In Linnean classification, an order is a group of related or similar organisms. An order contains one or more families. A group of similar orders forms a class. The Ordovician Period lasted from 505 to 438 million years ago.
Primitive plants appear on land, as do the first corals, primitive fishes, seaweed and fungi.
Graptolites, bryozoans, gastropods, bivalves, and echinoids also appear. There were high sea levels at first, then global cooling and glaciation, and extensive volcanism. North America was under shallow seas. The Ordovician ended in huge extinction, probably due to glaciation. For example, the ovary is an organ that produces ovules.