There was apparently a major extinction around 200 million years ago between the Triassic and Jurassic periods. This event (or events that lasted 10,000 years) led to the almost complete disappearance of many marine groups, such as some of the ammonoids; as well as the complete disappearance of some reptiles, including some types of archosaurs, phytosaurs, aetosaurs, and rauisuchians. Though many scientists speculate that this extinction was caused by an asteroid impact, the suspected resulting crater, Manicouagan in British Columbia, Canada, has been dated at 10 million years too early. There are thus heated debates as to the causes of this extinction event.

Some scientists think that this end-of-the-Triassic-period extinction event opened up more ecological niches (how a species fits into its environment) into which the dinosaurs dispersed, allowing them to flourish and become dominant. However, others feel that the dinosaurs were already on their way to dominance due to the major extinction event at the end of the Permian period. The real sequence of these events may never be known, but in any case, the dinosaurs did start to become dominant during the early Jurassic period.

There is another theory that tries to explain the extinction event between the Triassic and Jurassic periods: There were very large lava flows for approximately 600,000 years close to the division between the Triassic and Jurassic periods—one of the largest such events known to have occurred on our planet. The side effects of these flows, such as the emission of carbon dioxide and sulfur aerosols, may have contributed to the mass extinctions at this time by changing the atmosphere’s composition and/or climate.

Another suggestion is that an impacting asteroid or comet may have actually caused the lava flows, or worked in conjunction with the flows, to create an even harsher environment. The resulting environmental changes—from climate to vegetation—could have led to the mass extinction event between the Triassic and Jurassic periods.