THE PERMIAN WORLD
|The world of the Late Permian. Present-day continents are shown in outline: in the Southern Hemisphere (left to right), South America lies against Africa, with India to the right. Below are Antarctica, and to its right, Australia. Together these pieces (South America, Africa, India, Antarctica, and Australia) form Gondwana. In the Northern Hemisphere (left to right), North America is adjacent to Western Europe, though much of Europe is underwater. Siberia is further right. Together, they (North America, Europe, Siberia) form Laurasia. Pangea includes both Gondwana and Laurasia. The North and South China blocks are the large masses in the ocean, well to the right of Gondwana and Laurasia. The continents and the China blocks enclose the Tethys Ocean; surrounding Pangea is Panthalassa, the world ocean. The light blue color refers to continental shelves, underwater parts of the continents. (Map from Scholle, 1995, based on the work of Scotese and others, 1979.)|
A map of the Earth at the time in Permian time bears no resemblance to that of the present Earth. This is because continents have been moving back and forth across the surface of the planet, colliding to form mountain ranges, and breaking apart to form oceans, ever since the molten rock of Earth's earliest days first cooled sufficiently to allow large masses of rock to congeal. (This probably occurred about four billion years ago, some half billion years after Earth's formation, though the timing of the origin of the continents is disputed.) For further information on this process, see the Primer on Plate Tectonics:
APPENDIX 1: MOVING THE CONTINENTS; CREATING THE OCEANS
At times the continents that existed have been relatively dispersed across the face of the globe, as they are today. At other times, they have amalgamated into supercontinents, containing the equivalent of several of today's continents. On occasion, there have even been megacontinents, which comprise in one continuous mass almost all the continental material on the Earth's surface. Such was the case in the Permian, when a major feature of global geography included a global megacontinent we call Pangea, that is, "all-the-earth."
This megacontinent was comprised of two immense landmasses ("supercontinents"), one composed of what is now Antarctica, Australia, the Indian subcontinent, Madagascar, Africa and South America and called Gondwanaland, after part of India, Gondwana, the "land of the Gonds" [so Gondwanaland is actually redundant, meaning land of the Gonds land]. (The Gond kingdoms of Madhya Pradesh -- the "Middle Province" of central India -- flourished from about the 12th century on, and Gondwana was a powerful state in the 16th century.) The other immense landmass consisted of what is now North America, Greenland, Europe and a large part of Asia, and is referred to as Laurasia. This name derives from a combination of the Laurentia (northeastern Canada), geologically the most ancient portion of North America (and often referred to as the Laurentian or Canadian Shield) with Asia.
These two landmasses partially enclosed a large tropical ocean, centered on the equator, called the Tethys. Bounding the Tethys to the east were the North China and South China blocks, great islands somewhat like today's Greenland or New Guinea. Within the Tethys, north of Gondwana, was a string of large islands (comprising today's Turkey, Iran, Afghanistan, and Malaysia) that were part of the largely submerged Cimmerian continent. (The Lhasa Block, a large island that eventually became much of Tibet, may have been part of Cimmeria, or may have followed it: Wignall and Twitchett, 2002.) The Cimmerian continent had detached from Gondwana earlier in the Permian (or perhaps the Late Carboniferous), and had begun moving across the Tethys. The splitting of Cimmeria from Gondwana was caused by the development of the "Tethian rift," a mid-ocean ridge, which was eventually to transport Cimmeria all the way across the Tethys and attach it to Laurasia. (The Tethian rift, because it produced ocean floor, actually created a new ocean, labeled the Neo-Tethys, and helped destroy an older one, called the Paleo-Tethys, which subducted beneath Laurasia.)
Surrounding the entirety of Pangea as well as the North and South China blocks was an immense ocean, a hemisphere-sized ocean (therefore larger than today's Pacific): the "all-ocean," Panthalassa. This was the largest ocean of the Phanerozoic. Some paleoceanographers (those who study ancient oceans) believe that Panthalassa was a stagnant, highly stratified ocean, perhaps largely anoxic (lacking in the dissolved oxygen needed by all animals and other organisms): "dead," except for its upper reaches (Wignall and Twitchett, 2002). The evidence for this anoxic, stratified ocean, however, comes from a single locality (Isozaki, 1994; 1997a), and that source has indicated that the anoxia may have been the consequence of events at the end of the Permian (Isozaki, 1997b), rather than having been an ongoing condition of this ocean. Certainly careful studies of the organisms in the ocean waters off the northwest coast of Pangea reveal no such anoxia before the end of the Permian (Beauchamp and Baud, 2002).
The Tethys, with just a few relatively
narrow passages (up to about 1000 kilometers/600 miles wide) connecting
it to the larger ocean (Panthalassa), may have had more restricted
circulation, although today's Mediterranean, with only a single
outlet through the Strait of Gilbraltar, exchanges large quantities
of water with the adjoining Atlantic. The evidence for anoxia
in the Tethys, at least during the last million years or so prior
to the end of the Permian (Wignall and Hallam, 1992; Twitchett
and Wignall, 1996; Wignall, 1995; Kershaw, 1999) is somewhat better
than for Panthalassa. Being a warm and largely enclosed ocean
would have made the equatorial Tethys more susceptible to oxygen
depletion (warm water holds less dissolved gas) than Panthalassa,
which stretched from pole to pole, and undoubtedly was recharged
with oxygen in its colder regions. Before the last days of the
Permian, however, it seems likely that the Tethys was quite a
normal (unstratified, oxygenated) ocean, and that Panthalassa
also likely was so right to the very end.
Large landmasses produce extreme weather in their interiors. Whereas coasts tend to have climate conditions which are milder due to the moderating effects of nearby oceans, continental interiors have temperatures which are both hotter in summer and colder in winter than those on the seacoast. The midwestern regions of Canada and US, for example, can get quite hot in summer, producing violent thunderstorms and tornadoes. The same areas can be brutally cold in winter, with temperatures descending regularly into the minus thirties (°C, = minus twenties °F) in Missoula, Montana and into the minus forties (°C or °F) in Edmonton, Alberta. By contrast, Ketchikan, Alaska, though further north but on the Pacific coast, never has such extreme temperatures. The coldest spot on Earth during the Northern Hemisphere's winter is frequently around Yakutsk in the interior of Siberia, rather than places further north.
Large landmasses not only have climate extremes in their interiors, however, they also generate weather extremes on their periphery. The monsoons that bring seasonal rains to southern Asia, for example, are largely the creation of low pressure systems that form over the Asian continent. The heat of summer in the interior of Asia warms the air, causing it to rise and surface air pressure to drop. This low pressure region pulls in moisture-laden air from surrounding oceans, drawing major storms to the Asian coasts. Because of the great size of Pangea, it seems likely that its monsoons were proportionately greater than today's.
The surface layer -- about a hundred meters (yards) deep -- of Panthalassa would have been well stirred by such tempests, particularly near the shore. Wind generated waves would also have helped mix the surface layer, bringing nutrients to the surface and oxygen into the water. The size of waves on a given body of water is the uninterrupted expanse over which the wind can blow, a dimension known as fetch.
The reason that waves on a small lake
are smaller than waves on a larger lake, other things being equal,
is that the smaller lake provides less fetch for waves to build.
On Panthalassa, fetch would have extended for half the globe,
allowing wind waves to build to considerable heights. In addition,
great oceanic swells (ultimately the consequence of the rotation
of the Earth), known as Rossby waves, may pump nutrients from
deeper layers to the surface of the ocean and thereby feed photosynthetic
algae. The churning of the upper regions of the ocean by this
mechanism has been labeled "the Rossby Rototiller" (Siegel,
Toward the end of the Permian, two major volcanic events took place. One was an episode of significant pyroclastic volcanism in southern China. Pyroclastic volcanism is a particularly violent kind of volcanism, exhibited today around the "Ring of Fire," the lands which border the Pacific Ocean. In the Andes of South America, the Cascades of western North America, the Aleutian Islands of Alaska, the Kamchatka Peninsula of the Russian Far East, in Japan, the Philippines, Indonesia, and many volcanic islands of the South Seas, we regularly see highly explosive volcanoes called stratovolcanoes producing devastating eruptions of ash and lava.
In recent decades, we have seen such eruptions at Mount St. Helens in the state of Washington, Mt. Pinatubo in the Philippines, and Mt. Unzen in Japan. Stratovolcanoes, when they are not in their eruptive state, are often beautiful, symmetrical mountains, draped with evergreen forests on their slopes and capped with brilliantly white fields of snow and ice. When they erupt, however, they produce huge dark clouds of ash and gas which rain destruction on nearby communities. Some of these immensely hot ash and gas flows, called pyroclastic flows, race along at speeds of greater than a hundred miles per hour, hugging the ground, searing and suffocating all living things they encounter. Stratovolcanoes also erupt blobs of molten lava which fall as rocky bombs on areas close by. The heat produced during the eruptions of stratovolcanoes can melt their snow and ice fields, sending floods of water, mud, and rock (called lahars) surging down nearby river valleys. These lahars may carry volcanic debris a hundred or more kilometers (about 60 miles) from its source.
Particularly violent stratovolcanic eruptions can actually blast away the summits of their peaks, as happened with Mount St. Helens. Even more violent was the eruption about 10,000 years ago of Oregon's Mt. Mazama, which blasted away most of the mountain. Mount Mazama exists no longer; its rim forms the retaining wall of the body of water we now call Crater Lake. Yet more was the 1883 eruption of the Indonesian stratovolcano Krakatoa. Tens of thousands of Indonesians perished in the tsunamis it produced; the sound of its explosion was widely heard around the world. Thousands of others in the Roman cities of Pompeii, and Herculaneum died in the pyroclastic clouds unleashed in 79 CE during the eruption of the stratovolcano Vesuvius. The 1902 eruption of Mount Pelée on the Caribbean island of Martinique killed almost 30,000 people, including the entire population of the city of St. Pierre: when the pyroclastic clouds passed, a single human being was left alive in the city prison.
Despite such violence, however, stratovolcanic eruptions do not cause mass extinctions. They can cause climate effects, but even those are limited and short-term.
The pyroclastic eruptions of South China stratovolcanoes about 250 million years ago, therefore, are extremely unlikely to either have caused the end-Permian extinction, or even contributed in any significant way to that event. This is because, despite the fact that stratovolcanoes usually exist in long chains or arcs rather than as isolated volcanoes, they don't erupt all at once. Usually one volcano in a chain will erupt, then, some decades later, a different one. This is how the chains of volcanoes in Indonesia, or the Andes, or the Cascades behave, so our evidence is good. In addition, though each eruption does cause significant harm and disruption to nearby living things -- demolishing forests and their inhabitants, for example -- the effects are generally quite local and the affected organisms are likely to inhabit ecologically similar areas elsewhere in the volcanic chain. The likelihood of more than a few species -- if that -- going extinct in a major volcanic eruption, therefore, is very minimal.
There is, however, a second kind of volcanism that occurred about 250 million years ago which could have had much greater consequences. Near the end of the Permian, in northern Siberia, was the largest volcanic episode of the Phanerozoic. Instead of being produced by ordinary volcanoes, the eruptions likely occurred through great cracks or fissures, spilling huge quantities of syrupy, basaltic lava out over vast areas of the Siberian countryside.
Fissure eruptions are an uncommon type of volcanic eruption, generally unfamiliar even to those knowledgeable about other kinds of eruptions. The most recent large fissure eruption took place in southeast Iceland in 1783-84. There the Laki (Kagagigar) eruption ripped open a crack in the surface some 25 kilometers (15 miles) long, and sent hot, fluid basalt a distance of some 45 kilometers (27 miles). The lava eventually covered about 500 square kilometers (about 200 square miles; Duff, 1993, p. 210-11).
Laki fissure eruption. The map shows the areal extent of the eruption. The photo looks southwest along the fissures, with a cinder cone in front. (Duff, 1993, p. 210-11)
The eruption produced fluorine-laced ash, which contributed to crop failure and to the deaths of about 80% of Iceland's sheep population, largely from fluorine poisoning. The ensuing famine and fluorine poisoning led to the starvation of some 10,000 people, about 20% of Iceland's total population (Fisher, 1997, p. 170; Thordarson and Self, 2003). An acidic atmospheric haze created by the eruption reached the cities of northwestern Europe, where it was observed as a "dry fog" by the American ambassador to France, Benjamin Franklin (Brands, 2000, p. 626). (A similar acid fog is apparently a feature of the Hawaiian volcano Kilauea, where its acidity has created the adjoining Kau Desert: Schiffman, 2006).
The Siberian Traps fissure eruptions of 250 million years ago were accompanied by considerable pyroclastic activity. About 20% of the material erupted by Siberian Traps volcanism was in the form of pyroclastic debris, which produces distinctive rocks called tuffs. Some of this pyroclastic volcanism may have been of a particularly devastating variety called caldera explosions, where an area up to perhaps thirty miles in diameter suddenly settles into an enormous subterranean magma lake, lofting ash high into the atmosphere and carrying it far from its source. Ash from the Long Valley Caldera (California) explosion, which happened about 730,000 years ago, distributed ash over about two-thirds of what are now the contiguous forty-eight states. Caldera eruptions associated with today's Yellowstone, going back some 17 million years, regularly dropped ash over large parts of the country.
But fissure eruptions like those in Siberia at the end of the Permian extrude much of their output as fast flowing lava, much as do today's Hawaiian volcanoes. The thin, syrupy basalt flows in sheets across relatively flat terrain, eventually cooling into what are called "large igneous provinces" or LIPs. Much of the Columbia River valley in Idaho, for example, is occupied by such a LIP, providing farmers with fertile soil for potato crops and tourists with the Craters of the Moon National Monument.
LIPs are characterized by repeated, intermittent basalt flows, which settle and cool on each other like layers on a cake. These flows can look like giant steps, giving them the Dutch name "Traps," meaning steps (Alvarez, 1997, p. 168, fn. 24). In the northwest of India lie the famous Deccan Traps, which erupted about 65 million years ago -- roughly the same time as the end-Cretaceous extinction that killed the dinosaurs -- and have helped confuse the issue as to that extinction's cause (though it now seems to most scientists that the Deccan Traps eruption played no role in that extinction). Similar LIP-produced step-like formations may be found in Washington state's Grand Coulee area.
|A large igneous province (LIP) in Greenland. The layers in this flood basalt formation (presumably part of the widespread North Atlantic Igneous Province, or NAIP) are horizontal, which is the consequence of numerous thin, syrupy flows of basalt, each having cooled over one previously deposited. (Duff, 1993)|
There is much about LIPs which we don't know, because geology of necessity relies greatly on the principle that "the present is key to the past." In other words, geologists come to understand much of what occurred in the past by seeing similar processes occurring today. But if a geologic event is sufficiently rare (or even unique), it may be difficult to understand all the consequences of that event. Thus, while we can examine the consequences of the eruptions of ancient stratovolcanoes because one or another present-day stratovolcano is always erupting (actually, about twenty are erupting worldwide at any given time; Simkin and Siebert, 2000, cited by Wright and Flynn, 2004), we have no contemporary LIP eruptions to compare with the ones we know took place in geologic time. Today's closest similar eruptions may take place in Iceland, but as Iceland's volcanism is part of the general mid-oceanic ridge system's extrusive activities, it may not make for an appropriate comparison. Nonetheless, it is clear that LIPs are produced by the eruptions of huge blobs of mantle magma that have accumulated just below the crust of the Earth.
Rising from the Deep,
When lava erupts from volcanoes or along the mid-oceanic ridge, the iron particles in the lava, though initially pointing at random, orient themselves toward the poles like tiny compasses. Upon cooling, they therefore record in the rock the orientation of Earth's magnetic field at the time. When the tiny iron compasses in the rock point in the direction of today's north pole, the poles are said to exhibit normal polarity; when they point south instead, the poles are said to be reversed. Using sophisticated equipment, the periods of normal and reversed polarity are clearly seen in the rock record. Despite the use of the term normal to describe the current polarity of the Earth's magnetic field, however, there is nothing "abnormal" about reversed polarity: it is just as common in the rock record as "normal" polarity.
Polarity reversals occur every several thousand to many millions of years. When the periods of magnetic stability are relatively short in length they are referred to as chrons, but the few that lasted for extreme lengths of time are called superchrons. Towards the end of the Paleozoic, there was period when the magnetic polarity of the planet was reversed for some fifty million years (from 312 to 262 million years ago), the longest known period of magnetic stability of the Phanerozoic. This interval is known as the Pennsylvanian- Permian reversed superchron, for the two geologic periods during which it occurred. (The Pennsylvanian Period is the younger portion of the Carboniferous, which is divided into the Mississippian -- about 360-320 Ma -- and the Pennsylvanian -- about 320-290 Ma.)
Something was clearly somewhat different down in and near the planet's outer core. We don't know what that something was, but it has been suggested that it had to do with a disturbance at the boundary between the outer core and the overlying planetary layer, the mantle. It has been postulated that, on occasion, large blobs of mantle rock, heated by the fierce heat of the core at the core-mantle boundary, may rise through the mantle and cause eruptions on the Earth's surface. (Some have even suggested that the material at the bottom of the mantle, where the suggested blobs may originate, may be the remnants of old tectonic plates that, once subducted, ultimately drifted down that far.)
It is therefore possible that a great blob of material accumulated at the core-mantle boundary for the fifty million years of the Pennsylvanian-Permian reversed superchron, stabilizing the planet's magnetic field. When it became sufficiently buoyant, it lifted off and eventually made its way to the planet's surface, where it erupted as the Siberian Traps. This proposal thus explains both the existence of the Phanerozoic's longest period of magnetic stability, why it came to an end, and the origin of the Siberian Traps volcanism.
How We Know Where the Continents Were
How do we know what the configuration of the continents and oceans was like 250 million years ago? One major way is by the use of paleomagnetism. As lava is erupted, and iron-bearing particles orient themselves according to the direction of the magnetic poles, those particles record the position of the continent relative to Earth's magnetic field. The iron partilces in each successive lava flow, upon cooling, do the same. As a continent moves, therefore, the lava flows from its volcanic eruptions record the position of the continent at the time of eruption relative to the magnetic poles. Using these "frozen compasses," it is possible to trace the movement of a continent, or piece of continent, as it has moved across the surface of the planet. The work is complicated and difficult (magnetic reversals being one complicating factor), but by carefully examining these natural compasses to determine the original orientation of the ancient lava flows, scientists can determine the ancient positions of continents -- or pieces of continents.
Other evidence is also used. Well before the discovery and use of paleomagnetism, some geologists were struck by the similarity of fossils on continents that are now quite distant from each other. Similar Permian-age land plant fossils, for example, can be found in India, Madagascar, South Africa, South America, Antarctica and Australia, all places now separated by many thousands of kilometers (several thousand miles) of ocean. To explain these similarities, geologists were forced to posit some sort of physical connection (that would allow the movement of terrestrial organisms) between these areas.
For years geologists relied on the idea of "land bridges," like the famous Bering Strait Land Bridge, across which the original human inhabitants of the Americas crossed into the western hemisphere from Asia. But unlike the Bering Strait Land Bridge, which really did exist, and which was the result of the lowering of sea level during the most recent ice age, there was no evidence for most other proposed ancient land bridges. As we now know, large chunks of land do not spontaneously pop up out of the ocean when needed to allow the movement of terrestrial organisms from one place to another, and then conveniently disappear beneath the waves. Rather -- and probably even more amazing than the appearance and disappearance of purported land bridges -- the continents and pieces of continents themselves do move.
In the past, continents which are now widely separated were actually joined together, permitting the relatively easy movement of terrestrial organisms from one place to another. The similar Permian plant fossils of India, Madagascar, South Africa, South America, Antarctica and Australia, for example, can now be explained by the fact that for more than a hundred million years, these landmasses were all part of the supercontinent Gondwana. Thus one line of evidence of previous connections between continents is the similarity of their fossils; another is the similarity of their rocks.
The Permian was the final geologic period of the Paleozoic Era (543 to 250 million years ago). Named for the medieval kingdom of Permia, which occupied a small region between the Ural Mountains and the Volga River (Duff, 1993, p. 87) around what is now the city of Perm, Russia, where strata from this period are prominent, the Permian lasted from about 290 until about 250 million years ago. In some ways, the creatures of the Permian were beginning to resemble those of today. In other ways, they were very different.
On the land, there were forests. These forests were not like the forests of the preceding geologic period, the Carboniferous, during which fern and lycopod trees occupied swampy tropical lowlands and had begun to move into drier uplands. (Though there are no longer any lycopod trees, lycopods are still around, rising to the height of just a several centimeters (a few inches) above wet forest floors. Referred to as club mosses, they are not mosses at all, and an alternative common name, ground pines, gives a better sense of what these plants look like.) These trees became the source of much of the world's coal, conferring the name Carboniferous upon the period.
The forests of the Permian were different. Adapted to a drier world, Permian forests still provided the raw materials for present-day coalfields, but their constituent trees had changed. There are not any cycad trees around today, though their unusual descendants, the cycads, are with us yet. In appearance they are similar to ferns, though their fronds are much larger, and generally tougher and drier. During the Permian, cycad trees were a major component of forests, as were the conifers -- cone-bearing trees that were the ancestors of today's conifers, pines, firs and spruces.
Permian conifers apparently evolved in the northern regions of Pangea, in the supercontinent called Laurasia, and they had not yet spread to the southern supercontinent, Gondwana. In Gondwana, the broad-leafed Glossopteris and its relatives were the dominant trees. The Permian forests contained none of the deciduous flowering trees of today's forests -- those trees hadn't evolved yet -- but the northern coniferous forests would have seemed vaguely familiar.
In the forests and on the plains of the Permian world were vast numbers of a wide variety of reptiles, which had largely replaced an earlier (Devonian-Carboniferous) amphibian fauna. The giant insects of the Carboniferous -- millipedes with bodies up to half a meter (a foot and a half) in length, and dragonflies with three-quarter meter (two and a half feet) wingspreads -- were gone. But there were numerous lineages of reptiles.
Reptiles have a general competitive advantage over amphibians. This advantage is not sufficient to drive amphibians from every possible habitat: after all, there are still plenty of amphibians in the world, hundreds of millions of years since the Permian. Newts, salamanders, frogs and toads are quite plentiful, but, of course, they all need water or highly moist conditions in which to lay their eggs. But reptiles came up with an evolutionary innovation which freed them from that requirement: the amniotic egg. This egg, in essence, carried wet conditions with it. Having a relatively hard shell, it was resistant to drying, and allowed reptiles to move to areas like drier, upland forests, that were generally inhospitable for amphibians.
During the Permian, some reptiles also began to evolve a more efficient system of locomotion. Early reptiles, like many amphibians, possessed sprawling limbs. But some reptiles evolved the ability to place their limbs under their bodies, first the hind limbs, then later, the front. The fully upright stance was an evolutionary development that took many tens of millions of years. But as the Permian progressed, having upright limbs beneath the body became common, particularly among the more successful lineages of reptiles. This position of the limbs permitted the reptiles more rapid and efficient locomotion, increasing their effectiveness as predators.
Many reptiles were not predators, however. Unlike the amphibians, some reptiles developed the ability to process large amounts of vegetation. More efficient jaw muscles and more specialized teeth -- or, in some cases, horny beaks like turtles -- allowed them to crop and chew plant matter. The girth of some of these herbivorous reptiles testifies to the large guts they employed for digesting large quantities of vegetation, and their robust limbs indicate the physical strength required to transport these heavy creatures around.
Another evolutionary innovation freed many lineages of reptiles from obligatory cold-bloodedness. Most reptiles, like many other organisms are cold-blooded, that is, their body temperatures are just about the same as the world around them. This limits their ability to be up and about during the cooler hours of the day, and restricts them to warmer climates. From the early Permian on, however, various lineages of reptiles evolved solutions to this limitation. Most astonishing, perhaps, were the sails that were developed by particular herbivorous and carnivorous reptiles.
Labeled sails because that is what they look like, these features were actually constructs of vertebral bone, blood vessels, and skin that rose from the backs of reptiles like Edaphosaurus (an Early Permian herbivore), and Dimetrodon (a famous Early Permian carnivore). Most paleontologists believe that the primary function of these sails (there may have been additional functions, like attracting potential mates) was to catch the rays of the sun. This would allow their possessors to warm up quickly in the morning, and get on with the day's business earlier than their cold-blooded cousins. They would have also been able to stay more active during the day, and radiate away heat more effectively on hot days. Some paleontologists believe that these reptiles with solar panels may also have had systems that shut down most circulation from the sails during cooler periods, avoiding heat loss.
The sail system may have been just one of the ways that reptiles employed in attempting to regulate their body temperatures. In addition to the lineages that led to today's reptiles -- snakes, lizards, alligators and crocodiles, and turtles -- one highly successful reptile group -- the therapsids -- eventually led to the mammals. The therapsids were so successful, in fact, that they are estimated to have left behind about 90% of terrestrial vertebrate fossils in the Late Permian. Huge herds of these animals probably browsed the vegetation of southern Africa and elsewhere in Gondwana.
One key to their success may have been their thermoregulatory ability, the ability to regulate their body temperatures. Whereas reptiles have scaly skins, which provide little insulation, some therapsids may have evolved body coverings of hair. Though we do not have evidence they wore fur coats, some therapsid fossils do exhibit snout pits, dimples in facial bones that may have accommodated those particularly sturdy hairs we know as whiskers. If these therapsids had whiskers, they likely had some body hair as well. Later therapsids may also have been developing systems of internal thermoregulation, akin to mammalian warm-bloodedness.
While the mammal-like reptiles were thriving, other lines of reptiles were also evolving. Some of these reptiles would eventually return to the seas, and dominate the Mesozoic oceans as plesiosaurs and ichthyosaurs. Another reptile line would lead to the large and fearsome carnivorous monitor lizards of today's world, and also to the mosasaurs, another major group of large Mesozoic marine reptiles. All of these marine reptiles -- plesiosaurs, ichthyosaurs, and mosasaurs -- would meet their fates at or near the end of the Cretaceous.
The last reptile lineage worth noting here was that which eventually produced the crocodiles and alligators. This reptile line would lead, in the Late Permian and Triassic, first to the Archosauromorphs -- literally, the ruling lizard forms -- and then to the Archosaurs, the "ruling lizards" themselves. This descendants of this line of reptiles would take over the position of dominant terrestrial vertebrates from the mammal-like reptiles during the latter part of the Triassic. They would also put at risk those mammal-like reptile lineages that led, hundreds of millions of years later, to ourselves and all modern mammals. These archosaur descendants were literally the terrible lizards, the dinosaurs. They would dominate the land for over a hundred and thirty million years.
In the two Permian oceans, Panthalassa and the Tethys, fish and sharks were the dominant vertebrates. Whales and dolphins -- marine mammals -- were hundreds of millions of years in the future; after all, even true mammals were yet to evolve. But the armored fish -- fish with great bony plates on their heads -- of the earlier Paleozoic, particularly the Devonian, were a distant memory. Though the fish and sharks of the Permian oceans were not of types around today, their forms are familiar.
Not so the ammonites, which superficially resemble today's chambered nautilus of the southwestern Pacific. These often large, coiled mollusks were highly sophisticated organisms, quite unlike their distant cousins, the clams and oysters. Like their closer cousins, the octopi, ammonites were smart and stealthy predators. They hunted and consumed fish, and competed with larger fish for smaller prey.
On the shallow ocean floors of Permian continental margins, trilobites were declining in number and variety after thriving for hundreds of millions of years. Sponges were plentiful, and there were carbonate reefs, though not the structurally sophisticated reefs of the present. Two types of corals existed, the cone-shaped rugose corals, and the flatter tabulate corals, though neither were major reef constituents.
The single-celled marine organisms known as foraminifers (referred to by most scientists as forams) were flourishing, both in the water column and on the seafloor. These organisms construct shells of calcium carbonate (like many sea creatures, clams, for example) or build them by gluing together grains of sand and bits of shell. Like amoebae, they have pseudopods (literally, "false-feet": extensions of the cell body) for locomotion and feeding, and consume both phytoplankton (plant-like forms of plankton; they produce their own food) and zooplankton (animal-like forms of plankton; they consume other organisms). (Plankton is the name for organisms which passively drift in the sea. Though they may have limited means of locomotion, for the most part they are just carried along by ocean currents.) One type of Permian foram, the fusulinids, was especially notable. Perhaps with the cooperation of symbiotic algae, some fusulinids reached the size of large grains of rice.
The oceanic food chain, like today's,
must have ultimately depended on photosynthetic organisms. Many
of these organisms were relatively simple and provided fodder
for the zooplankton. Lacking hard parts, they left no evidence
of their existence beyond possible organic residues on the ocean
floor. Other photosynthesizers were more complex, and constructed
minute shells of silica (glass). These photosynthesizers, the
diatoms, have been extremely important in Earth's oceans, and
continue so today.
The end-Permian Extinction
The magnitude of the extinction at the end of the Permian is unparalleled in Earth's history. Although back in the dim reaches of time, during the Precambrian, there may have been extinctions which took out a higher percentage of individuals, and a higher percentage of the then existing groups of organisms, the fossil record for that time is extremely spare, and extinction events almost impossible to recognize. During the eon of visible life (the Phanerozoic), however, the geologic record has provided abundant fossils, making it possible to identify major extinction events. Based on the fossil evidence, therefore, the extinction at the end of the Permian exceeds all other mass extinctions of the Phanerozoic.
(The end-Permian extinction may have come in two stages, one several million years earlier than that of the final catastrophe. Some scientists consequently believe that the end-Permian was actually a kind of "double extinction," with a smaller but nonetheless major extinction coming perhaps five million years before the ultimate event [see, for example, Stanley and Yang, 1994; Racki, 2003]. This extinction came at the end of the next-to-last geological interval [in this case called an "age"] of the Permian, and is generally referred to as the end-Guadalupian extinction. However, it is clear that the devastation of this earlier event was not of the same scale as that at the actual end of the Permian, and it is that extinction which is our focus.)
According to some scientists (for example, Stanley, 1987, p. 99), the forests and terrestrial plants of the Permian are the group of organisms that best survived the end-Permian extinction. Plants do tend to be more resistant to extinction than other organisms, partly because they reproduce by means of spores and seeds, which can survive long periods of unfavorable climate conditions like drought. Plants also frequently are able to reproduce from roots or subsurface stems and tubers, which because of their location underground enjoy some protection from events on the surface. Consequently, terrestrial plants have been considered as relatively invulnerable to mass extinction (Knoll, 1984).
But terrestrial plants seem to have been hard hit at the end of the Permian, according to some systematic surveys of plant groups before and after the extinction. Over an extended period of millions of years, the number of terrestrial plant families dropped almost 50%, and the total number of species fell about 20% (Knoll, 1984, Figures 1 and 2). This represents the only significant loss of plant diversity in the entire Phanerozoic (there was a lesser loss in the Late Devonian).
More detailed studies show that many specific plants did become extinct at the end of the Permian; these included, most notably, the broad-leafed, small- to medium-sized tree Glossopteris, which had been ubiquitous throughout Gondwana. Plants related to and associated with Glossopteris (together called the Glossopteris flora) disappeared from many parts of the world. In North China, a pteridophyte (fern, horsetail and lycopod) flora replaced the conifers, while in South China the number of species of spores and pollen fell by almost one-half. This pattern was apparently worldwide, with similar changes reported from Greenland, Poland, Hungary, Pakistan, Madagascar, and Australia. The Early Triassic flora that replaced that of the Permian included far fewer species (Yang, 1992).
||Glossopteris. This tree and its close relatives were dominant members of Permian forests, but they were devastated by the end-Permian extinction. Nonetheless, some species did recover, and Glosssopteris trees were present in the Triassic, and some may have survived into the Jurassic. Some Glosssopteris trees stood about 4 meters (13 feet) tall. Artists' renderings of this tree vary considerably, however: Compare this drawing to that in Appendix 2. Part of the difference in the depictions can undoubtedly be traced to the fact that different parts of the tree (leaves, pieces of branch, trunks) are found separately, and the artist must reconstruct how they were actually assembled. (Stewart and Rothwell, 1993, Figure 26.6, p. 372)|
In the Sydney area of southeastern Australia, the plant extinction seems to have been particularly devastating: 97% of the Late Permian leaf species died off (Retallack, 1995). Though spore data from this area indicates that a higher percentage of plants may have survived than is indicated by the leaf fossils, it is clear that the change in vegetation was sudden and profound. Within only a short time, perhaps only a few thousand years, the Glossopteris flora had been replaced by an Early Triassic flora (Retallack, 1995). And there is no question that end-Permian plant mortality must have been high: fungi, which live on dead plant matter, did quite well at the time of the extinction.
We know this from the sudden increase in the numbers of fungal spores preserved in the fossil record. In fact, more than 95% of the spores found by one study at the boundary between the Permian and Triassic Periods are fungal. This "fungal spike" was worldwide in extent: similar evidence has been found in North America, Greenland, Europe, Asia, East Africa, Madacascar, and Australia (Eshet, 1995). The fungal spores are found in every type of formerly aquatic environment -- marine, lake, river -- an additional indication of how widespread the fungal spike was (Visscher, 1996). Though plants are presumed not as vulnerable to mass extinction events as other organisms, they are also presumed to be more vulnerable to changes in climate. Being rooted, they cannot escape when ecological conditions undergo rapid and/or substantial shifts (Knoll, 1984).
Terrestrial animals, by contrast, suffered extreme losses at the end of the Permian. Huge numbers of amphibians and reptiles died off. (Mammals and birds, remember, had not yet evolved.) More specifically, about three-quarters of the amphibian and reptile families became extinct (Vickers-Rich and Rich, 1993, p. 105). A family is a biological group containing many species, which in turn may be composed of millions of individuals. (One modern bird species contains only three individuals: this species is one of many on their way to extinction in today's world. Other species, such as those of bacteria, contain uncountable numbers. Mammal species, on average, have about 40,000 breeding individuals, though obviously this average includes some species with huge numbers of individuals, such as humans or field mice, as well as others which have far fewer, like tigers or polar bears.) For a biological family to go extinct, every individual member of every species in that family must die off.
Vertebrates like amphibians and reptiles have hard parts like skeletons and teeth which make for good fossils. Most other land animals have no such durable parts. Insects, for example, have external skeletons built of proteins, which generally do not survive for long lengths of time. The insect fossil record is therefore quite incomplete, as is the fossil record for most land invertebrates (creatures without backbones). Worms, slugs, and other less familiar invertebrates leave even fewer traces than insects. As a result, it is quite difficult to provide an estimate of the losses suffered by these organisms at the end of the Permian. There is no reason to suspect, however, that they fared any better than other land animals.
We do have a better understanding of the losses sustained by marine organisms, at least those which have hard parts and fossilize well. (In addition, the constant supply of sediment buries dead organisms and thereby helps preserve them.) Fish and sharks apparently suffered only minor losses: only about 10% of marine vertebrate families died off (Sepkoski, 1982). But among the marine invertebrates, the end-Permian devastation was comparable to that of the land vertebrates. It is estimated that between 90 and 96% of marine invertebrate species, and over 50% of marine invertebrate families, went extinct (Sepkoski, 1986). These marine extinctions hit forams, gastropod (snail-like) and bivalve mollusks, and echinoids (sea urchins, starfish, and their relatives) hard.
Even harder hit were the rugose and tabulate corals, which disappeared forever (though some believe that the tabulate corals went earlier in the Permian; see Boardman, 1987), ammonites, brachiopods (which superficially resemble clams but are vastly different), and bryozoans (colonies of organisms with lacy or platy skeletons). One arthropod group (the group that includes crabs, lobsters, and all insects) that had been very successful for more than two hundred million years, the eurypterids, also met its demise. Another extremely successful group, the blastoids, distant relatives of the starfish and the sea urchin, similarly came to its end. Closer cousins of the blastoids, the crinoids, many of which had stalks to lift them off the sea floor and allowed them to filter-feed in higher currents, survived the extinction with heavy losses (stalked crinoids are known as sea lilies, because they superficially resemble lilies, but they are animals, not plants). Only small groups of ammonites and bryozoans made it across the boundary between the Permian and Triassic Periods. In addition, the numbers of acritarchs -- enigmatic marine microfossils with thick organic coats that are probably algal spores or cysts, and are considered an "opportunistic survivor species" -- increased dramatically (Retallack, 1995; Eshet, 1995).
Marine organisms that were attached to or lived on the seafloor, including corals, bryozoans, echinoderms, and certain brachiopods (articulate brachiopods), fared poorly in the end-Permian extinction. These organisms, many of which are also characterized by heavy carbonate skeletons, weak circulatory systems and low metabolic rates, lost a high percentage of their members (Knoll, 1996). Other marine organisms, with higher rates of metabolism, stronger circulatory systems, and gills for better absorption of dissolved oceanic gases, had far fewer losses. The survival rates of these organisms -- mollusks, crustaceans, vertebrates and exotic creatures like sea squirts -- in contrast to the low survival rates of the others, has been attributed to their resistance to poisoning by carbon dioxide, which is called hypercapnia (Knoll, 1996).
If this extinction pattern and its suggested cause is correct, then end-Permian ocean waters did not merely lose their oxygen, as has been suggested by numerous scientists, but confronted unusually high carbon dioxide concentrations as well. But carbon dioxide -- proposed, in this case, to have come from ocean stagnation -- may not offer a unique solution to the observed extinction pattern. Methane would also have disrupted metabolic processes, and would have been rapidly converted by chemical reactions in the oceans to carbon dioxide itself.
Although paleontologists can examine the fossils of organisms that possessed hard parts, others without such hard parts would have left behind no fossils, except possibly biochemical traces in the sediments. However, these organisms -- the cyanobacteria (formerly known as blue-green algae, though they are not actually algae) -- are among the most important of marine organisms both because they conduct photosynthesis, providing the atmosphere with a good portion of its oxygen, and because they are a major food source for innumerable other creatures.
While paleontologists are using increasingly sophisticated techniques to identify such biochemical traces or "molecular fossils" (also referred to as biomarkers), we currently have no way of assessing what types of cyanobacteria inhabited Permian oceans, or how they might have fared during the end-Permian extinction event. This inability to obtain data is most unfortunate, because cyanobacteria are likely to have produced much of the Permian world's oxygen and used much of its carbon dioxide, just as they do in today's oceans. Obviously, a significant extinction among the cyanobacteria would have greatly compromised primary productivity in the oceans, and been a major factor in the extinction of those many other marine organisms which depended on them, directly or indirectly, for food. Other phytoplankton (some of which do leave hard parts or other traces of their presence) were indeed apparently badly hit in the end-Permian extinction (Payne, 2004).
If cyanobacteria and other phytoplankton had been so compromised, three additional consequences would have followed. Phytoplankton employ carbon dioxide in the process of photosynthesis; with less photosynthesis going on, atmospheric carbon dioxide levels would have risen even further. In fact, it has been estimated that if all phytoplankton died off, atmospheric carbon dioxide would have increased between 150 and 200 ppmv (parts per million by volume)(Falkowski, 2000). This extra amount of carbon dioxide would be negligible in a world where carbon dioxide levels were already high, merely compounding an existing problem.
In addition, certain phytoplankton (for example, Silicibacter pomeroyi) release into the atmosphere a compound known as DMS (dimethyl sulfide: CH¸3SCH¸3. According to Andrew Johnston of the University of East Anglia in England, it is DMS which provides the distinctive smell of the sea.) DMS is an extremely important cloud nucleating agent, meaning that it helps clouds form by facilitating the condensation of water vapor. (When DMS is oxidized, it releases its sulfur as sulfur dioxide, SO¸2. The sulfur dioxide is converted to sulfuric acid, H¸2SO¸4, which strongly attracts water.) Fewer DMS-producing phytoplankton translate into reduced DMS production; that in turn results in fewer clouds. In ocean areas, decreased cloud cover would have meant greater absorption of solar radiation by the dark waters, and therefore would have been another source of global warming. (Generally, however, the role of clouds themselves in the warming or cooling of the planet is still in dispute, and appears to vary depending on factors like the type of cloud, its altitude, droplet size, and so on.)
Finally, a major reduction of phytoplankton would have reduced the influx of oxygen to the atmosphere. Oxygen is a chemically active gas and readily combines with other elements and compounds. Without free oxygen continually entering the atmosphere after being produced by photosynthetic organisms, free oxygen would decrease and eventually disappear from the atmosphere. If phytoplankton -- or terrestrial green plants, or both -- suffered significant extinction at the end of the Permian, therefore, the supply of oxygen to the atmosphere would have been reduced, and many aerobic organisms would have felt serious consequences.
Biological classification involves the same sort of hierarchies. It is organized in the following way:
Each species belongs to a genus, each genus to a family, and so on. Every species has its own designation, consisting of two names, just like many humans. Joe is an individual's name, for example, and Smith tells us he is related to other Smiths. Similarly (although the order of the individual name and the larger group are reversed), the name Homo sapiens tells us that the species being referred to is the species "sapiens," or wise, of the genus "Homo," or man. Though not part of its name, Homo is a primate, a mammal, an animal, and a eukaryote (organisms with large, complex cells), in this manner (Latin names are typically used):
(Modified from Margulis and Schwartz, 1982, p. 3)
Each group in a biological classification is made up of organisms that share particular attributes. Thus, mammals are grouped together because they possess hair, mammary glands, and other features. Human beings (Homo sapiens) share these features with all other mammals, such as cats and dogs, monkeys, mice, zebras and whales. Therefore humans are mammals as well. Human beings do not possess scales or feathers, and therefore are not reptiles (which have scales) or birds (which have feathers). But, like reptiles and birds, we do have backbones and spinal cords, which make us all -- human beings, other mammals, reptiles and birds, together fish and amphibians and some other unfamiliar organisms -- chordates. In addition, of course, we human beings have our own special attributes which distinguish us from other creatures.
When evaluating mass extinctions, paleontologists often employ the higher groupings (groupings are called taxa) to get the best sense of how various organisms have been affected. Thus extinction compilations, though they often indicate how many species went extinct, also frequently include the impact at the family level. This helps provide a better sense of just how badly a given group of organisms was hit.
APPENDIX 2: A GALLERY OF PERMIAN-TRIASSIC ORGANISMS
Summary of Biological Changes, Permian (and before) through the Early Triassic
|Permian (and before)||end-Permian extinction||Early Triassic and thereafter|
|Marine||1/2 families gone||"Cosmopolitan" biota (see below table for explanation.)|
(found only in Tethys area by
|Other forams||Hit hard|
|Reefs||Decimated||Recover, but slowly and with different organisms|
|composed of alcareous algae and calcareous sponges, bryozoans.||Almost gone|
(found only in Tethys area by Djulfian time)
|Scleractinian corals||Survived||Thrived after extinction|
(found only in Tethys area by
|Hit hard||Only one of the five Permian orders survives but it thrives|
|Crinoids||Hit hard||Recovered, but never as plentiful as during Paleozoic|
|Hit, not badly|
|Gastropods||Hit, not badly|
|Trilobites "quite uncommon"||Trilobites already gone?||Major recovery. Ammonites become significant predators in the Mesozoic seas|
|Bony fish and sharks had replaced the placoderms (fish with bony armor plates)||Hit, not badly||Bony fish and
|Terrestrial flora||Lycopod (club moss) trees replaced by drier climate trees, the gymnosperms:||Hit||Today's lycopods are small but common plants|
Conifers (pines, firs, spruces)
|Survived extinction fairly well||Still around
Thrived in Mesozoic; lots around today
|Ginkgoes||Survived extinction fairly well||Thrived in Mesozoic; still around|
|Glossopteris flora in Gondwana||Gone||Survived elsewhere, but died out around the end of the Mesozoic|
|Terrestrial fauna||Big insects in Carboniferous||Gone; only smaller insects left|
Reptiles (amniote egg) evolve from:
reptiles later in Permian: limbs under body, heterodonty
greater thermoregulation: higher body temps.
Edaphosaurus (herbivore) and
Dimetrodon, (carnivore) in Early Permian. Pelycosaurs in Laurasia in Late Permian, but
therapsids (from Middle Permian) dominant on land
Richard Cowen (1990, p. 288) says 800 million fossil therapsids estimated in Karroo Basin alone.
Gone before end-Permian
Mammal and dinosaur ancestors survive and thrive.
|Dicynodonts: highly diverse, rat to cow size, such as:||Some make it thru||Survivors become quite abundant: Lystrosaurus herds.|
Sources: Stanley, Extinctions,
(Stanley believes that extinction took about 10 million years, and that cause was global cooling.)
Also Cowen, 1990; and Briggs and Crowther, 1990.
The Djulfian was the last stage of Permian (after the Guadalupian)
(In the Early Triassic, a large percentage of the few survivor species had a very wide, that is, "cosmopolitan" distribution. This is not surprising, because the survivors would have had little competition, enabling them to occupy extensive areas. Though cosmopolitan biotas are typically low in diversity, many of the end-Permian survivors -- ammonites, therapsids, sharks -- quickly diversified.)
In the mid-1990s, this is where our understanding of the end-Permian extinction stood. All paleontologists recognized that the riddle of mass extinctions generally had not been solved. There might have been just a single cause of most mass extinctions, or each may have had its own unique and unrepeatable cause.
At the University of California at Davis, those of us who were graduate students in paleontology had our own seminar, in which we discussed current articles in our field. It was very much our own seminar, though nominally under faculty guidance and advisorship. Faculty members were invited to attend, but most had other, more pressing obligations. Occasionally, when an article of particular relevance to a faculty member's own area of specialization was to be discussed, that faculty member was specifically invited to attend. But mostly we were on our own, and suggested, selected and examined a wide variety of articles, and had wide-ranging, sometimes quite animated, discussions.
Topics included the evolution of menstruation, how fast a new species could evolve, why the tiny roundworm Caenorhabditis elegans had been selected by biologists as a model organism for investigating the process of biological development in animals, oyster fossils from southern California mountains, why turtles have their appendicular skeletons (that part of the skeleton which includes pelvis, shoulders and limbs) inside their axial skeletons (spine and ribs), the evolution of flight (a perennial favorite of paleontologists), and so on.
At one of these noontime discussions, at my suggestion, we looked at a newly published paper by Paul Renne and his co-authors (1995) on the geologic dating of the end Permian extinction and Siberian Traps volcanism. Renne is a researcher at the Berkeley Geochronology Center, where they attempt to decipher the mysteries of geologic time, by determining the dates of origin for rocks and fossils.
Up to this point, the duration of the end-Permian extinction had been unknown. But the Renne paper indicated that the its duration was sufficiently short that it could be considered an "event," rather than something more protracted. The extreme shortness of the boundary event, however, severely limited the possible causes. Impacts were rapid events, of course, but the evidence then available suggested that there had been no impact at the Permian-Triassic boundary. Additionally, more exotic, and far more rare cosmic causes such as nearby supernova explosions which could partly or completely sterilize the planet, seemed to be ruled out because their nuclear residues were simply not found by end-Permian investigators.
But if extraterrestrial causes could be eliminated as possible contenders for the cause of the end-Permian extinction, that left only terrestrial candidates. Moreover, any real candidate would have had to do its work quickly, and leave few traces. This was an unexpected, startling, and even frightening possibility. Some terrestrial agent must have caused the greatest extinction of the Phanerozoic. What possibly could be the killer in our midst?
"A gas," I said. "Carbon
dioxide... No, methane."
I said methane for four reasons. First,
methane is an asphyxiating gas, and so can do similar kinds of
damage as carbon dioxide. Second, methane oxidizes to carbon dioxide
in just a short time (less than a decade), so with methane there
is a "two-fer": initially there is methane and later
carbon dioxide. With methane it is a case of, anything carbon
dioxide can do, methane can do better. Third, methane is a powerful
greenhouse gas, more powerful than carbon dioxide. Fourth, there's
a huge amount of methane sitting in the continental margins of
the world (presumably there was lots of it around in the Permian
as well), and, under the proper conditions, that methane could
have been released.
CONTINUE TO NEXT SECTION (Methane and Methane Hydrates)
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