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.
Global cooling seems to have been a primary cause and there may also have been some meteorite impacts during this time. 3) End Permian Extinctions. The mass extinction that occurred at the end of the Permian was the worst mass extinction in history. An estimated 96% of marine species became extinct.lobal cooling seems to have been a primary cause and there may also have been some meteorite impacts during this time. 3) End Permian Extinctions. The mass extinction that occurred at the end of the Permian was the worst mass extinction in history. An estimated 96% of marine species became extinct.