▁this ▁ir ides c ence ▁have ▁been ▁visible ▁during ▁the ▁animal ' s ▁life ; ▁additional ▁shell ▁layers ▁covered ▁it . ▁ ▁The ▁majority ▁of ▁am m ono id ▁spec im ens , ▁especially ▁those ▁of ▁the ▁Pale oz o ic ▁era , ▁are ▁preserved ▁only ▁as ▁internal ▁m olds ; ▁the ▁outer ▁shell ▁( com posed ▁of ▁a rag on ite ) ▁has ▁been ▁lost ▁during ▁the ▁foss il ization ▁process . ▁Only ▁in ▁these ▁internal - m old ▁spec im ens ▁can ▁the ▁s uture ▁lines ▁be ▁observed ; ▁in ▁life , ▁the ▁s ut ures ▁would ▁have ▁been ▁hidden ▁by ▁the ▁outer ▁shell . ▁ ▁The ▁am m ono ids ▁as ▁a ▁group ▁continued ▁through ▁several ▁major ▁ext inction ▁events , ▁although ▁often ▁only ▁a ▁few ▁species ▁surv ived . ▁Each ▁time , ▁however , ▁this ▁hand ful ▁of ▁species ▁divers ified ▁into ▁a ▁mult itude ▁of ▁forms . ▁Am mon ite ▁foss ils ▁became ▁less ▁abund ant ▁during ▁the ▁latter ▁part ▁of ▁the ▁Mes oz o ic , ▁with ▁none ▁surv iving ▁into ▁the ▁C en oz o ic ▁era . ▁The ▁last ▁surv iving ▁line ages ▁disappeared , ▁along ▁with ▁the ▁non - av ian ▁din osa urs , ▁ 6 6 ▁My a ▁in ▁the ▁C ret ace ous – P ale ogene ▁ext inction ▁event . ▁ ▁Ext inction ▁ ▁The ▁ext inction ▁of ▁the ▁am mon ites , ▁along ▁with ▁other ▁marine ▁animals ▁and ▁non - av ian ▁din osa urs , ▁has ▁been ▁attributed