Editing Paleozoic (section)

=== Ordovician Period ===
{{main|Ordovician}}

[[File:Ostracoderm digital recreation..jpg|thumb|[[Cephalaspis]] (a jawless fish)]]
The Ordovician spanned from 485–444&nbsp;million years ago. The Ordovician was a time in Earth's history in which many of the [[Class (biology)|biological class]]es still prevalent today evolved, such as primitive fish, cephalopods, and coral. The most common forms of life, however, were trilobites, snails and shellfish. The first arthropods went ashore to colonize the empty continent of Gondwana. By the end of the Ordovician, Gondwana was at the south pole, early North America had collided with Europe, closing the intervening ocean. Glaciation of Africa resulted in a major drop in sea level, killing off all life that had established along coastal Gondwana. Glaciation may have caused the [[Ordovician–Silurian extinction events]], in which 60% of marine invertebrates and 25% of families became extinct, and is considered the first Phanerozoic mass extinction event, and the second deadliest.{{efn|name=only-counting-Phanerozoic-extinctions-note}}<ref>{{cite web |title=Ordovician |website=www.ucmp.berkeley.edu |place=Berkeley, CA |publisher=[[University of California Museum of Paleontology]] |url=http://www.ucmp.berkeley.edu/ordovician/ordovician.php |access-date=2015-04-26 |archive-date=2015-05-02 |archive-url=https://web.archive.org/web/20150502201732/http://www.ucmp.berkeley.edu/ordovician/ordovician.php |url-status=live }}</ref>