Ordovician

Gastropoda and Bivalvia Bivalve, (class Bivalvia), any of more than 15,000 species of clams, oysters, mussels, scallops, and other members of the phylum Mollusca characterized by a shell that is divided from front to back into left and right valves. Gastropods are among the few groups of animals to have become successful in all three major habitats: the ocean, fresh waters, and land.	Cystoidea, Blastoides, Sea Urchins et al.	Brachiopods
Old Ammonites, Goniatites, Clymenias et al. The Cephalopoda is an ancient and very successful group of the Mollusca. Cephalopods have been among the dominant large predators in the ocean at various times in geological history.	Old Ammonites, Goniatites, Clymenias et al. Trilobite, any member of a group of extinct fossil arthropods easily recognized by their distinctive three-lobed, three-segmented form.	Graphtolites