World Paleogeography and Paleoclimate: From the Early to Middle Ordovician, the earth experienced a milder climate. Siberia and Baltica were separate, and the two continents moved north toward Laurentia (North America). Avalonia split from the northern margin of Gondwana, while the rest of that large continent (the future South America, Australia, Africa, India, and Antarctica) moved south over the South Pole. As Gondwana reached the South Pole during the Late Ordovician, massive glaciers began to form, causing sea level to drop. North American Paleogeography: During the Ordovician, Laurentia was located near the equator while it rotated about 45° counter-clockwise, closer to its present orientation. A shallow sea covered most of the continent, depositing limestones, shales, and sandstones. However, by the Middle Ordovician, its southern margin (now the East Coast) was uplifted due to a collision with an island arc. Glaciation at the end of the Ordovician resulted in a drop in sea level, so some rocks deposited earlier were exposed and eroded. Paleontology: The Ordovician fossil record contains a diversity of marine invertebrates, including graptolites, trilobites, brachiopods, and conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, early fish, cephalopods, corals, crinoids, and gastropods. A drop in sea level may have contributed to the mass extinctions that characterized the end of the Ordovician, in which perhaps 60% of all marine invertebrate genera went extinct.