ordovician

English Thesaurus

1. from 500 million to 425 million years ago; conodonts and ostracods and algae and seaweeds (noun.time)

instance hypernym	:	geological period, period,
definition	:	a unit of geological time during which a system of rocks formed (noun.time)

part meronym	:	paleozoic, paleozoic era,
definition	:	from 544 million to about 230 million years ago (noun.time)