During the cretaceous period, reptiles were the dominant species, and dinosaurs ruled earth. plants developed flowers and fruits. at the end of the cretaceous period, about 65 million years ago, 50% of the plants and animals on earth became extinct and dinosaurs were never to be seen again.

what is thought to have caused the mass extinction at the end of the cretaceous period?

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.