The Mass Extinction Event That Led to the Rise in Dinosaurs

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The Jurassic Period is one of the three prehistoric geological periods of the Mesozoic Era. It spans from 145 million to 201 million years ago. This period was preceded by the Triassic Period and followed by the Cretaceous Period. It was a time of significant changes for life on Earth, including the extinction of some species and the diversification of others. Here, we will explore what the Earth was like at this time, the plants and animals that existed, and the extinction events that they endured.

About the Jurassic Period

The Jurassic Period can be subdivided into three epochs. They are:

• Early Jurassic (201.4 million to 174.7 million years ago)
• Middle Jurassic (174.7 million to 161.5 million years ago)
• Late Jurassic (161.5 million to 145 million years ago)

It gets its name from the Jura Mountains, which were found between France and Switzerland. This is where a naturalist called Alexander von Humboldt first identified Jurassic rocks.

What Did the Earth Look Like During the Jurassic Period?

At this time, the arrangement of land on our planet was very different from today. There was a huge supercontinent called Pangaea that had already started to break down during the Triassic Period. Two smaller land masses were formed: Laurasia and Gondwana. Between them was a prehistoric ocean called the Tethys Ocean. As these continents further divided, what we now call the Atlantic Ocean also started to form.

Jurassic Period Climate

The climate during the Jurassic Period was warmer than it is today and is often described as having tropical greenhouse-like conditions. There was no ice at the North and South Poles. Sea levels were around 450 feet higher than current levels, and shallow inland seas covered what we now call the western United States.

Jurassic Animals and Plants

Woody trees became more common during the Jurassic Period with conifers and ginkgoes being much more diverse than they are today. At ground level, there were cycads (evergreen, cone-bearing, palm-like plants) and ferns. Low-growing mosses and liverworts were found in swampy moist areas. During the later Jurassic Period, some angiosperms (flowering plants) were also emerging.

When it comes to animals, dinosaurs dominated the land which is why this is commonly called the ‘age of the dinosaurs’. The largest dinosaurs of this time were the sauropods including Diplodocus, Brachiosaurus, and Apatosaurus. Other herbivores such as the plated stegosaurs were also roaming around. Hunting these were the carnivorous dinosaurs (carnosaurs) such as Allosaurus and the smaller coelurosaurs. The first birds, including Archaeopteryx were in the skies.

In the oceans, ichthyosaurs were thriving along with plesiosaurs. More familiar to us would have been the marine crocodiles and the sharks and rays. Ancient relatives of squids and octopi (cephalopods) called ammonites and belemnites would also have been found. Plankton was becoming both more numerous and diverse.

The mammals around at this time were very small and were mainly shrew-like creatures scuttling around the ground feeding on plants and insects and trying to keep out of the dinosaur’s way!

What Is an Extinction Event?

Extinction has happened ever since life began. It is estimated that 98% of all the organisms that ever existed on Earth are now extinct. Clearly, becoming extinct is catastrophic for some species, but it creates opportunities for others. They can exploit the ecological niches that have been left vacant. When there are simply ‘background’ levels of extinction, the situation remains quite stable as lost species are replaced by others. Experts have put the normal extinction rate for the planet at between 0.1 and 1 species per 10,000 species/100 years.

A mass extinction event is quite different. Here, the loss of species is so fast that they cannot be replaced for many millions of years. The generally accepted view is that if three-quarters of the world’s species are lost in less than 2.8 million years, we can call it a mass extinction event. That said, it is a lot more difficult to identify exactly when the event started and ended.

Jurassic Period Extinction Events

There was only one significant extinction event during the Jurassic Period and that was the one at the start. This is normally called the Triassic-Jurassic extinction event as it took place at the boundary between the two periods, which was 201.4 million years ago. Apart from this, there is evidence of a smaller extinction affecting the oceans around 183 million years ago. At this time, marine species such as ichthyosaurs and plesiosaurs thrived in the oceans. Then, volcanoes in what we now call South Africa released 20,500 gigatons of carbon dioxide (CO2) over 500,000 years. This heated the oceans and reduced the oxygen content, suffocating marine animals and triggering a mass extinction. Background extinctions also continued during the Jurassic Period with some species rising to dominance while others declined.

Triassic-Jurassic Extinction Event

You will find this mass extinction event referred to in a few different ways including the end-Triassic mass extinction (ETME) and Triassic-Jurassic (Tr-J) extinction event (TJME). It was a highly significant event and is classed as one of the ‘Big Five’ mass extinctions of the Phanerozoic Eon. This was a critical event in the evolution of life on Earth and ultimately led to the evolution of modern fauna (animals) and flora (plants). However, it did not take place in isolation. In many ways, it cleaned out some of the lineages that had been lingering from previous extinctions.

What Caused the Triassic–Jurassic (Tr-J) Extinction Event?

The most likely cause of this mass extinction was an increase in volcanic activity. At this time, the huge prehistoric supercontinent of Pangaea was starting to break up. The area called the Central Atlantic Magma Province (CAMP) was very active and released huge amounts (up to 100,000 gigatons) of carbon dioxide into the atmosphere. As a greenhouse gas, this led to intense global warming, with average temperatures increasing by as much as 18-27 degrees F.

Other scientists have pointed to the potential emissions of toxic mercury, intermittent cold periods, and the release of methane trapped in permafrost and under sea ice. There are also suggestions that it was caused by a meteor strike, and some argue that it was not even a mass extinction event at all. Perhaps, species were lost over a considerable period of time. There may have been two shorter-lived extinction pulses separated by an interval of several hundred thousand years.

Animals Lost During the Mass Extinction

Whatever the cause, or how quickly it took place, this extinction event resulted in the loss of many species of animals. The most severely affected were the terrestrial animals. Up to 42 percent of all terrestrial tetrapods became extinct at the beginning of the Jurassic Period. This included many amphibians, reptiles, and synapsids. Freshwater ecosystems were also affected. In this particular extinction, body size did not seem to affect your survival chances. Temnospondyls were large, crocodile-like animals and several species disappeared around this time. Plant-eating insects, however, emerged relatively unscathed.

Marine animals also experienced wide-ranging losses from the seafloors to the surface. The main driver of this was increased acidity caused by carbon dioxide from volcanic eruptions. Experts estimate that half of all bivalve genera were lost. Others survived by adapting to the change in conditions by either increasing or decreasing their size. Plankton diversity also dropped suddenly. Ammonites were very nearly wiped out. Conularidis completely died out, but around 4 percent of coral survived. On a more positive note, fish and marine reptiles did not experience a significant fall in populations.

Recovery From the Extinction Event

In the oceans, the recovery was relatively rapid. In most seas, it began immediately but in others, it took longer. The most notable feature of the recovery from this extinction event was the rise of the dinosaurs. On the land, they proliferated, filling many of the vacated ecological niches. Thanks to the Jurassic mass extinction, they went on to dominate the rest of the Jurassic Period, which became known as the ‘Golden Age of Dinosaurs’.

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