In the Lower Ordovician , trilobites were joined by many new brachiopods , bryozoans , planktonic graptolites and conodonts , and many types of molluscs and echinoderms , including the ophiuroids ( `` brittle stars '' ) and the first sea stars . In the Early Ordovician , trilobites were joined by many new types of organisms , including tabulate corals , strophomenid , rhynchonellid , and many new orthid brachiopods , bryozoans , planktonic graptolites and conodonts , and many types of molluscs and echinoderms , including the ophiuroids ( `` brittle stars '' ) and the first sea stars .