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.
Silurian Period. Note the expansion of life following the mass extinction at the end of the Ordovician. During the Silurian Period the first land plants appeared. Marine organisms once again expanded in diversity following the extinction of so many families in the late Ordovician. The Silurian was probably relatively warm even though pCO2 may have been lower. This is thought to be because there was no large land mass over the south polar region during the Silurian period.