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.