Ordovician
500 to 440 million years ago
In the Ordovician, the two parts of the future Asia are getting closer. The areas that Siberia and Central Asia underneath are crossed by the Equator, which also cuts through Canada. These areas had a rather warm climate, the southern continental block moves a little north and the South Pole is located for a long time in the middle of the Sahara. The presence of a large landmass in a cold region can account for a phenomenon which, on Earth, has occurred several times periodically. This is the phenomenon of glaciations. Snow and ice accumulate over millennia and vast glaciers form, resulting in a drop in temperature and often at a drier peak. Much of the continental waters that could become rain are trapped in very thick glaciers. The traces of the glaciations are evident: worn rocks, "planed" by the slow movement of glaciers, eroded by the rivers that descended from them. Similar evidence is evident in some areas on the edge of the Sahara whose rocks date back to the Ordovician. Trilobites continue to dominate in the seas, with many species.
Other arthropods also develop and soon triumph. The development of molluscs is very remarkable.
In addition to the forms similar to "snails" (Gastropods group, "foot-belly"), the Nautiloids are widespread. They are protected by a long conical shell crossed by a cavity wrapped like a gimlet.
