Evidence indicates that 251 million years ago a mass extinction of many life-forms occurred on earth. which form of life became extinct at this time?

251 million years ago, the Permian extinction occurred. It is the widest and most massive extinction that wiped out 85-90% of marine species and 70% of land vertebrates at that time. There were recorded low oxygen levels in deep and shallow oceans, extreme shift in the carbon cycle, massive eruption of a basal salt volcano in Siberia and the consecutive global warming that occurred evidently ended early prehistoric life.

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.