Triassic-Jurassic Extinction: Volcanism and the Rise of Dinosaurs

The Triassic–Jurassic extinction, also called the end-Triassic extinction, was a mass extinction about 201.4 million years ago that ended the Triassic Period. It wiped out an estimated three-quarters of marine species and many groups of land reptiles. Its likely cause was the Central Atlantic Magmatic Province, a vast outpouring of volcanic rock as the supercontinent Pangaea began to break apart. The event cleared the way for dinosaurs to dominate life on land through the Jurassic and Cretaceous.

Background: the Triassic world

The Triassic began about 251.9 million years ago, after the Permian–Triassic extinction, the worst crisis in the history of animal life. Over the following 50 million years, ecosystems recovered and new groups of animals appeared.

Land was still joined in the supercontinent Pangaea. Its climate was generally hot, with strong seasonal monsoons and wide dry belts. The dominant large land animals of the Late Triassic were not dinosaurs but pseudosuchians, the crocodile-line archosaurs. They included armoured plant-eaters called aetosaurs, crocodile-like phytosaurs living along rivers, and large predators known as rauisuchians.

Dinosaurs had appeared by about 230 million years ago, and possibly earlier, but in most places they were a modest part of the fauna. Early mammal relatives, the first turtles, early frog relatives and pterosaurs also lived in the Late Triassic. In the seas, ammonoids, conodont animals, reef-building corals and marine reptiles such as ichthyosaurs and placodonts flourished.

The Central Atlantic Magmatic Province

As Pangaea began to rift apart, enormous volumes of magma reached the surface along the zone where the Atlantic Ocean would later open. The resulting rocks are known as the Central Atlantic Magmatic Province (CAMP), a name introduced by Andrea Marzoli and colleagues in a 1999 paper in Science.

The province’s remnants are now scattered across four continents: eastern North America, north-western Africa, northern South America and south-western Europe. Its original extent is estimated at more than 10 million square kilometres, making it one of the largest volcanic provinces known, and its volume at around 2 to 3 million cubic kilometres. Well-known exposures include the lava flows of Morocco’s High Atlas, basalts in the Newark Basin of New Jersey, and the Palisades Sill, which forms the cliffs along the Hudson River opposite New York City.

In 2013 Terrence Blackburn and colleagues published uranium-lead dates from zircon crystals in CAMP rocks, combined with the astronomical cycles recorded in lake sediments. They showed that the first eruptions began about 201.6 million years ago, essentially at the same time as the extinction, and that activity came in several pulses over roughly 600,000 years. The breakup of the continent that produced these eruptions is described in the article on the breakup of Pangaea.

What happened

Carbon dioxide and warming

CAMP eruptions released huge amounts of carbon dioxide and sulphur gases. Studies of fossil soils in the Newark Basin, published by Morgan Schaller, James Wright and Dennis Kent in 2011, suggested that atmospheric carbon dioxide roughly doubled with the first pulses of eruption, from about 2,000 to more than 4,000 parts per million.

Fossil leaves provide another record. In 1999 Jennifer McElwain and colleagues showed that plants from Greenland and Sweden developed fewer stomata, the pores used to take in carbon dioxide, across the boundary. This change is consistent with a sharp rise in carbon dioxide and an estimated warming of around 3 to 4 °C. Sulphur gases from individual eruptions may also have caused short, sharp cooling episodes.

Crisis in the oceans

In the seas, rising carbon dioxide made seawater more acidic, which harmed organisms that build shells and skeletons from calcium carbonate. Reef ecosystems collapsed, and there is a marked gap in reefs at the start of the Jurassic. Parts of the oceans became oxygen-poor.

Conodont animals, which had existed for more than 300 million years, finally disappeared. Most ammonoid lineages died out, with only a small group surviving to give rise to the ammonites of the Jurassic. Many brachiopods, bivalves and marine reptiles were also lost.

Losses on land

On land, most pseudosuchian groups vanished. Phytosaurs, aetosaurs and rauisuchians disappeared, leaving only the crocodylomorphs, the ancestors of modern crocodiles. Many large amphibians were also lost. Plant records show a sudden turnover in pollen and spores, including a “fern spike” in some regions, where ferns briefly dominated disturbed landscapes.

Chemical evidence ties these changes to volcanism. Boundary rocks in many parts of the world record a sharp shift in carbon isotopes and raised levels of mercury, a marker of large volcanic eruptions.

Defining and dating the boundary

Geologists mark the start of each period at an agreed reference point in the rock record, called a Global Stratotype Section and Point. For the base of the Jurassic, the chosen section is at Kuhjoch in the Karwendel Mountains of Tyrol, Austria, ratified in 2010. The boundary there is defined by the first appearance of an early ammonite, Psiloceras spelae tirolicum, which lived shortly after the extinction.

Other important sections include the coastal cliffs of St Audrie’s Bay in Somerset, England, where grey and black mudstones record the changes in marine life and carbon isotopes across the boundary. Lake sediments in the Newark Basin of eastern North America preserve regular layers linked to cycles in Earth’s orbit, which allow scientists to count time in steps of around 20,000 years.

Combining these orbital cycles with uranium-lead dates from volcanic rocks, researchers have concluded that the main extinction happened very quickly. Many estimates place it within less than about 50,000 years, and possibly within a few thousand years for some groups, although the exact duration is still debated. The commonly cited age of the boundary is about 201.4 million years ago, with an uncertainty of a few hundred thousand years.

Sea level also changed across the boundary, and parts of Europe record a brief drop followed by a rise. Some researchers think these changes contributed to local extinctions, though they are seen as secondary to the effects of volcanism.

The rise of the dinosaurs

Dinosaurs came through the extinction relatively well, and in the Early Jurassic they spread into the ecological roles left empty by the crocodile-line reptiles. Footprint records from the Newark Supergroup in eastern North America, studied by Paul Olsen and colleagues, show that tracks of large meat-eating dinosaurs become more common and larger in size immediately after the boundary.

Within a few million years, Early Jurassic faunas included long-necked sauropodomorphs, armoured early ornithischians and theropods such as Dilophosaurus. Through the Jurassic, giant sauropods and large predators would follow, along with the first birds. The dinosaurs’ long reign lasted until the Chicxulub asteroid impact about 66 million years ago.

Why dinosaurs survived while their crocodile-line rivals did not remains debated. Ideas include a faster metabolism, insulating feathers or filaments in some lineages, and adaptations to cooler, more variable climates at higher latitudes. Other survivors included pterosaurs, turtles, lizard relatives, and the mammaliaforms, close relatives of true mammals.

Key people

Andrea Marzoli of the University of Padua led the 1999 study that defined CAMP as a single province. Paul Olsen of Columbia University’s Lamont–Doherty Earth Observatory spent decades studying the rocks, footprints and lake sediments of the Newark Basin, building one of the most detailed timelines of the boundary.

Terrence Blackburn, working with Olsen, Samuel Bowring and others, produced the high-precision zircon dates published in 2013. Jennifer McElwain’s work on fossil leaves helped show how plants record changes in carbon dioxide.

Myths and facts

It is sometimes said that dinosaurs “took over” because they outcompeted other reptiles. Many researchers now think that the extinction, rather than gradual competition, was decisive: dinosaurs expanded mainly after their rivals were removed.

An asteroid has also been blamed, but no impact structure of the right age has been confirmed. The 100-kilometre-wide Manicouagan crater in Quebec, once considered, formed about 214 million years ago, far too early. A small iridium anomaly near the boundary in the Newark Basin has been reported, but most scientists see volcanism as the main driver.

By the numbers

Timeline

• c. 230 million years ago: Early dinosaurs are present in South America and elsewhere on Pangaea.
• c. 214 million years ago: The Manicouagan impact crater forms in Quebec, millions of years before the extinction.
• c. 201.6 million years ago: The first CAMP eruptions begin as Pangaea starts to rift apart.
• c. 201.5 million years ago: The main extinction interval strikes marine and land life.
• c. 201.4 million years ago: The Jurassic Period begins.
• c. 201–200.9 million years ago: The last major pulses of CAMP volcanism end.
• 1999: Andrea Marzoli and colleagues define the Central Atlantic Magmatic Province.
• 2013: Terrence Blackburn and colleagues publish zircon dates linking CAMP to the extinction.

Why it matters

The Triassic–Jurassic extinction is the fourth of the “Big Five” mass extinctions and one of the clearest examples of how continental breakup and giant volcanic eruptions can reshape life. By removing the crocodile-line reptiles that dominated the Late Triassic, it gave dinosaurs the opportunity to become the leading land animals for the next 135 million years. It also links the history of life directly to the birth of the Atlantic Ocean.