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.
OrdovicianâSilurian extinction events (End Ordovician or O-S): 450â440 Ma at the Ordovician-Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 60% to 70% of all species. Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that became extinct.