Fascinating Facts about the Cretaceous Mass Extinction

J.T. Csotonyi

An artist’s impression of Cretaceous megafauna. Left to right: chasmosaurus, lambeosaurus, styracosaurus and ankylosaurus. These species were likely among the first casualties of the Cretaceous extinction event.

66 million years ago, one of the most devastating natural disasters ever to occur in the history of the Earth took place in what is now Mexico’s Yucatan Peninsula. The most widely accepted theory of the dinosaurs’ extinction concerns an impact event of truly apocalyptic proportions. Travelling tens of thousands of miles per hour, a meteor at least six miles wide landed in the ocean off the coast of Mexico, leaving behind the 112-mile-wide Chicxulub crater. This fateful day marked the Cretaceous extinction event, making way for a new geological period, the Paleogene.

As soon as the terrible impact befell the ancient Earth, the fate of around three quarters of all plant and animal species was already sealed. Releasing a billion times more energy than the combined atomic bombings of Hiroshima and Nagasaki, the impact would have sent megatsunamis over a mile in height sweeping across much of the world. An impact winter likely lasting many years followed, transforming the Earth’s climate beyond recognition and having a profound effect on evolution.

A Final Blow for Non-Avian Dinosaurs

Contrary to popular belief, the impact event, disastrous though it was, was likely not the only cause of the extinction of the dinosaurs. The previous million years had seen seen major climactic changes, which would have wreaked havoc on global ecosystems and likely wiped out certain species by itself. However, the Cretaceous mass extinction was undoubtedly the final nail in the coffin for most species, particularly the larger dinosaurs.

Larger dinosaurs of the late Cretaceous period, such as the famed tyrannosaurus rex and the triceratops, would have been the first to go. Because these enormous animals had far greater food and water requirements, they would have starved to death while also finding themselves struggling to adapt to the changes to the climate following the impact. It’s likely that most of these larger species would have died out within a few years. Of course, any species living anywhere close to the impact crater would have been rendered extinct within minutes, but the effects further afield took more time to occur.

Non-avian dinosaurs are undoubtedly the best-known victims of the Cretaceous mass extinction, but countless other species were also wiped out. Among the other casualties were plesiosaurs and pterosaurs (which were not actually dinosaurs). Avian dinosaurs, however, which are better known as birds, survive to this day.

An Impact Winter Transforms the Earth’s Climate

The Cretaceous world was a very different one to what we know today. The climate was much warmer, and it’s widely believed that there was significantly more oxygen in the atmosphere at about 30% compared to today’s 21%. Additionally, the poles had no permanent ice at them, and instead were cool temperate zones, while most of the rest of the world was humid sub-tropical or tropical.

Immediately following the Chicxulub impact, millions of tonnes of dust and dirt were kicked up into the atmosphere bringing about a rapid cooling of the Earth’s surface. It would have taken some years for the dust to settle, bringing about a devastating decade-long winter in the process. During this time, the amount of sunlight to reach the surface was halved, making it impossible for most species of plants and plankton to photosynthesize. Consequently, many species that relied directly on sunlight would have died within a year, thus also sealing the fates of any animals that relied on them for sustenance. Recent evidence also points to the impact event being responsible for a high release of sulphur into the atmosphere, which would have fallen as acid rain causing further damage to the global ecosystem.

The first decade or so after the impact would have also seen a period of intense cooling until the dust had finally settled and the Earth’s atmosphere finally had a chance to stabilize again. Eventually, the climate started to warm, giving rise to many new niches for life to evolve.

Mammals Take Over

While the Chicxulub impact presented the final blow for the dinosaurs, it also created many new niches for other species to fill, and thus mammals became the dominant species on the land. Although mammals had lived alongside the dinosaurs for around 150 million years, they were very small and not nearly as diverse as they became in the Cenozoic era that followed the mass extinction and continues to this day.

Following the extinction of the dinosaurs, the only survivors were small mammals, reptiles, birds and amphibians. Most of the mammalian genera that exists today came into existence in the first few million years of this era, with the Eocene period between 56 and 34 million years ago introducing the earliest direct ancestors to most modern animals. Nonetheless, mammals weren’t the only large creatures to replace the dinosaurs. Others included the 30-foot-long titanoboa snake and the carbonemys, a turtle with a 6-foot-long shell.

It Wasn’t the Only Major Extinction Event

Though undoubtedly one of the biggest if such events to strike the Earth, the Cretaceous mass extinction certainly wasn’t the only one. In fact, the most devastating by far was the Permian mass extinction 248 million years ago, which preceded the dinosaurs by around 17 million years. The end of the Permian period saw the extinction of 96% of Earth’s species over a few million years. The causes remain unknown, although theories include another asteroid impact, severe volcanism, methane release and plummeting oxygen levels. Other major extinction events include the Ordovician-Silurian event 443 million years ago and the Late Devonian event 359 million years ago, which saw the end of three quarters of Earth’s species.

In total, there were five major extinction events in the Earth’s history as well as numerous smaller ones that are typically attributed to climate change. However, we are currently living in what some scientists describe as the sixth major extinction event or the Holocene extinction. Partly attributed to the overpopulation and encroachment of humans, the Holocene extinction has seen thousands of species disappear in the last 10,000 years, including famous cases such as the dodo, quagga, thylacine, auroch, moa and mastodon among countless others.

Avian Dinosaurs Still Live among Us

Perhaps the most common misconception of all regarding the extinction of the dinosaurs is that they ever really went extinct at all. Dinosauria is the name given to a clade of organisms that all include a common ancestor, and both birds and dinosaurs all share a common ancestor that being the archaeopteryx, which lived some 150-million years ago.

Modern birds and their descendants are all theropods, an enormously diverse group of animals that includes everything from pigeons to stegosauruses. Despite the devastation that befell the Earth some 66 million years ago, palaeontologists can now confidently say that birds continue the lineage of the dinosaurs to this day.

Conclusion

It’s a universal rule that the demise of something opens up a new niche for something else to take its place, whether it’s an ecosystem left open for a new species to explore or the dust of dying stars coming together to form new planets. While the dinosaurs might seem far removed from the world we live in today, the highly successful species present an integral and inseparable part of the evolution of life on Earth. There’s something to think about next time you’re feeding the pigeons in Trafalgar Square.