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.
3. PermianâTriassic extinction event (End Permian): This one occurred right about 251 million years ago. It is the largest extinction event that is currently known. It led to the extinction of â57% of all families, 83% of all genera and 90% to 96% of all species.rdovicianâSilurian extinction event (End Ordovician): It occurred about 450â440 million years ago. It actually consisted of two different events, in total they annihilated 27% of all families, 57% of all genera and 60% to 70% of all known species.