<P> Uncertainty exists regarding the duration of the overall extinction and about the timing and duration of various groups' extinctions within the greater process . Some evidence suggests that there were multiple extinction pulses or that the extinction was spread out over a few million years, with a sharp peak in the last million years of the Permian . Statistical analyses of some highly fossiliferous strata in Meishan, Zhejiang Province in southeastern China, suggest that the main extinction was clustered around one peak . Recent research shows that different groups became extinct at different times; for example, while difficult to date absolutely, ostracod and brachiopod extinctions were separated by 670 to 1170 thousand years . In a well - preserved sequence in east Greenland, the decline of animals is concentrated in a period 10 to 7001600000000000000 ♠ 60 thousand years long, with plants taking several hundred thousand additional years to show the full impact of the event . An older theory, still supported in some recent papers, is that there were two major extinction pulses 9.4 million years apart, separated by a period of extinctions well above the background level, and that the final extinction killed off only about 80% of marine species alive at that time while the other losses occurred during the first pulse or the interval between pulses . According to this theory one of these extinction pulses occurred at the end of the Guadalupian epoch of the Permian . For example, all but one of the surviving dinocephalian genera died out at the end of the Guadalupian, as did the Verbeekinidae, a family of large - size fusuline foraminifera . The impact of the end - Guadalupian extinction on marine organisms appears to have varied between locations and between taxonomic groups--brachiopods and corals had severe losses . </P> <Table> <Tr> <Th> Marine extinctions </Th> <Td> Genera extinct </Td> <Td> Notes </Td> </Tr> <Tr> <Td_colspan="5"> Arthropoda </Td> </Tr> <Tr> <Td> Eurypterids </Td> <Td> 100% </Td> <Td> May have become extinct shortly before the P--Tr boundary </Td> </Tr> <Tr> <Td> Ostracods </Td> <Td> 59% </Td> <Td> </Td> </Tr> <Tr> <Td> Trilobites </Td> <Td> 100% </Td> <Td> In decline since the Devonian; only 2 genera living before the extinction </Td> </Tr> <Tr> <Td_colspan="5"> Brachiopoda </Td> </Tr> <Tr> <Td> Brachiopods </Td> <Td> 96% </Td> <Td> Orthids and productids died out </Td> </Tr> <Tr> <Td_colspan="5"> Bryozoa </Td> </Tr> <Tr> <Td> Bryozoans </Td> <Td> 79% </Td> <Td> Fenestrates, trepostomes, and cryptostomes died out </Td> </Tr> <Tr> <Td_colspan="5"> Chordata </Td> </Tr> <Tr> <Td> Acanthodians </Td> <Td> 100% </Td> <Td> In decline since the Devonian, with only one living family </Td> </Tr> <Tr> <Td_colspan="5"> Cnidaria </Td> </Tr> <Tr> <Td> Anthozoans </Td> <Td> 96% </Td> <Td> Tabulate and rugose corals died out </Td> </Tr> <Tr> <Td_colspan="5"> Echinodermata </Td> </Tr> <Tr> <Td> Blastoids </Td> <Td> 100% </Td> <Td> May have become extinct shortly before the P--Tr boundary </Td> </Tr> <Tr> <Td> Crinoids </Td> <Td> 98% </Td> <Td> Inadunates and camerates died out </Td> </Tr> <Tr> <Td_colspan="5"> Mollusca </Td> </Tr> <Tr> <Td> Ammonites </Td> <Td> 97% </Td> <Td> </Td> </Tr> <Tr> <Td> Bivalves </Td> <Td> 59% </Td> <Td> </Td> </Tr> <Tr> <Td> Gastropods </Td> <Td> 98% </Td> <Td> </Td> </Tr> <Tr> <Td_colspan="3"> Retaria </Td> </Tr> <Tr> <Td> Foraminiferans </Td> <Td> 97% </Td> <Td> Fusulinids died out, but were almost extinct before the catastrophe </Td> </Tr> <Tr> <Td> Radiolarians </Td> <Td> 99% </Td> <Td> </Td> </Tr> </Table> <Tr> <Th> Marine extinctions </Th> <Td> Genera extinct </Td> <Td> Notes </Td> </Tr> <Tr> <Td_colspan="5"> Arthropoda </Td> </Tr>

When did the last mass extinction occur answers