<P> The Paleozoic spanned from roughly 541 to 252 million years ago (Ma) and is subdivided into six geologic periods; from oldest to youngest they are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian . Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Pannotia and at the end of a global ice age . Throughout the early Paleozoic, the Earth's landmass was broken up into a substantial number of relatively small continents . Toward the end of the era the continents gathered together into a supercontinent called Pangaea, which included most of the Earth's land area . </P> <P> The Cambrian is a major division of the geologic timescale that begins about 541.0 ± 1.0 Ma . Cambrian continents are thought to have resulted from the breakup of a Neoproterozoic supercontinent called Pannotia . The waters of the Cambrian period appear to have been widespread and shallow . Continental drift rates may have been anomalously high . Laurentia, Baltica and Siberia remained independent continents following the break - up of the supercontinent of Pannotia . Gondwana started to drift toward the South Pole . Panthalassa covered most of the southern hemisphere, and minor oceans included the Proto - Tethys Ocean, Iapetus Ocean and Khanty Ocean . </P> <P> The Ordovician period started at a major extinction event called the Cambrian--Ordovician extinction event some time about 485.4 ± 1.9 Ma . During the Ordovician the southern continents were collected into a single continent called Gondwana . Gondwana started the period in the equatorial latitudes and, as the period progressed, drifted toward the South Pole . Early in the Ordovician the continents Laurentia, Siberia and Baltica were still independent continents (since the break - up of the supercontinent Pannotia earlier), but Baltica began to move toward Laurentia later in the period, causing the Iapetus Ocean to shrink between them . Also, Avalonia broke free from Gondwana and began to head north toward Laurentia . The Rheic Ocean was formed as a result of this . By the end of the period, Gondwana had neared or approached the pole and was largely glaciated . </P> <P> The Ordovician came to a close in a series of extinction events that, taken together, comprise the second - largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct . The only larger one was the Permian - Triassic extinction event . The extinctions occurred approximately 447 to 444 million years ago and mark the boundary between the Ordovician and the following Silurian Period . </P>

2.2 is plate tectonics caused by a change in earths size