DECLINING IMPORTANCE OF TRILOBITES IN ORDOVICIAN NEARSHORE PALEOCOMMUNITIES - DILUTION OR DISPLACEMENT

Declining importance of trilobites was a key feature of Ordovician com munity evolution. Previous work has shown that replacement of trilobit e-dominated paleocommunities by mollusc- and brachiopod-rich paleocomm unities was diachronous, and began in nearshore environments. However, the processes responsible for these changes remain unclear. New data from northern North America indicate that trilobite species diversity in nearshore settings maintained a constant, low level between the Lat e Cambrian (Marjuman) and Middle Ordovician (Whiterockian). As the tot al number of species of other groups, especially molluscs, increased i n, nearshore environments, the relative importance of trilobites in th ese paleocommunities declined through a process of dilution,, rather t han actual displacement. The apparent offshore retreat of trilobite-do minated paleocommunities is also at least partly a rejection of this d ilution process: through the Cambrian and Ordovician, trilobite assemb lages tended to be most speciose in, offshore environments, so that th e apparent rate of dilution proceeded more slowly than in the nearshor e. Thus, trilobite-dominated paleocommunities maintained their integri ty longer in, the offshore, and this produced an overall pattern of di achronous replacement. Similarly, progressive offshore replacements of other community types in younger strata may also be dilution phenomen a related to species diversity gradients.