Twelfth.

Returning to a post I made about the Permian/Triassic mass extinction and its effect on dinosaur evolution, I recently came across this article:

The end Triassic extinction coincided with the rise of the Dinosaurs. The world was different back then, having just suffered the worst extinction in the history of the living world, and the continents were amassed into one single supercontinent, Pangea. Jessica Whiteside and her team (2010) used fossil evidence and the carbon signature found in fossil leaves and wood deposited in lake sediments containing basalts, which indicated volcanic activity, to construct a climate record marking at the Triassic/Jurassic boundary. 

The researchers proposed that a large spike in carbon dioxide levels in the air due to volcanism resulted in half of land plants going extinct, whilst also marking the end of the Triassic. The volcanism is thought to have originated from the drifting North American and African plates, leading to 9 million square kilometres of lava being generated. Pollen counts and C12 and C13 isotope concentrations showed just how badly this effected the plant life in terrestrial environments. The rise in carbon dioxide concentrations also destroyed crurotarsan populations, cousins to the crocodiles, that competed for terrestrial dominance with the early dinosaurs.  

Crurotarsans

Freed from their crurotarsan competitors, the carnivorous theropod dinosaurs dramatically increased in number (see Olsen, 2002, who documented this rise). Whether this rise was due to adaptive evolution or just luck, is still unknown, and many never be discovered, but once again shows that climate change had its hand in creating the most dominant species the world has seen. 

Jessica H. Whiteside, Paul E. Olsen, Timothy Eglinton, Michael E. Brookfield, and Raymond N. Sambrotto. (2010) Compound-specific carbon isotopes from Earth's largest flood basalt eruptions directly linked to the end-Triassic mass extinction. Proceedings of the National Academy of Sciences