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.
Scientists have narrowed down several of the most likely causes of mass extinction. Flood basalt events (volcano eruptions), asteroid collisions, and sea level falls are the most likely causes of mass extinctions, though several other known events may also contribute. These include global warming, global cooling, methane eruptions and anoxic eventsâwhen the earth's oceans lose their oxygen.