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.
Life responds quickly following the Cambrian extinction. In fact, species exhibit far greater diversity in the Ordovician than in the explosive preceding period. Among the new animal groups to appear are the reef-building, , , and stalked. While few fishes appear in the fossil record from this time, there is fossil evidence that jawless and heavily armored fishes swim the seas. Later in the period, the first sharks evolve. In the plant kingdom, pioneering green algae adapt to open-air living on land.