Mass Extinctions Michael J

MASS EXTINCTIONS MICHAEL J. BENTON INSTANT EXPERT 00 Month 2010 | NewScientist9 | 1 THE BIG FIVE DEATH ON A MASSIVE SCALE (OR IS IT SIX, Every now and again, life on Earth faces a crisis. At least five times in OR SEVEN?) the past 540 million years, half or more of all species have been wiped We now recognise that there have out in a short space of time. These mass extinctions are important been several mass extinctions over punctuation marks in the history of life, as once-dominant groups are the past 600 million years – the period over which macroscopic life has swept away and replaced with new ones. What triggers this wholesale existed in relative abundance. The regime change? How does life recover? And are we in the middle of a first was about 540 million years ago, mass extinction of our own making? Woolly rhinos and at the end of the Neoproterozoic era mammoths died out (see geological timescale, below), in an extinction event when the enigmatic Ediacaran EDIT Cr 11,000 years ago animals disappeared. Some palaeontologists also identify the late Given how important mass extinctions are to This all began to change in the 1960s, a time of Cambrian as another time of mass understanding the history of life, it may seem ferment and revolution for geologists when ideas of WHAT IS A MASS EXTINCTION? extinction. surprising that no one was much interested in the idea an immobile Earth were rejected in favour of the Three further mass extinctions until the 1970s. Of course, the great Victorian dynamic reality of plate tectonics. Extinction is a normal part of evolution. Species come There have been many such punctuate the Palaeozoic era. The late palaeontologists such as Richard Owen and Thomas That decade also saw the birth of impact geology. and go continually – around 99.9 per cent all those that “extinction events” through the Ordovician, between 450 and 440 Huxley were aware that dinosaurs and other ancient Gene Shoemaker of the California Institute of have ever existed are now extinct. The cause is usually history of life. million years ago, saw substantial creatures were extinct, but they did not see any role Technology in Pasadena identified rare minerals, such local, for example a lake might dry up, an island might Occasionally extinction events are losses among the dominant animals of for sudden, dramatic events. as coesite and stishovite, in the floor of Meteor crater, sink beneath the waves or an invasive species might global in scale, with many species of the time: trilobites, brachiopods, Following Charles Darwin, they argued that and argued that these were evidence of an impact. At outcompete another. This normal loss of species all ecological types – plants and corals and graptolites. The late extinction was a normal process: species originated at the time such minerals were unknown in nature and through time is known as the background rate of animals, marine and terrestrial – dying Devonian mass extinction, beginning some point by splitting from existing species, and at had only been created in the lab using enormous extinction. It is estimated to be around 1 extinction per out in a relatively short time all over around 375 million years ago, was some point they died out. temperatures and pressures. million species per year, though it varies widely from the world. This is a mass extinction. another long and drawn out affair. This mindset can be traced back to Charles Lyell, Shoemaker also investigated a large circular group to group. There is no exact definition of a Armoured fish known as placoderms Arizona’s Meteor who in the 1830s argued that the foundation of sane depression called Nördlinger Ries in Bavaria, Germany. The vast majority of species meet their end in this mass extinction. The loss of 40 to 50 and ostracoderms disappeared, and Crater, the birthplace geology was uniformitarianism. This holds that “the There he found coesite and stishovite, along with way. Most dinosaurs did not die out in the asteroid per cent of species is about the norm, corals, trilobites and brachiopods of impact geology present is the key to the past”: all geological suevite, a type of rock composed of partially melted strike – after 165 million years of evolution, hundreds but this is only the upper end of a suffered heavy losses. The Palaeozoic phenomena can be explained by material. The depression is now considered to be an or thousands of species had already been and gone. spectrum of extinction events. There ended with the enormous end- processes we see today, extrapolated impact crater some 16 million years old. Sometimes many species disappear together in a is no set timescale either: some Permian mass extinction (see page v). over enormous periods of time. Around the same time, palaeontologist Norman short time. At the end of the ice ages 11,000 years extinctions happen relatively quickly, Another 50 million years or so In fact, until quite recently, Newell of Columbia University in New York began ago, for example, mammoths, woolly rhinos, cave like the KT event, others take several passed before the next mass geologists were conditioned against building the case that the fossil record contained bears and other large mammals adapted to cold million years, as in the late Ordovician. extinction, at the end of the Triassic. seeing any evidence of major crises. evidence of large-scale extinctions. With his work the conditions died out across Europe and North America. It all depends on the cause (see p vi). Fishes, molluscs, brachiopods and Woe betide anyone who believed in concept of mass extinctions began to gain currency. other marine groups saw substantial past impacts and explosions, the Even so, when Luis Alvarez of the University of losses, while extinctions on land marks of an unscientific catastrophist! California, Berkeley, and his colleagues proposed in ” ntil quite recently, geologists opened the way for the dinosaurs. Until the 1950s geologists even 1980 that the dinosaurs had been killed off by an They dominated for 145 million years denied that the Earth had been hit by asteroid impact the world was still not ready to believe were conditioned against before being wiped out in the most meteorites, arguing, for example, it. Opposition to the idea was substantial, and it took seeing evidence of major recent extinction, the Cretaceous- that Meteor crater in Arizona was a another decade to convince the world that this EDIT crises of any kind ” Tertiary (KT) event (see page vii). Cr volcanic collapse feature. massive catastrophe really did happen. Extinction Neoproterozoic Late Ordovician Late Devonian Permian Triassic KT Date 540 mya 450-440 mya, in two pulses 375-360 mya, possible in a series of pulses 252 mya 200 mya 65 mya Cause Unknown Glaciation? Anoxia? Flood basalt Unknown Asteroid strike Genera extinct Unknown 57 per cent 50 per cent 83 per cent 50 per cent 50 per cent Major groups lost Ediacarans none Armoured sh Trilobites, eurypterids Large non-dinosaur archosaurs Dinosaurs, pterosaurs, marine reptiles, ammonites 540 252 199 65 Ediacaran Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Palaeogene Neogene Q NEOPROTEROZOIC ERA PALAEOZOIC ERA MESOZOIC ERA CENOZOIC ERA 600 million years ago 500 400 300 200 100 0 ii | NewScientist | 5 March 2011 5 March 2011 | NewScientist | iii THE DEATH OF THE DINOSAURS THE terrible TWO The extinction of the dinosaurs 65 million years the Cretaceous-Tertiary boundary at Gubbio in Italy. “shocked” quartz grains, glassy ago, at the Cretaceous-Tertiary (KT) boundary, is the But at the boundary itself they found a sharp spike in spherules of melted rock and the Extinction events punctuate the history of life but two really stand out, most recent of the major mass extinctions and the one iridium, 10 times the normal amount. If they had stuck sudden extinction of many groups of one for its sheer scale and the other for its sudden, spectacular and most amenable to study. Rocks from before, during to their original hypothesis, they would have plankton worldwide. Around the and after the event are more abundant, detailed and concluded that the rocks were laid down by unusually Caribbean they also found ancient shocking cause datable than those for older events. So its cause was slow sedimentation over a vast time span. But they tsunami debris, and in 1991 the crater just waiting to be resolved. rejected that in favour of the idea that the spike itself was identified at Chicxulub on Up to the 1970s the best evidence suggested that indicated a sudden influx of iridium from a very large Mexico’s Yucatán peninsula (see map). the dinosaurs – along with pterosaurs, mosasaurs, meteorite or asteroid. This, they argued, was what had As predicted, it was 130 kilometres plesiosaurs, pliosaurs, ammonites and many other caused the mass extinction. across. groups – declined slowly over some 10 million years as The team reasoned that such an impact would have There are still some serious loose a result of cooling climates. sent up a vast cloud of dust that encircled the globe, ends to tidy up, not least the role Then came the bombshell. In 1980 Luis Alvarez, blacking out the sun, preventing photosynthesis and played by massive volcanic eruptions who had already won a Nobel prize in physics, his so causing massive loss of life. They calculated that a on the Deccan plateau of India around One mass extinction truly dwarfs all the others. of the dominant reef-builders, the rugose and tabulate geologist son Walter and other colleagues published crater some 100 to 150 kilometres in diameter was the time of the extinction. A handful WHEN LIFE Whereas earlier and later events each seem to have corals, the Earth was cleared entirely of reefs.

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