It’s the end of the Triassic, and it’s another mass extinction. It was one of the 5 largest extinction events, and some measures make it the second worst, after the event at the end of the Permian. About half the species living at the time disappeared, including a third of the marine genera and many land animals, from amphibians and reptiles to the ancestors of mammals. The conodonts, the little eel-like animals whose teeth are found throughout the geologic record from the Cambrian onward, survived the Permian extinction, but the group was totally eradicated by this one at the end of the Triassic.
Unlike some mass extinctions, which seem to have spanned quite a period of time, a million years or more, this one appears to have had a sudden onset, and to have lasted for as few as 10,000 years – just an instant in geologic time.
And it’s an event that has no really clear cause. Over the course of these podcasts you’ve heard many causes for extinctions, from voluminous volcanism to glaciation and other climate change, changing sea levels and atmospheric chemistry to meteorite impacts. All those are possibilities for the end-Triassic event, but except for an impact, they are gradual processes, generally, and are challenging to use to explain the suddenness and short duration of the extinction.
There’s no evidence for a large impact crater at this time, and there’s no geochemical evidence such as an iridium layer, either.
|Pangaea showing areas of volcanism near the end of the Triassic.|
Map by Williamborg.
There isn’t any doubt that they are related to the break-up of Pangaea, but there are some issues with the timing. It’s pretty close to the end-Triassic extinction, and might have been a factor. But why the very sudden onset of extinction? These igneous rocks span about 600,000 years over a million or more years total. One idea is that one specific volcanic event created a tipping point, a volume of CO2 or acidic aerosols that was devastating to life on a global scale. Another is that greenhouse conditions that might have resulted from volcanic eruptions could have warmed the earth so that methane hydrates, methane frozen into water ice within the sea floor, could have melted. If it were released catastrophically, that might account for the suddenness of the extinctions. There are problems with that, too, in that the volcanic rocks closest in age to the extinctions are often separated from the extinction layers by sedimentary rocks indicating many hundreds of thousands of years. In many ways, this all comes down to the accuracy of age dating, which does have error bars for dates 200 million years ago.
I think this extinction is the least understood of all the Big Five extinctions. The Atlantic volcanism seems to be the best candidate as the cause, one way or another, but the jury is still out. One thing is clear, though. While dinosaurs were definitely impacted by the event, it was the dinosaurs that survived that were able to exploit the niches vacated by other forms of life. Tomorrow, when the Jurassic begins, we will really begin to see the Age of Dinosaurs.
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Robert Folk was born September 30, 1925, in Cleveland, Ohio. He is a professor emeritus at the University of Texas where he taught for many years. He’s probably most noted for establishing a detailed classification system for carbonate rocks, but he’s also worked in the diverse fields of archaeological geology and on bacteria in sediments, research that has had implications for the study of structures in Martian rocks.
—Richard I. Gibson
Map by Williamborg, used under Creative Commons license.