The Earth’s Worst-Ever Extinction Event Wiped Out 90% of All Animal Species

Everyone knows that dinosaurs are extinct, and most people have some idea about how it might have occurred. But the exact periods in history when it happened are less well known. Was it a single extinction event or a series of catastrophic changes? Exactly which animals were affected? Here, we will look at the animals that lived on the Earth during one particular geological time: the Mesozoic Era. We will look at the extinction events that took place during that era and how those events ultimately created the world we live in today.

The Mesozoic Era and Geological Timescales

Earth has existed for so long (about 4.5 billion years) that scientists needed a reference and communication system that could describe vast periods. They created the geological timescale to do this, enabling geologists in particular to describe and convey their findings in a way that fellow geologists understand.

Eons are the broadest category. From oldest to youngest, they are the Hadeon, Archean, Proterozoic, and Phanerozoic eons. Each eon is subdivided into eras. The Phanerozoic Eon is divided into the three eras. The oldest is the Paleozoic Era, which means ‘ancient life’ and occurred between 541 and 252 million years ago. Next is the Mesozoic Era, which means ‘middle life’ and spans 252 to 66 million years ago. Finally, there is the Cenozoic Era, which began 66 million years ago and in which we live today.

The Mesozoic Era can be subdivided into three periods, namely the Triassic, the Jurassic, and the Cretaceous. It is important to note that even though the geological timescale is the result of hundreds of years of work, it is still a work in progress and may be changed again in the future.

Animals on Earth During the Mesozoic Era

The names Paleozoic, Mesozoic, and Cenozoic all describe the types of fossils that have been found relating to those periods. In the Mesozoic Era, there are fossils that represent both extinct groups and modern groups of animals and plants.

During the first period of the Mesozoic Era (Triassic Period), many new types of animals evolved to fill the available ecological niches created by a mass extinction event that took place at the end of the Paleozoic Era and the start of the Mesozoic Era. Land was dominated by reptiles, and the first dinosaurs began to appear along with the first marine reptiles, tortoises, and lizards. There were mammals but they were not a dominant group. Crocodiles, on the other hand, were widespread.

The Jurassic Era was when giant plant-eating and smaller carnivorous dinosaurs emerged. Reptiles gained the ability to fly resulting in the first birds. Mammals were present but were still no bigger than rats while the oceans were dominated by whale-sized marine reptiles.

In the Cretaceous period, the dinosaurs dominated the land, and marine reptiles dominated the seas. New types of dinosaurs were appearing along with new types of birds. By now the three groups of mammals that we know today (monotremes, marsupials, and placentals) had evolved. Meanwhile, there were also plenty of insects including some that we would recognize today such as ants, bees, aphids, and grasshoppers.

Extinction Events

We, quite rightly, worry about how the activities of humans are leading to the extinction of animal species alive today. However, extinction is not a modern phenomenon. All through geologic time there have been extinctions mostly occurring at a fairly stable background rate. Occasionally, far more extinctions than we would expect have occurred and these are often called ‘extinction events’ or ‘mass extinctions’. Some of these took place during the Mesozoic Era.

Early Mesozoic Mass Extinction Event

The Mesozoic Era began with the Earth’s worst-ever extinction event. It is referred to as the Permian-Triassic extinction event because it spanned these geological Periods. You may also see it called the Great Dying. The significance of this event, which took place around 252 million years ago cannot be overstated. The cause is not fully understood although there are several theories. These include an asteroid impact, catastrophic volcanic eruptions in what we now call Siberia, methane releases from the oceans, increased acidity, sea level changes, or a combination of some or all of these.

An estimated 90 percent of all animal species on the Earth were wiped out. Many groups of insects and mammal-like reptiles disappeared. The trilobites (a type of marine arthropod whose closest living relative is the horseshoe crab) were totally wiped out having been on the planet for 300 million years.

Interestingly, no coal seams were laid down at this time, indicating that plants were also destroyed. It probably took about 10 million years for the planet to recover.

The End-Triassic Extinction

The second extinction event of the Mesozoic Era is the end-Triassic extinction or the Triassic-Jurassic extinction event. Again, the exact cause is not certain but it was likely triggered by volcanic activity along what would become the Atlantic coast of Africa, Europe, North America, and South America. This is the Central Atlantic Magmatic Province (CAMP), and the activity was part of the ancient supercontinent of Pangea breaking up. As the land mass divided, lava was released, emitting massive amounts of carbon dioxide and sulfur dioxide. These gases had profound effects on both the climate and ocean chemistry.

Sea levels rose and the oceans became more acidic, leading to mass extinctions of creatures such as conodonts (marine jawless eel-like vertebrates), conulariids (small coral-like animals), and some groups of coiled-shelled squid-like ammonoids.

Things were no better on land where numerous groups went extinct. All of the Triassic archosaurs (the ruling reptiles) who had evolved since the last great extinction were made extinct apart from the dinosaurs, pterosaurs and crocodiles. Other victims were phytosaurs (semi-aquatic reptiles) along with many other groups of reptiles. Not all of the mammals survived, for example the dicynodont mammal relatives disappeared.

Cretaceous Minor Mass-Extinctions

A minor mass extinction may seem like a contradiction in terms but compared to the catastrophic events described so far, the early and middle part of the Cretaceous Period were fairly stable. However, as the continents shifted, seas grew and shrank, and flowering plants evolved, there was a constant turnover of species. New ones emerged and some older ones went extinct. For example, most groups of sauropods were replaced by titanosaurs. Stegosaurs also disappeared. The number of medium and large carnivorous theropods reduced so that only tyrannosaurs survived in Asia and North America. However, caution is needed when interpreting the fossil records as some of these ‘disappearances’ may in fact be sampling artifacts and may not represent the true picture. That said, there is evidence of two minor mass-extinctions during the middle Cretaceous. One of them resulted in the extinction of the ichthyosaurs.

Cretaceous-Paleogene (K-Pg) Extinction Event 

Perhaps the most well-known extinction is the Cretaceous-Paleogene (K-Pg) extinction event, which took place 66 million years ago. We all know about it because it was the event that wiped out the dinosaurs (except for a few birds) and started the evolutionary process that led to life on Earth as we know it today.

The truth is that some dinosaurs may have already become extinct during the ongoing extinction processes earlier on in the Cretaceous Period described above. However, it is impossible to be sure about this from the rocks studied so far.

That said, there is no doubt that the end-Cretaceous extinction wiped out many major animal groups including every land creature that weighed more than a few pounds. Things were no better in the oceans with mosasaurs, ammonites, belemnites, giant inoceramid bivalves, reef-forming rudist clams, and plesiosaurs all disappearing.

What Caused the End-Cretaceous Extinction?

The most likely explanation is an asteroid measuring 6 to 10 miles across landing on the Yucatan Peninsula in southeast Mexico forming what we now call the Chicxulub crater. This giant impact (the crater is 90 miles across) would have produced enough energy to create a megatsunami in the Gulf of Mexico. Just as importantly, huge quantities of the Earth’s crust were hurled into the atmosphere and then fell back to the ground at elevated temperatures. The ambient air temperature would have been like the inside of an oven! This alone would have been enough to kill most creatures but things were about to get even worse.

Not all of the dust fell quickly to the ground and what was left in the atmosphere cut off the sunlight. Without light, plants could not photosynthesize and the fundamental basis of many food webs was destroyed. In the absence of plants the herbivores perished and without herbivores, the carnivores could not survive either.

Meanwhile, carbonate- and sulfate-rich marine rocks had been vaporized by the impact. This released gases into the atmosphere making the rain acidic. As it fell on the seas, the water became acidic enough to stop animals from being able to grow shells and killing them off. Without sunlight, the phytoplankton could not photosynthesize either. Marine food chains were affected as much as those on land. It took many years for all of these environmental impacts to reverse and by then, 80 percent of the planet’s animal and plant species were dead.

There are other factors that may also have contributed to the mass extinction. These include lava pouring into oceans, sea level changes and climate changes.

Survivors of the End-Cretaceous Extinction

The story of the end-Cretaceous extinction is not complete without looking at what survived and even benefitted. The loss of so many living things created new opportunities for others.

 New research has shown that flowering plants escaped relatively unscathed. In fact, it was this mass extinction that allowed them to become the dominant plant type on the Earth today. The ancestors of what we now call orchids, magnolia, and mint all survived the end-Cretaceous extinction. They were able to do this because of their remarkable abilities to adapt. They had a variety of seed dispersal and pollination methods and evolved new ways to photosynthesize.

In the animal kingdom, there were also many survivors. Not even all the dinosaurs died – the avian dinosaurs stayed alive and went on to become what we call birds. Some frogs, salamanders, lizards, snakes, and insects also survived.  

Mammal Survivors

Even though mammals undoubtedly benefited from the K-Pg  event and went on to dominate life on the Earth, they did not have an easy ride. Around 90 percent of mammal species also died out. The survivors were small and their lack of size was the key to their survival. They required less nutrition (and were not fussy about what it was), reproduced more quickly, and were able to get underground where it was safer. As conditions became more favorable, these few mammals were able to evolve and diversify to the incredible range we see today – including humans. So, without this extinction event, we would not be here!