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.
The Cambrian was a time of mild, equitable climate, when high sea levels covered extensive continental margins. Sediments deposited on the margins of one of the Cambrian continental masses, Avalonia, now form the type section in Wales. Modern dating techniques have shown that the Cambrian was of shorter duration than previously thought: the currently accepted duration is ~56 Ma, from 541 to ~485 Ma ago.