 In each burial event layer the commonest species generally has several times as many individuals as the second most common , and accounts for 15 % to 30 % of individual fossil animals . The more common a species is in one layer , the greater the number of other layers it appears in . These " recurrent " species account for 88 % of the individual specimens , but only 27 % of the number of species . This suggests that the majority of species were in existence for much shorter periods than the " recurrent " ones . Species that cover shorter periods of time occur mainly in the higher , younger layers . The GPB shows an overall trend of increasing diversity as time progresses . 
