Just over 200 million years ago — almost exactly 50 million years after the great Permian extinction in which more than 90 percent of all marine species and nearly three fourths of all terrestrial vertebrate species were expunged — a great extinction event marks the boundary between the Triassic and the Jurassic: the event which eventuated in the rise of the dinosaurs.
For 135 million years after the Triassic-Jurassic extinction, the dinosaurs were the dominant vertebrate species on the planet.
We almost certainly know the reason for the extirpation of the dinosaurs 65 million years ago (see tomorrow’s post), and we aren’t yet at all certain of the reason for the earlier Permian extinction. (At least three good possibilities are: massive volcanism; multiple asteroid or comet impacts; or – the one I find most intriguing — the catastrophic release of methane hydrates from the ocean floor. Perhaps it was a combination of any two, or all three, of these eventualities.) As for the end Triassic extinction, speculation has heretofore concentrated largely upon volcanism.
Now, in a study published in the Proceedings of the National Academy of Sciences, a team of scientists led by paleobiologist Jessica Whiteside has found strong evidentiary support for the hypothesis that “massive, widespread volcanic eruptions led to a spike in atmospheric carbon dioxide and other greenhouse gases that wiped out half of plant species and marked the end of the Triassic, one of the five great mass extinctions of Earth history.”
Moreover, “the team also established through the fossil record that the abrupt rise in atmospheric gases decimated crurotarsans [crocodile-like creatures], which had competed vigorously with the earliest dinosaurs during the Triassic. Thanks to the climatic catastrophe, those early, small dinosaurs were freed from their main competitors to become the dominant force in the animal world.”
For considerably more detail, see this press release from Brown University, or its near duplicate How Dinosaurs Rose to Prominence in Science Daily, and also Dinosaur Rise Linked to Volcanism in the BBC News.
(You’ll find the abstract of the PNAS article at Compound-Specific Carbon Isotopes from Earth’s Largest Flood Basalt Eruptions Directly Linked to the End-Triassic Mass Extinction.)
Update: Wired magazine produces an excellent recapitulation of this study in Dinosaurs Rode Volcanic Armageddon to Victory.