Triassic–Jurassic extinction event
The Triassic-Jurassic extinction event, occurring around 201 million years ago, is recognized as one of the most significant mass extinction events in Earth's history, with an estimated 80% of known species becoming extinct. Marine life was particularly affected, with many species, such as ammonites and conodonts, suffering severe losses, while land-based species also faced significant declines. This extinction paved the way for new species to thrive, notably the dinosaurs, which began their rapid expansion in the aftermath. Although the exact cause of the extinction remains uncertain, prevailing theories suggest it was linked to increased volcanic activity associated with the breakup of the supercontinent Pangea. This activity likely released large amounts of greenhouse gases, leading to global warming and drastic changes in the planet's ecosystems. The Triassic-Jurassic extinction event is also referred to as the End-Triassic Extinction, highlighting its place in the geological timeline. Understanding this event provides insights into the resilience of life and the cyclical nature of extinction and evolution on Earth.
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Triassic-Jurassic extinction event
The Triassic-Jurassic extinction event was a period of mass extinction that occurred approximately 201 million years ago. The event is among the most deadly in the planet’s history, with an estimated 80 percent of all known species at the time becoming extinct. Marine life was especially hard hit, although the majority of land-based species were also impacted. The mass extinction did provide an advantage to new species and set the stage for the eventual growth and spread of the dinosaurs. Although the exact cause of the extinction is uncertain, many scientists believe it was triggered by increased volcanic activity that changed the world’s climate. This event is also known as the End-Triassic Extinction.
Background
Life has existed on Earth for more than 3.7 billion years and has undergone an extraordinary amount of change and development during that time. Scientists have attempted to document these changes using broad categories based on the eras of the planet’s geological development. During each geological period, different forms of life appeared, evolved, and disappeared for a variety of reasons.
Many of the geological periods of life on Earth are marked by extinction events, or times in which the majority of the planet’s life suddenly died off. Paleontologists determine a prehistoric species has gone extinct by studying its disappearance from the fossil record. An organism that was abundant in fossils from one period and absent in later fossils is considered to have most likely become extinct.
The causes of extinction events vary greatly. Most were likely caused by sudden shifts in climate, changes in sea or oxygen levels, asteroid impact, volcanic activity, and other natural factors that made conditions on the planet uninhabitable for many organisms. These events also range in size and severity. At least five such events in Earth’s history are considered major. Major extinction events are categorized as the loss of more than 75 percent of the planet’s living species.
The first major extinction event known to modern scientists occurred about 443 million years ago, toward the end of the geological period known as the Ordovician. That was followed by a major extinction at the end of the Devonian Period, about 375 million years ago, and at the end of the Permian Period, around 251 million years ago. Those would ultimately be followed by the Triassic-Jurassic extinction, approximately 201 million years ago.
Each extinction saw a majority of the planet’s species eradicated. However, following each extinction, surviving species generally multiplied, and new species appeared that were better suited to the ever-changing environmental conditions of Earth.
Overview
Hundreds of millions of years ago, landforms were clustered more closely than they are today. Most of the planet was covered by unbroken seas and oceans. The majority of life during these early periods in Earth’s development lived in the seas and oceans. In the first major extinction events, marine life suffered the most catastrophic losses.
For example, during the event at the end of the Permian Period, greenhouse gases filled the atmosphere, causing the oceans to heat to dangerous levels and become unlivable for many species. This led to the extinction of a majority of all life. The Permian extinction was the most deadly in the planet’s history, killing more than 90 percent of all living species, including about 96 percent of all marine life and about 70 percent of land-based species.
Life rebounded and flourished after the extinction, repopulating the planet during the following geological era, the Triassic Period, which began about 252 million years ago. Again, most of the life was concentrated in the oceans, though land animals, including dinosaurs, became considerably more common. These species would ultimately experience another extinction event, known as the Triassic-Jurassic extinction, or the End-Triassic Extinction, about 201 million years ago.
The Triassic-Jurassic extinction saw the demise of approximately 80 percent of all known species of living things. Many species that had regularly appeared in the fossils of earlier eras became rare or absent altogether. Marine creatures such as shelled cephalopods known as ammonites were nearly wiped out. Eel-like vertebrates called conodonts disappeared completely. Previously, conodont teeth and jawbones were commonly found in the fossil record. Other hard-hit marine creatures included brachiopods, bivalves, sponges, corals, and several species of reptiles.
On land, many kinds of vertebrate animals were reduced in number and variety. Tetrapods—four-limbed creatures that include amphibians, reptiles, and birds—in what is today eastern North America lost a significant amount of their population. Phytosaurs, reptiles that resembled crocodiles, completely disappeared from the fossil record. However, in general, terrestrial organisms escaped ruinous losses. In particular, dinosaurs proved resilient to the conditions that followed the extinction.
Although many of the prehistoric extinction events have unclear causes, many scientists consider the Triassic-Jurassic extinction one of the most mysterious. Experts have been unable to agree on one likely cause. The most widely accepted theory posits that the extinction was related to the breakup of the huge landmass known as Pangea, formed long ago from a connected mass of all the modern continents.
Volcanoes formed as segments of Pangea broke apart and drifted away from one another to form today’s continents. These changes in the structure of Earth triggered massive volcanic eruptions that emitted large amounts of so-called greenhouse gases, mainly carbon dioxide, which filled the atmosphere. Global warming resulted, heating the oceans and offsetting the delicate balance of life. Much as in the Permian extinction, increasing temperatures in the planet’s oceans and seas led to disastrous changes in ecosystem and widespread death.
Regardless of the cause or causes of the extinction, it had a clear aftermath. The reduction in species, shifting of populations, and changes in ecosystems ultimately proved an advantage to dinosaurs. Dinosaurs only began to appear during the Triassic Period. They not only survived the extinction, but also began to expand at a rapid rate afterward. Throughout the coming Jurassic Period, dinosaurs were the dominant form of life on Earth.
Bibliography
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