But sometimes, the real reason is unclear. It may have been due to competition from other species, dramatic climate changes, or the impacts from an asteroid/meteor (the last one being quite a popular one). Yet some questions remain unanswered. Why was it that some species died out, and others survived, some to this day (famous example: the coelacanth).
During the extinction of the dinosaurs, crocodiles and turtles were around, but they survived, even to this day, while the dinosaurs, the pterosaurs, the marine reptiles and others died out.
To this day, they are a mystery, and without a time machine, we may never know. Other popular theories include:- flood basalt events, smaller asteroid showers, global warming/cooling, sea level drops.