On what basis it was presumed that dinosaurs were subjected to mass extinctions?

Explanation

Dinosaurs were believed to be the dominant group of organisms in the Jurassic period of Mesozoic era. Mass extinction of these ani mals took place in the latter period of that era. One factor that might have led to the extinc tion of dinosaurs could be meteorites. Basis for this belief is the presence of high concentration of iridium in the Earth.s crust. Iridium is not an original component of the earth.s crust. These iridium deposits were traced back to the same time period in which dinosaurs became extinct.