induces the evolution of others, just as the evolution of legs also brought about the evolution of lungs and hips.

    Endothermy, warm-bloodedness, likely came about due to the extra energy animals had when they used the high walk or the erect-limb gait rather than the sprawling-limb gait. It likely came about through shivering, animals generating body heat through the mechanical action of twitching their muscles. Every morning, when dawn’s first light spilled over the horizon and filled the land, these creatures woke from their nightly torpor and shivered to warm themselves in preparation for getting an early start on their daily adventures. But easy movement and shivering don’t go well together, so natural selection would promote any mutation that enabled a creature to convert chemical energy directly into heat. Thus those creatures became properly endothermic with shivering kept as an emergency warming system (which we still have).

    If an endothermic animal loses heat too rapidly, it won’t do well in the evolution sweepstakes. It must either eat more food to replace the lost energy or evolve some means of inhibiting heat flow to its surroundings. Like the horned lizard, these creatures may already have had their scales modified by predatory pressure into stubby, thorn-like spines, which then evolved into the longer kind of spines that we see on echidnas and hedgehogs. Those would not have provided much thermal protection at first, but as they evolved further into hair or feathers their insulating property would have increased, thereby promoting the mutations that made them into the body coverings that we see today.

    Thus the tendency of some creatures on the prairies of Pangaea to use the high walk led to the erect-limb stance and produced two new traits to go along with it.

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