Geologists use a series of extinction events that occurred around 490 million years ago to define the end of the Cambrian period and the beginning of the Ordovician. These events led to the demise of many types of marine animal. The brachiopods (marine mol-lusks resembling bivalves) were very numerous before this event, but whatever occurred all that time ago had a drastic effect on their numbers. The trilobites, ancient forerunners of today's numerous creepy crawlies, could also be found in profusion before this event, but the Cambrian-Ordovician mass extinctions heralded a slow decline of these organisms that lasted for millions of years.
What caused this series of extinction events almost 500 million years ago? No one can be sure, but many scientists suggest it was a lengthy series of glaciations. By far the most important source of energy for life on earth is the sun. Its heat, reaching out over millions of miles of space, ensures that the earth has a balmy climate—well, some of the time. The problem is that our planet does not travel around its star in a perfect orbit. There are rhythmic variations, not only in how the earth goes around the sun, but also in the way the earth spins on its axis (see the "Extinction Insight" in chapter 5 for more information). All of these anomalies have a huge effect on the earth's climate. For example, small wobbles in the earth's spin can reduce the amount of solar radiation that strikes the Northern Hemisphere, and temperatures can drop—not by a massive amount, but enough to result in the formation of huge glaciers that can lock up much of the planet's water. These ice ages, as they are known, have a huge effect on the earth's inhabitants, which is not surprising as life generally fairs better in a greenhouse than in a refrigerator. It has been suggested that a sequence of ice ages was responsible for the Cambrian-Ordovician extinction events. Life at this time was at its most diverse in the numerous shallow seas that surrounded the earth's landmasses. If the earth did indeed enter a long phase of ice ages, the water from these shallow lagoons and seas disappeared as the world's moisture was locked up in the growing glaciers.
Another possibility is that the action of bacteria living in the mud on the sea floor led to the depletion of oxygen in the ocean, which in itself is due to climate change. All animal life at this time was marine—there were no land-dwelling creatures—and all animals require oxygen. Deprived of oxygen, animals would have gone into a steep decline.
In scarcely no geological time at all after the Cambrian-Ordovician mass extinction, the fossil record tells us that there was another big die-off of species around 450 million years ago. It is likely that this Ordovician-Silurian mass extinction was also a series of events which occurred quite close together—in geological time, at least. This mass extinction is widely considered to be the second largest the world has ever seen, and it resulted in the loss of around 50 percent of the animal types that were around at that time.
Again, we can only make educated guesses at the culprit, but climate change is a definite possibility, such are the vagaries of earth's motion through space. A series of ice ages and warmer periods led to the cyclical rise and fall of sea levels. Before this series of changes, the shallow seas would again have been the focus of animal activity, but global cooling deprived these creatures of the habitat they required. In the intervening warm periods, the creatures that evolved to live in the new habitats provided by the cool conditions were doomed. So this cycle continued for hundreds of thousands, if not millions, of years, with animal diversity decreasing all the while.
The late Devonian extinction happened around 360 million years ago, and instead of one event, it seems the decline in animal species, which marks the beginning of the Carboniferous period, was also a series of events that lasted for around 20 million years. Again, we can never be sure of the underlying causes that resulted in the loss of around 70 percent of all species, but numerous theories have been suggested, including a large asteroid impact and the evolution of the plants from small, surface-hugging forms, no larger than 30 cm, to giants 30 m tall. These new plants had well-developed roots that penetrated bedrock and led to the eventual formation of thick layers of soil. Rainwater running through this soil carried huge quantities of minerals to the sea, completely changing its chemistry and creating algal blooms, which sucked the oxygen out of the water. Starved of oxygen, marine animals perished. This is just a theory, but it is an event that could have conceivably been played out over millions of years. The profusion of land plants may have also caused extended periods of glaciation by removing carbon dioxide, an important greenhouse gas, from the atmosphere.
The mother of the major extinctions is the one that occurred at the end of the Permian period (about 250 million years ago), an event which defines the beginning of the Triassic. The Permian-Triassic extinction killed off around 96 percent of all marine species and about 70 percent of land-dwelling species. Many theories for the cause of this event have been suggested, and some are more credible than others. The usual suspect of an asteroid strike (or even multiple strikes) has been proposed, but in the absence of definite crater(s), we cannot be sure if this was the case. At around the right time to coincide with the Permian-Triassic event, there appears to have been a massive increase in volcanic activity. The Siberian Traps are the lasting reminders of this colossal outpouring of basalt from the earth's mantle. In this scenario, a plume of hot magma from the deep mantle rose up and ruptured the crust, appearing as a series of eruptions over a huge area. Eruptions of this type do not end after a few days, and it appears that the basalt of the Siberian Traps was spewed out over a million years. Imagine all the dust and gases ejected into the atmosphere by an outpouring of 3 million km3 of lava (for comparison, the largest eruption in very recent history occurred in Iceland, and it produced 12 km3). Mount Pinatubo ejected only a tiny fraction of the gases and dust produced by the Siberian Traps eruptions, yet this was enough to lower global temperatures by 0.5 degrees Celsius, which is not an inconsiderable drop for living things.
Imagine how the earth's climate was cooled by the Siberian eruptions. The effect must have been like a nuclear winter, and photosynthesizing organisms, the basis for all food chains on land and most in the oceans, died en masse. Huge amounts of noxious gases pouring into the atmosphere acidified the moisture in the air, and thousands of years of global acid rain made the oceans more acidic, dissolving corals and countless other organisms that secrete a shell of calcium carbonate. As the eruption occurred over an area the size of Europe, molten rock heated seawater, and immense storms may have formed. These hypercanes, with winds in excess of 800 km per hour, sucked dust, debris, and gases into the high atmosphere, eroding the ozone layer until the earth was stripped of its protection from ultraviolet radiation.
We know that this huge, prolonged volcanic eruption occurred in Siberia about 250 million years ago, but there is a possibility that it may have been triggered by a huge asteroid impact. An errant cosmic body, bigger than the largest mountain, slamming into the earth at 15 to 20 km per second generates an unimaginably huge amount of energy. Is this enough to disturb the currents of molten rock that flow through the earth's mantle, causing the creation of a gigantic plume of molten rock that bursts from the surface and wreaks millions of years of havoc? Possibly, but until we find the remnants of a crater of the right age and size, the trigger of the Siberian eruption will remain a mystery.
Another very interesting proposed cause of the mass extinction at the end of the Permian is the release of huge quantities of natural gas from below the seabed. Beneath the seabed, this gas (mostly methane) is locked away within the crystal structure of frozen water, and a huge impact or an increase in ocean temperatures due to a colossal eruption may have been enough to melt these extensive reserves, releasing huge quantities of methane into the atmosphere. Methane is one of the most potent greenhouse gases of all, and billions of tonnes of it released all at once could have triggered a runaway greenhouse effect that turned the earth into a sweltering sphere for thousands of years. Any one of these events (flood eruption, asteroid impact, or an enormous release of methane) would be very bad news for all life, but if all three were perhaps linked, it must have been as close to the end as life has ever come.
The next mass extinction after the Permian event is the one that divides the Triassic period from the Jurassic: the Triassic-Jurassic extinction event. This was minor compared to the event that went before it, but it is significant enough to have been preserved in the fossil record, with the disappearance of many marine forms as well as a range of land animals. Some scientists have challenged whether this was actually a real event or just a reduction in the appearance of new species. An asteroid impact has been proposed as a possible cause, but no crater of the right age or size has been found. This is definitely not the case for the next major extinction, and perhaps the most famous of them all, for it is when the dinosaurs disappeared from the earth.
Dinosaurs have fascinated us since the first species was described in 1824, yet almost all of them disappeared rather abruptly around 65 million years ago along with countless other species. I say almost all because birds are the direct descendents of these animals. Every time you look at your bird feeder or see a flock of geese heading south for the winter, you are see ing the lasting reminders of these reptiles. What happened to the rest of the dinosaurs? The event that ended their dominance is known as the Cretaceous-Tertiary extinction event, or the K-T event for short (Cretaceous is traditionally abbreviated as K, derived from the German word for chalk, kreidezeit), and it is the only mass extinction for which there is definite evidence of an asteroid impact. In numerous sites around the world, geologists saw that rock strata laid down in the Cretaceous were topped off with a thin layer of grayish material. This layer turned out to be ash, and further analysis showed that it contained a high concentration of the very rare metal, iridium. Iridium may be rare on earth, but it is much more abundant in certain types of asteroid. For years, skeptics argued that the iridium could have originated deep in the earth's mantle and been ejected by intense volcanic activity. Also, they argued, how could there have been an asteroid impact with no crater? Then, in 1990, geologists formally identified the crater from observations made many years before. The site is known as Chicxulub, and it is on the very edge of the Yucatan Peninsula in Mexico. The crater is half on the land and half under the sea, but after 65 million years, the portion on land has been eroded and the submarine half is buried under hundreds of meters of sediment. With that said, it is possible to get an idea of its size, and it is truly immense, with a diameter as large as 300 km. The space rock that formed this crater was at least 10 km across and was traveling at around 15 to 20 km per second. Such an enormous thing hitting the earth at such a high speed generated a huge amount of energy—at least 2 million times more energy than the largest nuclear bomb ever detonated. Huge waves ravaged the earth's low-lying areas, and the huge quantities of dust and gas ejected into the atmosphere plunged life into darkness for months, if not years. With insufficient sunlight, plants and other photosynthesizing organisms everywhere died, and the animals followed. Some geologists have suggested that the earth was hit by several asteroids around 65 million years ago, but the other craters are yet to be found. As most of the planet is covered by water, lots of impact craters may be buried beneath the waves and hundreds of meters of sediment.
At around the same time as the Chicxulub crater was formed, the earth was struck by a second terrible event, a second huge volcanic flood eruption that produced the Deccan Traps in India. Again, it's feasible that the impact triggered an outpouring of basalt on the other side of the world, and the effects of both together spelled disaster for all life.
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