Endemic ammonite faunas evolved from cosmopolitan faunas in a series of successive episodes over about 35 million years of the Cretaceous of the Gulf Coast of the United States. During basin-basin-margin tectonic adjustments the Cretaceous barrier reef was inundated or circumvented so that a cosmopolitan fauna entered the back-reef area. Gradual isolation of the fauna behind the barrier produced endemism. With the next basin adjustment the endemic fauna became extinct, and a new cosmopolitan fauna migrated into the back-reef area, likewise evolving into an endemic fauna in its turn. Six cosmopolitan-endemic cycles have been identified. Geological evidence suggests two or three additional cycles.