▁This ▁is ▁thought ▁to ▁be ▁because ▁the ▁female ▁required ▁a ▁larger ▁body ▁size ▁for ▁egg ▁production . ▁A ▁good ▁example ▁of ▁this ▁sexual ▁variation ▁is ▁found ▁in ▁B ifer ic eras ▁from ▁the ▁early ▁part ▁of ▁the ▁Jur ass ic ▁period ▁of ▁Europe . ▁ ▁Only ▁recently ▁has ▁sexual ▁variation ▁in ▁the ▁shell s ▁of ▁am mon ites ▁been ▁recognized . ▁The ▁macro con ch ▁and ▁micro con ch ▁of ▁one ▁species ▁were ▁often ▁previously ▁mistaken ▁for ▁two ▁closely ▁related ▁but ▁different ▁species ▁occurr ing ▁in ▁the ▁same ▁rocks . ▁However , ▁because ▁the ▁dim orph ic ▁sizes ▁are ▁so ▁consist ently ▁found ▁together , ▁they ▁are ▁more ▁likely ▁an ▁example ▁of ▁sexual ▁dim orphism ▁within ▁the ▁same ▁species . ▁ ▁Vari ations ▁in ▁shape ▁▁ ▁The ▁majority ▁of ▁am mon ite ▁species ▁feature ▁ ▁plan isp iral , ▁flat - co iled ▁shell s , ▁but ▁other ▁species ▁feature ▁nearly ▁straight ▁( as ▁in ▁b ac ul ites ) ▁shell s . ▁ ▁Still ▁other ▁species ' ▁shell s ▁are ▁co iled ▁hel ically , ▁similar ▁in ▁appearance ▁to ▁some ▁g ast rop ods ▁( e . g ., ▁T urr il ites ▁and ▁B ost ry ch oc eras ). ▁ ▁Some ▁species ' ▁shell s ▁are ▁even ▁initially ▁un co iled , ▁then ▁partially ▁co iled , ▁and ▁finally ▁straight ▁at ▁m atur ity ▁( as ▁in ▁Austral ic eras ). ▁These ▁partially ▁un co iled ▁and ▁totally ▁un co iled ▁forms ▁began ▁to ▁divers ify ▁mainly ▁during ▁the ▁early