Leukonychia (or leuconychia), also known as white nails or milk spots, is a medical term for white discoloration appearing on nails. It is derived from the Greek words leuko (white) and onux (nail). The most common cause is injury to the base of the nail (the matrix) where the nail is formed.
Extinction Events. 1  There were many small-scale, sporadic extinction events during the Cambrian Period. 2  A large-scale mass extinction event occurred approximately 500 million years ago. 3  Scientists believe was caused by a significant decrease in global temperature. A large-scale mass extinction event occurred approximately 500 million years ago. 2  Scientists believe was caused by a significant decrease in global temperature. 3  Decrease in global temperature created an ice age-further eliminated many of the oceanic species.
Clues from the fossil record give us information about what species went extinct and when, but how the extinction happened in the first place has been a puzzling question for scientists. One of the biggest theories involves a series of volcanic eruptions, which took place about 250 million years ago in Siberia. According to Smithsonian's Mass Extinction, these eruptions spewed enough lava to bury the continental US under a layer 1,000 feet deep. Wikimedia Commons Part of the Siberian traps.