▁to ▁the ▁K - P g ▁ext inction ▁event , ▁mark ing ▁the ▁end ▁of ▁the ▁C ret ace ous ▁Period . ▁ ▁E ight ▁or ▁so ▁species ▁from ▁only ▁two ▁families ▁made ▁it ▁almost ▁to ▁the ▁end ▁of ▁the ▁C ret ace ous , ▁the ▁order ▁having ▁gone ▁through ▁a ▁more ▁or ▁less ▁steady ▁decl ine ▁since ▁the ▁middle ▁of ▁the ▁period . ▁Six ▁other ▁families ▁made ▁it ▁well ▁into ▁the ▁upper ▁Ma astr icht ian ▁( upp erm ost ▁stage ▁of ▁the ▁C ret ace ous ), ▁but ▁were ▁ext inct ▁well ▁before ▁the ▁end . ▁ ▁All ▁told , ▁ 1 1 ▁families ▁entered ▁the ▁Ma astr icht ian , ▁a ▁decl ine ▁from ▁the ▁ 1 9 ▁families ▁known ▁from ▁the ▁C en oman ian ▁in ▁the ▁middle ▁of ▁the ▁C ret ace ous . ▁ ▁One ▁reason ▁given ▁for ▁their ▁dem ise ▁is ▁the ▁C ret ace ous ▁am mon ites , ▁being ▁closely ▁related ▁to ▁cole oid s , ▁had ▁a ▁similar ▁re product ive ▁strategy ▁in ▁which ▁huge ▁numbers ▁of ▁eggs ▁were ▁laid ▁in ▁a ▁single ▁batch ▁at ▁the ▁end ▁of ▁the ▁lif es pan . ▁These , ▁along ▁with ▁ju ven ile ▁am mon ites , ▁are ▁thought ▁to ▁have ▁been ▁part ▁of ▁the ▁pl ank ton ▁at ▁the ▁surface ▁of ▁the ▁ocean , ▁where ▁they ▁were ▁killed ▁off ▁by ▁the ▁effects ▁of ▁an ▁impact . ▁N aut il oid s , ▁exempl ified ▁by ▁modern ▁na util uses , ▁are ▁convers ely ▁thought ▁to