Triassic-Jurassic Mass Extinction
The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 201.3 million years ago, and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas, a whole class (conodonts) and 34% of marine genera disappeared. On land, all archosaurs other than crocodylomorphs (Sphenosuchia and Crocodyliformes) and Avemetatarsalia (pterosaurs and dinosaurs), some remaining therapsids, and many of the large amphibians became extinct.
At least half of the species now known to have been living on Earth at that time became extinct. This event vacated terrestrial ecological niches, allowing the dinosaurs to assume 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. In the area of Tübingen (Germany), a Triassic-Jurassic bonebed can be found, which is characteristic for this boundary.
There are several different hypotheses on what caused this particular mass extinction at the end of the Triassic Period. Since the third major mass extinction actually is thought to have occurred in several small waves of extinctions, it is entirely possible that all of these hypotheses, along with others that may not be as popular or thought of as of yet, could have caused the overall mass extinction event.
There is evidence for all of the causes proposed.
One possible explanation for this catastrophic mass extinction event is unusually high levels of volcanic activity. It is known that large numbers of flood basalts around the Central America region occurred around the time of the Triasssic-Jurassic mass extinction event.
These enormous volcano eruptions are thought to have expelled huge amounts of greenhouse gases like sulfur dioxide or carbon dioxide that would quickly and devastatingly increase the global climate. Other scientists believe it would have aerosols expelled from these volcanic eruptions that would actually do the opposite of the greenhouse gases and end up cooling the climate significantly.
Other scientists believe it was more of a gradual climate change issue that spanned the majority of the 18 million year time span attributed to the end of the Triassic mass extinction. This would have led to changing sea levels and even possibly a change in the acidity within the oceans that would have affected species living there.