End-Triassic extinction event

The End-Triassic extinction event marks the boundary between the Triassic and Jurassic periods, 201.3 million years ago. However, many of the extinctions occurred before then in the Upper Triassic.
Overall, this was one of the major extinction events of the Phanerozoic eon. It profoundly affected life on land and in the oceans. At least half the species now known to have been living on Earth at that time became extinct. It was a series of extinctions when life was hit by volcanic events.
A whole class, the conodonts (extinct chordates); 20% of all marine families; all large crurotarsans (non-dinosaurian archosaurs); some remaining therapsids; and many of the large amphibians such as the temnospondyls were wiped out. 
The event emptied many ecological niches, and allowed the dinosaurs to take over the dominant roles in the Jurassic period. This event happened in less than 10,000 years, and occurred just before Pangaea started to break apart. 
Scientists have suggested several explanations for this event, but all have unanswered challenges:
Recent evidence.
A recent paper in PNAS shows fairly clearly that mass volcanism was involved. It was probably the primary cause of the extinction. Mercury is present in volcanic rocks found at the end-Triassic boundary. 
Mercury is emitted in gaseous form during volcanism, and deposited in sediments. The paper says "Such episodic volcanism likely perturbed the global environment over a long time and strongly delayed ecological recovery".