Notes: 1  In 1997, MLB retired Jackie Robinson's number 42 meaning no other player in MLB could ever wear it. However, it was grandfathered to anyone who was currently wearing the number. 2  Mariano Rivera is the only Yankee to appear as an active player while wearing his retired number.
The Ordovician Period. The Ordovician Period lasted almost 45 million years, beginning 488.3 million years ago and ending 443.7 million years ago.* During this period, the area north of the tropics was almost entirely ocean, and most of the world's land was collected into the southern supercontinent Gondwana.
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.