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 spans from 541 million years to 485 million years ago and is the first period of the Paleozoic and of the Phanerozoic Eon. The Cambrian sparks a boom in evolution in an event known as the Cambrian Explosion in which the largest number of creatures evolve in the history of Earth during one period.he Paleozoic Era ended with the largest mass extinction in Earth's history, the PermianâTriassic extinction event. The effects of this catastrophe were so devastating that it took life on land 30 million years into the Mesozoic to recover. Recovery of life in the sea may have been much faster.