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The Triassic-Jurassic extinction event 200 million years ago caused the extinction of 50% of all species, including many marine and terrestrial animals. The cause is unknown, but hypotheses include volcanic eruptions and meteorite impacts. The event paved the way for dinosaur dominance, but if therapsids had thrived instead, mammals could have evolved much earlier.
The Triassic-Jurassic extinction event, which occurred worldwide about 200 million years ago, is one of the five major mass extinctions in the last 600 million years. It is estimated that 50 percent of all species have gone extinct. Some paleontologists call the Triassic-Jurassic extinction the second largest mass extinction in prehistory. It occurred in the geological blink of an eye, in no more than 10,000 years.
This extinction event wiped out about 20% of marine families and 30% of marine genera. It wiped out many therapsids, which have also been called “mammal-like reptiles,” although they were neither mammals nor reptiles. Also wiped out were all the large crurotarsans – non-dinosaur archosaurs, the ancestors of modern crocodiles, alligators and gharials – and most of the large amphibians, which had hitherto been the dominant terrestrial fauna.
The Triassic-Jurassic extinction occurred just 50 million years after the largest extinction event in the era of animal life, the Permian-Triassic extinction about 250 million years ago. The first mass extinction of the Mesozoic era, the Triassic-Jurassic extinction event, is often seen as setting the stage for dinosaur dominance. Before the mass extinction, dinosaurs represented about 1-2% of the terrestrial fauna, but after it, occupying niches of extinct species, they came to represent about 50-90% of the fauna.
The cause of the Triassic-Jurassic extinction event is unknown. Unlike some of the other mass extinctions of the past, little evidence has coalesced around any particular interpretation. Some hypotheses include the impact of meteorites and volcanic traps or massive eruptions sustained over a million years.
Volcanic eruptions may have triggered secondary and tertiary effects such as global warming or cooling, methane hydrate release, reduced oxygen levels in the oceans, and more. Until more evidence emerges, scientists can’t be sure. This may not happen, however. Oceanic crust recycles about every 50 million years, so it’s likely that all major meteor or comet impact craters have been obliterated.
The period immediately following the Triassic-Jurassic extinction event was crucial, because the vacant niches may have been filled either by reptiles such as dinosaurs or by therapsids, which include mammalian ancestors. In the end, the dinosaurs won out, but if the therapsids had thrived and diversified, mammals could have evolved more than 150 million years earlier than they actually did. It may have just been a fluke that delayed mammalian evolution until the dinosaurs themselves succumbed to another mass extinction some 135 million years later.
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