<Tr> <Td> Ordovician </Td> <Td> Late </Td> <Td> Hirnantian </Td> <Td> Invertebrates diversify into many new types (e.g., long straight - shelled cephalopods). Early corals, articulate brachiopods (Orthida, Strophomenida, etc .), bivalves, nautiloids, trilobites, ostracods, bryozoa, many types of echinoderms (crinoids, cystoids, starfish, etc .), branched graptolites, and other taxa all common . Conodonts (early planktonic vertebrates) appear . First green plants and fungi on land . Ice age at end of period . </Td> <Td> 445.2 ± 1.4 </Td> </Tr> <Tr> <Td> Katian </Td> <Td> 453 ± 0.7 </Td> </Tr> <Tr> <Td> Sandbian </Td> <Td> 458.4 ± 0.9 </Td> </Tr> <Tr> <Td> Middle </Td> <Td> Darriwilian </Td> <Td> 467.3 ± 1.1 </Td> </Tr>

How is the age of an event defined using relative time