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.
Each period is separated by a major geological or palaeontological event, such as the mass extinction of the dinosaurs which occurred at the boundary between the Cretaceous period and the Paleocene epoch. 1  Archean era 3.8 billionâ2.5 billion years ago. 2  Cryogenian period 850 millionâ635 million years ago.