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The end-Permian Extinction

Clouds over South Table Mountain

What happened 251 million years ago?

By Jack Barkstrom

A question of survival...

Dimetrodon, the name for a lizard-like, but relatively unknown, prehistoric animal, suffers when compared to that of more familiar names, such as Brontosaurus or Triceratops.  However, it is universally recognized by the large dorsal fin or sail rising out of its back.  By the time the dinosaurs arrived it had disappeared.  It belonged to a group of prehistoric animals known as synapsids, for an opening in the skull behind the eyes. Along with its cousin, Edaphosaurus, it belonged to a subgroup called pelycosaurs.  Both were large, but not on a scale equal to that of the dinosaurs. Dimetrodon was about 3.5 meters (11 and 1/2 feet) from head to tail, while Edaphosaurus was somewhat smaller, at 3 meters (10 feet).  Around 260 million years ago, a new synapsid subgroup, called therapsids, replaced the pelycosaurs.  Thought to resemble modern mammals, they differed in one significant way from the pelycosaurs -  they had lost the dorsal sail.  Moschops, one of the therapsids, at 5 meters (17 feet), was also larger.  Still another group of therapsids was the dicynodonts, which appeared between 260 and 250 million years ago. They varied in size  The short-tailed, walrus-faced Lystrosaurus, at one meter (39 inches) was much smaller than the pelycosaurs - about the size of a pig.  The earliest forms of Dicynodon began small, about a meter, but its descendents increased in size. Later forms reached 3 meters (9 and 1/2 feet), with a weight of close to a metric ton.

What happened to Moschops, Lystrosaurus, and Dicynodon 251 million years ago is a matter of speculation.  Whether there was a meteor strike, a major volcanic eruption, or a dramatic change in climate is unknown.  While the date - 251 M.Y.A. - has been pinpointed from the fossil and geological data as significant, it is not known whether a single "event," such as a meteor impact or volcanic eruption, took place on a specific day, or whether a cumulative series of events took place over hundreds of thousands of years - roughly 251 MYA. Whatever took place, it was more than an ordinary life-threatening event.  It has been estimated that only 10 percent of the then-existing species survived what is known as the end-Permian mass extinction (or the Permo-Triassic extinction), a far greater impact than the asteroid which devastated the dinosaurs when it struck 65 million years ago.  That impact is estimated to have destroyed half (50 percent) of the existing species.

Most of the dicynodonts, including Dicynodon, died as a result of the events of 251.  Lystrosaurus, for some reason, survived.  Maybe its diet provided it with some advantage or perhaps the geographical region where it lived offered a refuge from catastrophic geological events - or maybe it was just lucky.

Mass Extinctions - The Big Five

Five major mass extinctions have been identified from the fossil record.  The biggest is the Permo-Triassic or end-Permian of 251 MYA.  The other four are the Late Ordovician (440 M.Y.A.), the Late Devonian (370 M.Y.A.), the end- or post-Triassic (200 M.Y.A.), and the Cretaceous-Tertiary (K-T), the extinction which wiped out the dinosaurs.  There are also some "smaller" extinctions, including the Quaternary or end-Pleistocene, which took place at the end of the last Ice Age, between 12,000 and 10,000 years ago.  Some 73 percent of large mammals in North America and 80 percent of those in South America became extinct, including the mammoths, saber-toothed cats, giant sloths, and camels.  Another "minor" event occurred around 225 MYA, (the end of the Carnian stage), during the Late Triassic, at the breakup of the supercontinent Pangaea, as was the Eocene-Oligocene event of 34 MYA.

251 Million Years Ago...

Either 95 percent of all species may have died out immediately 251 million years ago - or took 800,000 years to complete the process, depending on which scenario is to be believed.  Another scenario has the extinction process taking place in three stages: 1) the end of the Capitanian Stage (named for the Capitan reef in Texas); 2) the end of the Changhsingian Stage (named for the Meishan rock formations of China); and 3) the Olenekian Stage (named for the River Olenek in the South Urals). The Capitanian Stage is dated between 260 and 255 million years ago. The Olenikian Stage is dated between 245 and 240 million years ago.

In February 2001 a team of scientists made headlines when they announced that they had found evidence the earth had been struck by an asteroid or meteorite, measuring 6 to 12 kilometers across, 251 million years ago. What they announced was not the discovery of a large impact crater from 251, but evidence of an ancient chemical reaction.  They had found traces of two of the noble gases, helium and argon, trapped inside molecules called fullerenes or buckyballs, comprised of 60 to 200 carbon atoms, named for their resemblance to the geodesic dome invented by Richard Buckminster Fuller.  The buckyballs were contained in geological samples from China, Japan, and Hungary, within the boundary of Permo-Triassic formations.  They are known to form in meteorites and forest fires.  When the samples were analyzed, their chemical composition was found to be the same as that found in meteorites.  The problem was that other scientists had difficulty duplicating the results and there was some question as to the age of the samples used.  Were they actually from 251 million years ago or from a different point in time?  Other scientists did find evidence, in the form of shocked quartz, i.e., grains of compressed sand, and geochemical shifts, which might have resulted from a meteorite impact.  Yet the evidence which would establish conclusively that an impact had occurred, i.e., a massive crater, had not been found.  To date, it has not been found.

The fact that no crater has been discovered does not mean that the impact did not occur. But even if an asteroid did strike the earth, it has been suggested, the impact by itself, would not have been enough to exterminate 95 percent of all life.  Other factors may have caused, or at least contributed to, the catastrophe.  Some combination of events magnified the scale of what occurred.

One such alternative explanation for the end-Permian extinction is the Siberian Traps, a huge (3.9 million square kilometer or 2.4 million square mile) region of eastern Russia covered by basalt lava.  (The word "trap" is roughly translated from the Swedish "trapp" as "step" or staircase, which is what the formations resemble.)  The lava has been dated to 251 million years ago.  Since the depth of the lava varies between 300 and 3,000 meters (from 975 to 9,750 feet), it is estimated that the volcanic eruptions produced between 2 and 3 million cubic kilometers of lava.  The eruptions probably would have produced large amounts of sulfur dioxide and carbon dioxide, which would have created problems in two ways.  Since both compounds are 'greenhouse gases,' their introduction into the atmosphere would have led to a warming of the atmosphere, a prehistoric version of the now current phrase 'global warming.'  There are questions as to whether eruptions in Siberia, perhaps extending over a time period of a million years, would have introduced enough carbon dioxide into the atmosphere to cause global warming.

It has been postulated that a second source of carbon might be gas hydrates, such as methane, trapped and compressed in frozen ice and sediments at the bottom of the world's oceans, estimated at billions of tons.  It has been suggested that if conditions, or some event, released this gas, it would create a so-called methane burp at the surface.  Such a release may have occurred about 55 million years ago, increasing the global temperature by 2 degrees centigrade over 10,000 years. It was not enough to permanently change climate, and it did not lead to an extinction event, but it did kill a large number of species.

Another possible scenario involves complications caused by the release of chlorine gas, sulfates, and carbon dioxide into the atmosphere.  Combined with water, these compounds would have generated hydrochloric acid, sulfuric acid, and carbonic acid.  The resulting acid rain would have destroyed forests and led to the death of the animal life dependent on the forest plants.  In the oceans, the decay of organic matter, plant and animal, would have released hydrogen sulfide - and consumed the shrinking supply of oxygen.  Ocean waters, at greater depths, would have slowly turned anoxic, i.e., a condition in which oxygen is absent, as oxygen levels fell.

Definitive proof of what actually happened during the end-Permian period is still lacking.  Without an impact crater or overwhelming evidence pointing to the Siberian Traps or a massive methane release, the mechanism may not be identified.

In the aftermath of  251, Lystrosaurus, representing the dicynodants, and making up as much as 95 percent of the surviving fauna, was very much alone on land.  New forms eventually appeared. New hunters, the cynodants, or "dog-toothed" synapsids arrived.  Flesh-eating archosaurs, such as Chasmatosaurus (Proterosuchus) and the two-foot long Euparkeria, ancestors of crocodiles, dinosaurs, and pterosaurs, were among the new arrivals.  Among the crocodile predecessors were the rauisuchians, five-meter flesh-eaters.

If the dicynodant line had survived 251 by virtue of Lystrosaurus, the dicynodants, along with groups known as chiniquodonts and rhynchosaurs, failed to escape destruction 225 million years ago, when another extinction occurred.  What emerged, following the extinction event, were the first of the dinosaurs, small by later standards, but large in comparison to what had preceded them.  One of these was the five to seven meter long, metric ton (1 and 2/3rd ton) plant eater Plateosaurus.  The extinction of rauisuchians eliminated one line of predators, but that line was replaced by initially small, but increasingly larger predators, such as the 3 and 1/2 ton Allosaurus or the 7 and 1/2 ton Tyrannosaurus.

Suggestions for further reading.

Michael J. Benton, "When Life Nearly Died: The Greatest Mass Extinction of All Time," Thames & Hudson, (London 2003).
Stanley Chernicoff and Donna Whitney, "Geology: An Introduction to Physical Geology, 3rd ed." Houghton Mifflin Company, (New York, NY 2002)
David E. Fastovsky and David B. Weishampel, "The Evolution and Extinction of the Dinosaurs," Cambridge University Press, (New York 1996).
Kirk R. Johnson and Robert G. Raynolds, "Ancient Denvers: Scenes from the Past 300 Million Years of the Colorado Front Range," Denver Museum of Nature & Science, (Denver 2003).
David Lambert, "The Ultimate Dinosaur Book," Dorling Kindersley, (New York 1993).
Douglas Palmer, "The Earth, Its Wonders, Its Secrets: Life Before Man," Reader's Digest, (New York 1998).
Steve Parker, "Dinosaurus: The Complete Guide to Dinosaurs," Firefly Books, (Buffalo, NY 2003).
Hazel Richardson, "Dinosaurs and Prehistoric Life," DK Publishing, Inc., (New York 2003).