Ordovician period (500 to 435 million years ago) This period commenced after Cambrian about 500 and ended 435 million years ago. It is before Silurian. In this period, above equator was ocean and most of lands in located in southern region (Gondwanaland). Gondwanaland has vastly developed toward South Pole within Ordovician. In fact, continents were mostly located between equator and Antarctic. Calcareous sediments indicate the existence of big seas and shallow water having warm and transparent water at that time. It was an appropriate environment for growth of algae, which were the food of big group of animals, for instance; -Corals, sponges, bryozoa, algae producing lime (reefal structures). -Trilobites, brachiopods, gastropods and echinoderms. -Straight cephalopods' shells (Nautiloidea) and Nautilus which chambers are full of gas and were active swimmers and they were probably the first predators. This period is famous for these specifications; marine invertebrates for instance graptolites, trilobites, corals, crinoids and gastropods. Evidences of tetrahedral spores, which are similar to primitive plants' spores, occurred in this period explaining displacement of plants toward land. The Earth experienced a mild climate from early to middle Ordovician that was warm and had a lot of humidity. Any way, when Gondwanaland located near the South Pole within late Ordovician, big ice massives produced relatively shallow sea. It seems that it caused a big extinction at the end of Ordovician. During this period 60% of marine invertebrates and 25% of all families were extinguished.