Ronald Guthrie, DPhil in Environmental Research (2016)
Life on Earth has been punctuated sporadically by mass extinctions. Evidence of these events are preserved in the rock record. A change in the environment can result in a sudden and widespread decrease in biodiversity, increasing extinction levels above the background rate, leading to a mass extinction. “The Big 5” is a collective name for the five most commonly recognised mass extinction events. Their various causes can be a point of some debate and the focus of many a PhD thesis, whilst some extinctions are better known than others. The dinosaurs died out during the end-Cretaceous extinction event, in which a ten kilometre wide meteorite is theorised to have collided with the Earth’s surface in what is now the Yucatán Peninsula of Mexico. Others events are more contentious and have competing theories about what was the exact trigger and kill mechanisms. One such scrutinised event is the end-Permian mass extinction, otherwise known as “The Great Dying”.
The end-Permian mass extinction occurred 252 million years ago, marking the boundary between the Permian and Triassic periods, and has been described as the most severe of all recognised extinctions. Over 96% of marine species and 70% of terrestrial vertebrate species all went extinct. Unique creatures such as the trilobites, which had been present for the previous 270 million years, suddenly vanished. On land, very few coal deposits have been found until mid-way through the Triassic, suggesting coal-forming plants took tens of millions of years to re-establish their dominance. Yet there is no single smoking-gun for the Permian extinction, no significant impact crater has been found that could have caused such a calamity for life. Instead, there are several, which makes the problem much more difficult to discern and much more interesting for geologists.
The environments of the past – palaeoenvironments – can be preserved as rock. Through looking at the physical structures, geochemical characteristics and fossil content, rocks can tell us a lot about what was occurring before, during and after extinction events. Permian marine rocks transitioned from having a recognisable fossil assemblage to a complete absence of fauna in much darker, organic-rich mudrocks. Analysing their geochemistry, we can tell that the water in which they were deposited suddenly changed and became acidic. But that does not tell us how this happened. Was large scale volcanism playing a role? Did the atmospheric composition change? Could changing climate patterns affect how the oceans are mixed? How did this affect plants and animal species on land? The latest research suggests a combination of several of these issues.
An interesting element theorised to have been a factor in the magnitude of the marine extinction is the actual biological make-up of species. Throughout the evolutionary history of life, the chemical composition of species’ skeletons has changed to make best use of the changing environments. But with a change in the Permian environment occurring so quickly, species did not have time to adapt. Analysis of fossils shows that the extinction primarily affected organisms with calcium carbonate skeletons, especially those reliant on stable CO2 levels to produce their skeletons. These organisms were particularly susceptible to the effects of the ocean acidification that resulted from increased atmospheric CO2. The make-up of their skeleton, which had given them an advantage over others before the extinction event, ultimately contributed to their demise. How atmospheric CO2 levels increased is a further complication of “The Great Dying” and currently studied with great intrigue.
Extinction isn’t always bad for life on an evolutionary timescale. Without the Permian extinction, the early ancestors of dinosaurs would not have become the dominant form of vertebrate during the Triassic Period. If the meteorite that killed the dinosaurs had been smaller and less severe, then it isn’t certain that mammals would have been able to evolve beyond small creatures scurrying between the feet of Tyrannosaurus rex. In fact, it is only through looking back in time at extinction events that we can predict how severe changes to the present environment can affect modern-day biodiversity. Some scientists believe that we are in a sixth mass extinction – one of our own making – and by analysing how quickly past environments are able to respond to change, then we might begin to recognise the severity of our actions.
Through studying these episodes – the run-up to, the actual event and the recovery period – we can gain a better understanding of how life and our planet’s environments cope with extreme circumstances. Extinction has shaped the evolution of life for over 540 million years and will continue to do so. It is part of nature, but, through its study, we might be able to learn something both of the past and warning signs for the future.