The maintenance of evolutionary equilibrium in Late Ordovician benthic marine invertebrate faunas

An upper evolutionary limit on species equilibrium number (S of MacArthur & Wilson) has been postulated but never demonstrated. Results of the present study indicate that an S of about 10 was attained very rapidly in a Late Ordovician benthic marine invertebrate faunal succession. This S value was then maintained for perhaps five million years within a marine environment that did not remain uniform but changed progressively from relatively deep to shallow water. Most importantly, significant taxonomic and presumed trophic changes that accompanied the gradual habitat alteration seem to have had a negligible effect on the plateau-like limit to species equilibrium number. We conclude from these preliminary results that the maintenance of evolutionary equilibrium in an open ecosystem strongly suggests a long-term component of biotic resiliency, at least within this one marine ecosystem, and perhaps in many others. The reasons for this resiliency remain to be explored.