The “fern spike” is a layer of rock filled with fern spores; it occurs after a major cataclysmic or localized extinction. Scientists believe that after a major mass extinction, most plants would be wiped out. The first plants to recover are the ferns, which spread their spores into the air: thus, the “fern spike.” This evidence in rock layers is often used to determine the line between the time before and after a massive global or local extinction.