Extinction research: lessons from prehistoric catastrophes
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We are said to be in the midst of the world’s sixth mass extinction event, 66 million years after the fifth one. While we are mostly aware of what is going on now, how much do we really know about the previous five, how did we find out and are we learning more as time goes on?
Determining the number of species existing at any given time is difficult given that estimates of current living species goes up to 30 million but some are an order of magnitude lower. That makes counting extinctions problematic, but fossil records do show ‘spikes’ where an unusually large number of these species have died out over the course of a short geological time, which is the definition of a mass extinction event.
Some fossil evidence suggests that cyanobacteria – the photosynthesising bacteria that were the earliest form of life on Earth – go back to as much as 3.7 billion years ago, although there is more consensus around a figure of about 3.4-3.5 billion years. However, such organisms belong firmly in the Precambrian eon, the age that covers the birth of the Earth 4.6 billion years ago until the hard-shelled creatures became abundant 541 million years ago. From this latter time until the present is the Phanerozoic eon; the age of visible life. While whatever went on before in terms of life on Earth remains more of a mystery, it is during this most recent period that fossils have revealed the path evolution has taken.
Evolution has included five mass extinction events as defined above. Arguably, without the last mass extinction, including the disappearance of the non-avian dinosaurs, mammals would not have proliferated and homo sapiens would not be here today. In fact, ‘arguably’ and ‘debatably’ are words that could litter such a discussion because the time spans are so vast that the remaining evidence is both scarce and hard to quantify. The resulting information is open to interpretation.
We have known about the existence of mass extinctions for a long time – they are shown by compiling occurrences of fossil taxonomy through time, and thus estimating fluctuations in diversity. By the mid-19th century a British geologist called John Phillips catalogued diversity through time using fossils and identified at least two of the ‘big five’ mass extinctions: the end-Permian and Cretaceous-Palaeogene.
The identification of the ‘big five’ mass extinctions came in the 1980s in a paper by David Raup and John Sepkoski from Chicago. In this manuscript, Raup and Sepkoski compiled a database of 3,300 fossil and living taxa (the bulk, 2,400, were fossils) organised into families (a biological taxonomic rank). Most fossils were assigned to geological time units called stages.
For these analyses Raup and Sepkoski estimated extinction rates as the number of families that disappeared in a stage divided by the length of the stage in millions of years. By calculating these rates, they identified five stages that had statistically significant higher extinction rates compared to the rest of the Phanerozoic. These higher rates of extinction were the ‘big five’: the late Ordovician (439 million years ago), late Devonian (364 million years), end-Permian (251 million years – also known as ‘the Great Dying’), Triassic-Jurassic (201 million years), and the end-Cretaceous (66 million years).
Mark Puttick, Royal Commission for the Exhibition of 1851 Research Fellow based at the University of Bath, says: “Subsequent analyses have built on this database and have used more sophisticated techniques to estimate rates of extinction but have generally reached the same conclusions about the ‘big five’.”
Puttick’s work includes analysis of these extinction events to see if there are any common elements and anything we can learn with respect to current changes in climate. However, the main information source, fossils, is not always that reliable. Puttick continues: “A big issue in documenting past diversity is accounting for the variation of fossil preservation through time. Some time periods, geographical regions and taxonomic groups do not preserve fossils equally through time. Notably the ‘quality’ of the fossil record declines further back in time.”
‘Some aspects of palaeontology are potentially beyond the limits of our current understanding. It is possible entire groups did not fossilise.’
A number of techniques have been developed to analyse these differences in fossil record preservation as well as the diversity of the fossilised species, and much of this work has been pioneered by John Alroy of Macquarie University in Australia.
Such techniques have built on the classic palaeontologist’s work of collecting fossils in the field and documenting and collating findings. Puttick says: “The 1970s saw the dawn of numerical palaeontology, as pioneered by Raup, Sepkoski and others. A large amount of current research is numerical and statistical and integrates biology alongside palaeontology. Thus, palaeontological breakthroughs are as likely to be computer-based as fieldwork.”
As well as palaeontology, areas of research such as geochemistry and volcanism are vital to understanding past extinctions. While fossils are found in sedimentary rocks, there is a role for igneous rocks as they are the ultimate source of radiometric dates, and many extinction events are linked to large-scale volcanism (e.g. the end-Permian, Triassic-Jurassic and end-Cretaceous), as evidenced by volcanic rock deposits called ‘large igneous provinces’.
The end-Permian event was caused by the massive volcanic activity that created the Siberian Traps. They are called ‘traps’ from the Swedish word for stairs, as the hills the igneous rocks formed today look like steps. The basalt from these eruptions today covers an area equal to the size of Western Europe but the original eruption could have been three times larger.
The Triassic-Jurassic event is linked to the Central Atlantic Magmatic Province, which formed before the separation of continents that created the Atlantic Ocean. The end-Cretaceous is linked to the eruptions that made India’s Deccan Traps.
The trigger for mass extinctions related to volcanic activity is generally believed to be a cascade of inter-related events leading to global warming as a result of greenhouse gas emissions, including carbon dioxide. These geologically sudden changes in climate can be assessed by changes in chemical isotopes from rocks, and other effects such as anoxia (lack of oxygen), which is shown by certain types of rock preservation.
Why is it important to understand mass extinction events?
“Additionally, in evolutionary biology we seek to understand how modern biodiversity arose. Mass extinctions undoubtedly played a large role in evolution, so by studying them we can understand this pattern of evolution. A counter-intuitive recent consensus has shown that extinctions are disastrous in terms of diversity, but the long-term effects are somewhat minimal. For example, types of ecosystems do not disappear – they emerge during the recovery.
“In terms of evolutionary biology, mass extinctions are also interesting as determining what died and what did not die may be unpredictable. During normal ‘background’ times extinction can be explained by selection acting from the environment and/or interactions with other species. At mass extinctions it is not clear if the same rules apply. [David] Raup summarised this by saying it is possible loss of species at mass extinctions is largely random, so extinction is like a ‘field of bullets’.”
So was it volcanic activity that did for the dinosaurs? “The general consensus is that the Deccan Traps volcanism caused increasing extinction by global warming, but an asteroid impact was the trigger,” says Puttick. “The evidence for the asteroid here is strong – in the 1980s [it was] shown that rocks from around the world precisely dated at the boundary had a marked increase, or ‘spike’, of the element iridium.
Iridium could only come from an extra-terrestrial cause, and this was proved by the discovery of an impact crater at Chicxulub in the Mexico Yucatán peninsula in the 1990s.”The effects of this impact would have been disastrous: tsumanis, a ‘nuclear winter’ blocking photosynthesis causing food webs to collapse, sulphur injected into the atmosphere causing acid rain, and potentially global wildfires.
Studying mass extinctions is multifaceted, as it involves geology, ecology, palaeontology, geochemistry and other fields, with improvements in all these methods contributing to an improved understanding.
For example, radiometric dating has made it possible to define the end-Permian mass extinction to a precise 200,000 years. Furthermore, sophisticated mathematical modelling techniques have improved the analysis of the fossil record to elucidate the size of extinction and account for different rates of preservation of fossils.
An extension of this is the ‘molecular clock’ – a technique that studies how different living species evolved from a common ancestor by taking genetic molecular information and analysing differences in DNA sequences. It is possible to use this information to estimate when groups of species first arose, the rate at which they evolved, how the environment influenced their evolution and vice versa, how aspects of their morphology change through time, and how they evolved through extinction. This molecular clock is ‘calibrated’ using fossil information. Puttick says: “Using this technique to understand mass extinctions is controversial but could become a useful future tool.”
Analysis of plants is more difficult than animals, but techniques are developing. It seems possible that plant diversity was not hit as much by mass extinctions as animals. The Triassic-Jurassic mass extinction 200 million years ago was well-studied for plants by Professor Jenny McElwain of University College Dublin. McElwain used the size of stomata (the part of a plant’s leaves that allows carbon dioxide to enter) to study temperature and carbon dioxide patterns across the Triassic-Jurassic mass extinction.
The relevance to our modern day situation is that current carbon dioxide levels are at 0.04 per cent, perilously close to the 0.05 per cent that would cause the temperature to rise sufficiently for the ice caps to destabilise, potentially causing global flooding. And, of course, if such levels caused mass extinction in the past, they may well do so in the present or future as well.