The answer is trilobites. They are among the life forms that became extinct 250 million years after a calamity in the Permian-Triassic extinction event. It is presumed that nearly 95% of world species were wiped from the face of the earth and this gave rise to the reign of the dinosaurs (that also became extinct 65 million years ago due to asteroid strike). The cause of this major extinction event is yet to be established.