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.
Cenozoic Era. The Cenozoic Era began 65 million years ago with an asteroid impact that killed off a majority of the dinosaurs and ends at the present day. The Cenozoic is commonly divided into three periods: Paleogene (65.5 to 23.03 million years ago) Neogene (23.03 to 2.6 million years ago) Quaternary (2.6 million years ago to present) KT Extinction