▁Distribution ▁ ▁Starting ▁from ▁the ▁mid - Dev onian , ▁am m ono ids ▁were ▁extremely ▁abund ant , ▁especially ▁as ▁am mon ites ▁during ▁the ▁Mes oz o ic ▁era . ▁Many ▁gener a ▁evol ved ▁and ▁ran ▁their ▁course ▁quickly , ▁becoming ▁ext inct ▁in ▁a ▁few ▁million ▁years . ▁Due ▁to ▁their ▁rapid ▁evolution ▁and ▁w ides p read ▁distribution , ▁am m ono ids ▁are ▁used ▁by ▁ge olog ists ▁and ▁pale ont olog ists ▁for ▁bi ost rat ig raph y . ▁They ▁are ▁excellent ▁index ▁foss ils , ▁and ▁it ▁is ▁often ▁possible ▁to ▁link ▁the ▁rock ▁layer ▁in ▁which ▁they ▁are ▁found ▁to ▁specific ▁ge olog ic ▁time ▁periods . ▁ ▁Due ▁to ▁their ▁free - sw imming ▁and / or ▁free - f lo ating ▁hab its , ▁am mon ites ▁often ▁happened ▁to ▁live ▁directly ▁above ▁se af loor ▁waters ▁so ▁poor ▁in ▁o xygen ▁as ▁to ▁prevent ▁the ▁establishment ▁of ▁animal ▁life ▁on ▁the ▁se af loor . ▁When ▁upon ▁death ▁the ▁am mon ites ▁fell ▁to ▁this ▁se af loor ▁and ▁were ▁gradually ▁buried ▁in ▁accum ulating ▁sed iment , ▁b acter ial ▁decomposition ▁of ▁these ▁corps es ▁often ▁t ipped ▁the ▁del icate ▁balance ▁of ▁local ▁re do x ▁conditions ▁sufficiently ▁to ▁lower ▁the ▁local ▁sol ub ility ▁of ▁min er als ▁dissol ved ▁in ▁the ▁se aw ater , ▁not ably ▁ph osph ates ▁and ▁carbon ates . ▁The ▁resulting ▁sp ont aneous ▁concent ric ▁precip itation ▁of ▁min er