Neal Monks (UK)
The extinctions at the Cretaceous-Tertiary (K/T) boundary make up what is probably the most famous geological event in popular culture. This is the point when the great reptiles that characterise the Mesozoic went extinct. Alongside the dinosaurs, the giant marine reptiles died out too, as did the pterosaurs, and a whole host of marine invertebrates, including the ammonites and belemnites.
What happened? Some geologists argue the climate changed over a period of a million years or more, thanks to the massive volcanism that created the Deccan Traps in India. Others maintain that the K/T extinctions happened suddenly, pointing to evidence of a collision between the Earth and an asteroid. Perhaps there wasn’t a single cause, but rather a variety of factors: volcanism, climate change, asteroid impact, underlying changes in flora and fauna, and perhaps even variation in the output of the Sun and resulting weather patterns.
That life on Earth can be wiped out this way is the stuff of disaster movies as much as TV documentaries. However, what comes as a surprise to many people is that there wasn’t just one mass extinction at the K/T boundary, but a whole series of them that can be observed throughout the fossil record. One of them, the Permo-Triassic extinctions, appear to have been even more catastrophic than the K/T extinctions, and at least three other extinction events are comparable in scale. In between these five big extinctions were lots of smaller extinctions that aren’t well studied, but had profound long-term effects on the Earth’s marine and terrestrial ecosystems. Even more scarily, it’s arguable that man himself has been the agent of no fewer than two further mass extinctions, the first about 10,000 years ago, and the second probably going on right now.
The Big Five extinctions
Extinctions happen all the time, just as new species are evolving all the time. What makes a mass extinction distinctive is that the number of species that die out far exceeds the number of species that appear during the same period of time. This sounds easy enough to measure, but it’s actually a slippery thing to pin down. The fossil record is patchy and our record of animals, in particular, is biased towards marine organisms with preservable hard parts – shells, bones and teeth. Soft bodied animals like earthworms and sea anemones have essentially no fossil record at all, and even the insects aren’t particularly well represented, with good fossils available only at localities and geological horizons where their delicate bodies happen to be preserved, such as the Baltic amber of the Eocene.
The landmark study into mass extinctions was carried out in the early 1980s by two American scientists, Jack Sepkoski and David Raup. They looked at the diversity of marine animals from the Cambrian to the present. The overall trend they saw was that extinction rates were declining, but at five key points, extinction rates shot up dramatically. Often called the ‘Big Five’ extinctions, these occurred at the Ordovician/Silurian boundary, during the Late Devonian, at the Permian/Triassic boundary, at the Triassic/Jurassic boundary and at the K/T boundary.
What is a mass extinction?
Sepkoski and Raup measured diversity in two ways – at genus level and at family level. Genera are groups of closely related species that differ only slightly in appearance, anatomy and ecological niche. For example, the big cats form a genus, Panthera, within which zoologists place lions, tigers, leopards and jaguars. Families are bigger groups that encompass more diversity, but the species placed within families still share much in common. The cat family, Felidae, is one of the more familiar genera and, while cheetahs, housecats and lions are clearly different in important ways, they’re distinctly different from the dog family, Canidae, or the bear family, Ursidae.
Why measure genera and families instead of species? Defining a species from the fossil record is subjective and difficult to do consistently. If you had a jumble of lion and tiger bones, would you recognise them as coming from two different species? Or males and females of one species? Or geographical variants of one species? Or even just one species that happened to be quite variable? Humans, after all, are very variable, but form a single species, whatever their anatomical differences might suggest. Genera and families are more securely defined, making them a harder currency when it comes to measuring what’s going on at a particular extinction event.
In any case, at the Big Five mass extinctions, palaeontologists noted massive declines in marine animal diversity. Up to half of known marine families died out and, at the genus level, up to 80% of known genera died out. Land animal diversity is a bit more difficult to pin down because animal remains are so much less likely to fossilise. So, while some conclusions can be drawn from the fossil records of things like terrestrial reptiles and mammals, the margins of error are greater than they are for marine organisms like clams, corals and ammonites.
What about extinctions before the Cambrian? Unfortunately, this part of the fossil record is difficult to decipher. One problem is that Pre-Cambrian animals didn’t have hard parts and, when you go back further than 630 or so million years, there weren’t any multicellular animals anyway – or at least none that have left recognisable fossils. With that said though, there are some tantalising hints that mass extinctions happened in these very ancient Pre-Cambrian times.
The so-called ‘Oxygen Catastrophe’ is one of the most dramatic and happened about 2.4 billion years ago, when the only life on Earth were various types of bacteria. Photosynthetic bacteria had been releasing oxygen into the atmosphere as a waste product. At that time, bacteria were anaerobic, so they didn’t use this oxygen at all. Instead, it reacted with certain chemicals in the environment, particularly iron, leaving behind distinctive ‘banded iron’ sediments that resemble vast layers of rust. Eventually, these rusty sediments stopped being formed. The usual interpretation is that there wasn’t anything left for the oxygen to react with, so it was left to accumulate in the atmosphere.
Over quite a short period of time – perhaps just a few thousand years – atmospheric oxygen concentration reached something similar to the modern level (about 20%). Oxygen is reactive stuff and lethal to anaerobic organisms, damaging their biochemistry. It’s likely that the oxygen-rich atmosphere was poisonous to them, and only in places like anoxic mud and deep underground could they survive.
They’re still there, in their anoxic refuges, but other bacteria learned to survive in an oxygen-rich environment. Subsequently, different bacteria came together, forming the first eukaryotic cells. The cells of all animals, fungi and plants are descended from these, and within their cells are mitochondria able to neutralise oxygen and, in the process, release energy from food very efficiently. In fact, our cells are really colonies of Pre-Cambrian bacteria, survivors of the terrible Oxygen Catastrophe.
This extinction event happened between 450 and 440mya and saw tremendous changes in marine invertebrate diversity. About half the marine invertebrate families around during the Ordovician died out at this point, including the majority of brachiopod families as well as many families of trilobites.
Although quite well studied, the causes of the Ordovician–Silurian extinction event are not clear at all, though two distinct extinction ‘pulses’ have been recognised. One theory suggests that the first set of extinctions occurred when glaciation started, which not only meant that mid to polar climates got colder, but also led to a decline in sea level. With water locked up in the ice caps, sea level goes down, and the lower global sea level, the less sea there is to cover the very productive continental margins. This is a key thing about the sea – while it’s a big place, most of the action happens in shallow water around the edges of the land and, when these habitats are lost, marine diversity plummets.
The second phase of extinctions happened shortly afterwards. Life had only just adapted to the new cooler climate when things warmed up again, wiping out the cool climate species. Making things worse was the fact oceanic circulation seems to have stagnated, meaning that much of the sea floor was starved of oxygen, making it hard for animals to survive.
The overall effect of this two-phase extinction was dramatic in terms of taxonomy, with lots of species going extinct. However, what is remarkable about the Ordovician–Silurian extinction event is that animal life after it didn’t seem all that different to what preceded. Silurian marine communities contained brachiopods, corals, bryozoans, trilobites and most of the other creatures we’d associate with Ordovician communities. This isn’t always the case with mass extinctions, as we’ll see. Often, what happens is a clearing of the decks, with entirely new groups of animals diversifying into the ecological niches vacated by those groups that went extinct.
The Late Devonian extinctions also occurred in two phases, the first affecting marine communities and the second affecting both marine and terrestrial communities. The first phase is sometimes called the ‘Frasnian-Famennian boundary event’ after the two Devonian stages that bound it, while the second phase, the Hangenberg event, sits right at the very end of the Devonian.
As is often the case with mass extinctions, the causes are somewhat uncertain, though various theories have been proposed, including an asteroid impact similar to that associated with the end-Cretaceous extinctions. Another explanation is the rapid expansion of true forests around the globe for the first time. The classic Devonian ‘trees’ belong to the genus Archaeopteris and some grew to heights of 10m or more. Like modern trees these would have absorbed massive amounts of carbon dioxide from the atmosphere and, in doing so, reduced the overall greenhouse effect, making the Earth a significantly colder place than before.
Usually referred to as the Permo-Triassic extinction event, this was the most devastating of the Big Five extinctions and unusual in that it affected terrestrial communities just as hard as marine communities. Several marine groups were wiped out completely, including most of what we’d recognise as classic Palaeozoic groups – trilobites, sea scorpions, fenestrate bryozoans, tabulate and rugose corals, orthid and productid brachiopods, and all but one class of crinoid. On land, there were similarly severe exterminations of numerous vertebrate and invertebrate groups, including reptiles, amphibians and insects. While these groups obviously did survive, the families that diversified through the Mesozoic were not the ones the ones that had prospered during the Permian.
Lots of ideas have been put forward to explain the Permo-Triassic extinctions, but the hard part is finding an explanation that works for both land and marine organisms at the same time. Perhaps inevitably, both asteroid impact and large-scale volcanism have been put forward, but other, more subtle mechanisms that have been postulated include sea level changes and calamitous drops in the oxygen concentration in the sea. On land, the fusion of the continental plates into the single land mass, Pangaea, would have produced rather dry and strongly seasonal conditions inland, restricting those species that needed warm, wet conditions to a fringe along the continent’s coastline.
Then there’s the problem of explaining why some groups of organisms were wiped out while other, closely related groups, sailed through. The brachiopod order, Strophomenida, is a case in point. Most died out during the Permo-Triassic extinctions, including all of the spiny, deep-shelled productids. But a few of the flat-shelled types survived and, though they never regained their previous status in terms of diversity or abundance, they managed to persist until well into the Jurassic. Another example of this sort of thing can be seen among the ammonoids – while the goniatites failed to survive the Permo-Triassic extinctions, the ceratites passed through without major problems, giving rise to all the ammonites that populated Triassic, Jurassic and Cretaceous seas.
Yet again, the Triassic-Jurassic extinction event sees large-scale changes on land and in the sea, and again, there’s no real consensus about how these extinctions came about. The usual suspects are put forward – asteroids, volcanism and sea level changes, but without much evidence to convincingly implicate one over the others.
The faunal changes are significant, particularly on land, not least because reptile and amphibian diversity crashed, leaving open lots of ecological niches that were eventually filled by dinosaurs. Primitive, mammal-like animals called therapsids survived, but, whereas the therapsids had been ecologically significant during the Permian and Triassic, they were relegated to a very minor role by the Jurassic. It would only be after the extinction of the dinosaurs at the K/T boundary that the surviving therapsids – the mammals – would have a chance to reclaim their previous status.
In the sea, perhaps the best-known victims were the conodonts, the whole class of which vanished at this time. These wormy, fish-like animals were probably vertebrates of a sort and had been around since the Cambrian. They had even managed to survive the preceding three mass extinctions.
Close: in our time…?
As should be clear, mass extinctions may be easy to recognised, but they’re very difficult to explain. In every case, multiple theories have been put forward to explain them. If anything, explaining why extinctions would happen is the easy part; the tricky part is explaining why certain groups survive. Returning to the last of the Big Five extinctions, we’re confronted with precisely this problem. If the asteroid impact was big enough to kill off the dinosaurs, then why did the crocodiles survive? Or the turtles? Or the frogs? If climate change caused by volcanism was driving ammonites and belemnites to extinction, then why did squids and nautiloids survive?
Perhaps, there’s something to be learned from human history. When humans left Africa, they destroyed practically every big mammal and flightless bird species they encountered. Only in modern Africa can you see something approximating to a rich fauna of large mammals similar to what you’d have seen in the Americas 50,000 years ago. Before humans got there, the Great Plains of North America would have been home not just to bison and wolves, but also elephants, horses and even camels. But, soon after humans arrived, the diversity of large mammals declined dramatically. The obvious conclusion is that humans either hunted them to extinction or else so changed their habitat that these mammals couldn’t survive, perhaps through the use of fire to burn away scrub and trees.
In our own time, we’re seeing what some call the ‘sixth’ mass extinction. Frogs are often described as the key indicators of this extinction event, with a third of all frog species threatened with extinction. What’s killing the frogs is a fungal plague, but zoologists believe that the frogs have become acutely vulnerable to the fungus because of climate change. There’s a very scary thought there, that if we want to understand mass extinctions in the fossil record, we first need to look around at the modern world around us.