Life at the start of the Ordovician remained confined to the seas with new animals evolving in place of those that didn't survive the Cambrian. Chief among them were the squidlike nautiloids ...

The Ordovician world was a watery one in which ... They come from arthropods — the group of animals including insects, spiders, scorpions, shrimp, cnidarians (jellyfish and corals), plus algae ...