Most people can name the asteroid that wiped out the dinosaurs. Far fewer know about the extinction that came before it — the one that actually set the stage for dinosaurs to take over in the first place. Two hundred million years ago, a catastrophe tore through life on land and in the ocean, and the culprit wasn't a rock from space. It was fire from inside the Earth itself.
Ask someone to name a mass extinction and they'll almost certainly say the one that killed the dinosaurs. The Cretaceous-Paleogene event, 66 million years ago, with the asteroid and the crater in Mexico — it has a clean story, a villain, a smoking gun. It's the one that got turned into movies. But there's an older one. Quieter in reputation, messier in cause, and in some ways more consequential for the shape of life that came after it. At 201.4 million years ago, right at the seam between the Triassic and Jurassic periods, something went catastrophically wrong for life on this planet. Marine genera disappeared by the hundreds. On land, the dominant reptile groups — the ones that had ruled Triassic ecosystems for tens of millions of years — were suddenly gone. And when the dust settled, a group of animals that had been living in their shadow for millions of years found themselves with an open world. Dinosaurs. Pterosaurs. The ancestors of every crocodilian alive today. The Triassic-Jurassic mass extinction didn't get an asteroid. It got something arguably stranger: a continent tearing itself apart.
The Central Atlantic Magmatic Province — the largest known large igneous province by area — unleashed volcanic activity across what is now eastern North America, northwestern Africa, and parts of South America and Europe, flooding the atmosphere with carbon dioxide and poisoning the oceans.
Before We Get to the Dying — What Was the Triassic Actually Like?
The Triassic is one of those periods that tends to get skipped over in popular prehistory. It sits between two extinctions — coming after the Permian-Triassic event that nearly sterilized the planet, and ending with the one we're about to get into — and so it tends to be treated as filler. A transitional time. Something that happened between more interesting things. That's not really fair to it. The Triassic, particularly in its later stages, had a genuinely diverse fauna. The dominant land animals weren't dinosaurs — they were archosauromorphs of other kinds, particularly the pseudosuchians (the lineage that leads to modern crocodilians) and phytosaurs, which looked something like modern crocodilians despite not being closely related to them. The superficial resemblance is just convergent evolution — when you're a large semi-aquatic ambush predator, a long snout full of teeth turns out to be a useful shape regardless of your ancestry. Among amphibians, the dominant forms were huge, crocodile-like temnospondyls — nothing like modern frogs or salamanders, which did exist by this point but were small and peripheral. In the oceans, marine reptiles were at their most diverse. Conodonts — eel-like vertebrates whose hard tooth-parts make them invaluable to geologists for dating rock layers — had been a fixture of the seas for 300 million years. All of this was about to go wrong.
The Culprit: A Province the Size of a Continent
Pangaea — the supercontinent that had assembled over hundreds of millions of years — was breaking apart. And the process was not quiet. As the landmass began to rift along what would eventually become the Atlantic Ocean, an enormous volume of magma forced its way to the surface across an area that now spans parts of eastern North America, northwestern Africa, northeastern South America, and southwestern Europe. Geologists call it the Central Atlantic Magmatic Province, or CAMP, and it's one of the largest known large igneous provinces on Earth — the flood basalts eventually covered millions of square kilometers. The volumes of gas released were staggering. Carbon dioxide poured into the atmosphere in pulses — the scientific evidence shows at least two major pulses that each roughly doubled atmospheric CO₂ levels. Before the extinction, CO₂ was already around 1,000 parts per million (already considerably higher than today's 420 ppm). During the extinction, it quadrupled. Global temperatures spiked by 3 to 4 degrees Celsius on average, with some regions experiencing rises as great as 10 degrees. But CO₂ and heat weren't the only problem. CAMP volcanism also released sulfur dioxide in enormous quantities — causing short-term cooling that created violent swings in climate on top of the underlying warming trend. And it discharged toxic mercury into the environment in amounts that show up clearly in the fossil record as mercury anomalies, layers of sediment where mercury concentrations are far above normal. Fossil spores from the extinction interval show elevated mutation rates that researchers link directly to this mercury poisoning. The magma didn't just erupt onto the surface. It also intruded through organic-rich sediments underground, essentially cooking them — releasing additional carbon into the atmosphere through what geologists call contact metamorphism. The warming signal from this secondary carbon release helps explain why the carbon isotope excursions at the Triassic-Jurassic boundary are as large as they are.
CAMP basalts now appear on four continents — a testament to how much of the rifting Pangaea supercontinent was touched by volcanic activity at the Triassic-Jurassic boundary 201 million years ago.
What the Oceans Lost
The carbon dioxide flooding the atmosphere didn't stay there — much of it dissolved into the oceans, dropping seawater pH in what we now call ocean acidification. For organisms that built their shells and skeletons from calcium carbonate, this was a serious problem. The chemistry of the water was working against them. Corals took the worst of it. Around 96% of coral genera disappeared. The Tethys Ocean — the ancient sea that stretched across much of what is now Eurasia — lost its coral populations almost entirely, leaving what researchers call a "coral gap" at the start of the Jurassic. Reef ecosystems, which had been thriving, collapsed. Megalodontid bivalves, large and aragonite-shelled, declined in part because of the acidification eating away their ability to calcify. Between 23 and 34 percent of all marine genera went extinct. Ammonites — those coiled, chambered cephalopods that are now among the most recognizable fossils — came extremely close to total extinction. The dominant Triassic ammonite group, the ceratitidans, vanished entirely. The ammonite groups that would repopulate the Jurassic seas were survivors from smaller, less prominent Triassic lineages. And then there were the conodonts. These had been present in Earth's oceans for 300 million years — survivors of every previous extinction, including the catastrophic end-Permian event that killed 90% of marine species. The Triassic-Jurassic extinction finished them. After 300 million years, conodonts simply stopped appearing in the fossil record at the Triassic-Jurassic boundary and never came back. Adding to all of this: anoxia. The rapid warming stagnated ocean circulation in many regions, cutting off oxygen delivery to deep waters. In some shallow sea areas, particularly in what is now northwestern Europe, water columns became salinity-stratified, making them prone to developing anoxic — oxygen-depleted — conditions. High nutrient runoff from accelerated continental weathering fed algal growth that further stripped oxygen from the water. Layers of black shale deposited during this period are the geological fingerprint of these dead zones. Not everything suffered equally. Fish mostly came through without a mass extinction event, though the patterns of which fish groups thrived shifted — more modern bony fish types started replacing more primitive lineages. Gastropods lost diversity gradually rather than in a sudden crash. Benthic foraminifera — tiny single-celled organisms that live on the seafloor — were hit relatively lightly.
The Land Losses Were Worse
Terrestrial ecosystems, if anything, took a harder hit than the oceans did. Estimates suggest around 42% of all terrestrial tetrapod species — animals with four limbs — disappeared at the end of the Triassic. The archosauromorph reptiles that had dominated Triassic land ecosystems were largely wiped out. Phytosaurs, aetosaurs, rauisuchids, drepanosaurs, trilophosaurids — gone. Procolophonids, a group of small lizard-like reptiles that had survived the Permian-Triassic extinction — gone. The large crocodile-shaped temnospondyl amphibians that had been a fixture of freshwater ecosystems for tens of millions of years were mostly wiped out too. A few lineages clung on into the Jurassic and even the Cretaceous, but the age of temnospondyls as major ecosystem players was over. What survived? The groups that would go on to define the Mesozoic. Dinosaurs came through with minimal losses. So did pterosaurs, crocodylomorphs (the ancestors of modern crocodilians), and mammals — which at this point were still small, mostly nocturnal, and living in the margins of a world dominated by bigger things. The warming periods associated with CAMP volcanism may have actually favored endothermic (warm-blooded) animals like dinosaurs, pterosaurs, and early mammals, which were better insulated against temperature swings than the large ectothermic pseudosuchians. With the ecological competition removed, the Early Jurassic saw dinosaurs radiate into the niches that other groups had vacated. Crocodylomorphs also underwent a rapid adaptive radiation in the extinction's aftermath. The world that emerged from the Triassic-Jurassic boundary was, in terms of its dominant land animals, fundamentally different from the one that had existed before it.