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 end of the Cambrian Period is marked by evidence in the fossil record of a mass extinction event about 490 million years ago. The Cambrian Period was followed by the Ordovician Period.ossils of the Cambrian Period. Scientists find some of the best specimens for the âevolutionary experimentsâ of the Cambrian Period in the fossil beds of the Sirius Passet formation in Greenland; Chenjiang, China; and the Burgess Shale of British Columbia.