Environmental Stress: The True Cause of Mass Extinction

Literally hundreds of theories have been proposed for the death of the dinosaurs – some with an element of sense, but mostly quite fanciful. In June 1980, Walter Alvarez and colleagues published their now famous paper on the topic in the journal Science. Their work in central Italy had uncovered an unusually high concentration of iridium in a thin clay layer that marked what is known as the KT boundary. The clay was deposited on the floor of the long vanished Tethys Ocean some 65 million years ago, precisely the time at which the dinosaurs finally disappeared from Earth, together with around 50% of all known species. There are several ways of concentrating iridium, but the Alvarez favoured an extraterrestrial source and so proposed a gigantic asteroid impact as the primary cause for the KT mass extinction event. Fanciful, but now firmly rooted in popular belief – yes. True – I don’t believe so. Let me explain.

How fossil records at sea uncover the real extinction facts

The real problem for the asteroid impact, or for any other catastrophist theory, is in explaining the actual extinction facts. These we can glean from the fossil record on land and at sea, but especially that recovered from the Tethys Ocean, which dominated the world during the reign of dinosaurs and on which I have worked now for several decades. For some organisms, it was a slow and painful decline; for others it was rapid but not abrupt. But many plants and animals were barely affected at all – most land plants, for example, survived. Most molluscs, sharks, bony fishes, placental mammals and all amphibians were also completely unscathed. And so on.

The marine world suffered most. For example, most coral species died out between 10 and 20 million years before the KT boundary. Their place was taken for a time by strange bivalves, the rudists, but these too showed a very marked decline over at least the last 2 million years. The same was true for many other types of clams, sea snails and lamp shells, as well as the beautifully coiled ammonites and cigar-shaped belemnites. These and many other organisms did not like the changes that were clearly afoot in the ocean world.

The slow decline of the marine world

Today’s coral megacities cover just 0.2% of the ocean floor but support around 25% of all known marine species; we assume it must have been similar for the Tethys coral and rudist reefs. When environmental stress occurs, such as warming oceans today, coral polyps expel their microbial symbionts and both die – we call this coral bleaching. Removing the very fabric of life from these cornucopias of the past would have had a very significant onward effect on the whole complex community. Even more significant were the changes at the very base of the food chain in the oceanic plankton. These microscopic organisms were in turmoil; progressive extinctions decimated and ultimately changed their world for good. The results were inevitably catastrophic throughout the ocean food webs, up to and including the very top predators of the day.

At the top of the food chain were the marine reptiles, some far larger and more fearsome than even the infamous dinosaur Tyrannosaurus rex. Most of these had been on the decline for several millions of years, as had the dinosaurs and pterosaurs on land. Signs of environmental stress were everywhere. Those dinosaurs that had been living in Africa, Asia and South America disappeared long before the KT boundary. The final rump of survivors was confined to western North America, boasting high diversity some 10 million years before the end, but then reducing to a mere handful of species as the KT boundary was approached. In fact, the ultimate demise of these magnificent beasts was more of a whimper than a bang.

Global environmental stress from super-volcano to warmer climates

There is abundant evidence for a combination of environmental drivers. There were catastrophic eruptions from a super-volcano, somewhere in the middle reaches of the Tethys Ocean, just preceding the KT boundary event. The scale and duration of this event, which is now marked by half-a-million square kilometres of Deccan Traps in western India, would have led to global environmental stress. There were extremely high sea levels and a universally warm climate had bathed the world in a prolonged state of benign tranquillity. This led to the removal of carbon dioxide from the oceans (and hence atmosphere) as calcium carbonate skeletons of marine plankton and ultimately into the chalk rocks that accumulated so widely on the seafloor. This, in turn, would have significantly affected ocean chemistry. As sea levels fell, so temperatures dropped progressively and dramatically, a feature we can measure and even quantify from the oxygen isotope record in fossil shells. The evidence for thermal stress amongst different groups has shown up repeatedly in our examination of this time period. Coastal habitats diminished and land bridges opened up – the spread of diseases between animal groups, already subject to enhanced competition, would have increased.

It is difficult, therefore, to avoid concluding that the KT extinction event was due to a complex combination of environmental factors that triggered extreme biological stress. This occurred over a contracted period of time, but was in no way instantaneous. Everywhere, the base of the food chain either altered or suffered greatly – the battle between angiosperms and gymnosperms on land; the fall of coral reefs and then rudist reefs in shallow seas; and the fundamental overhaul of primary producing plankton at sea. These individual turmoils spelled disaster for some and brought opportunity for others.

In fact, this is but the natural backdrop to the rich history of life on planet Earth. We find evidence in the Tethys Ocean record of two previous extinction events, the earliest of which came perilously close to wiping out all life for good. In neither case can we call upon extraterrestrial events; in both cases, prolonged environmental stress was the ultimate cause. Fortunately for us, all three mass extinction events have been followed by an exuberant radiation of new life forms.

 Dorrik Stow is author of Vanished Ocean