Regression of epicontinental seas could cause extinction of benthic marine organisms simply by destruction of their habitat, but many widespread regressions have occurred without a concomitant event of extinction. Important events of extinction can be shown to correspond to relatively rapid regressions that occurred when epicontinental seas had previously attained large dimensions.
Sustained transgression, such as is associated with the formation of a cratonic sequence, allows migration of marine organisms into widespread epicontinental seas, where they adapt and become stenotopic to variable degrees. Thus, these organisms compose perched faunas (new term) in the sense that their supporting environment is destined to disappear. Relatively rapid regressions that occur under these circumstances result in extinction. Relatively slow regressions, such as occur at the end of cratonic sequences, allow speciation and extinction to become diffuse and thus not recognizable as a distinct event.