▁have ▁been ▁modified ▁for ▁cop ulation ▁due ▁to ▁changes ▁in ▁the ▁h ox ▁gen es ▁h ox d 1 3 , ▁the ▁origin ▁of ▁the ▁m ating ▁org ans ▁in ▁pla cod erm s ▁most ▁likely ▁re lied ▁on ▁different ▁sets ▁of ▁h ox ▁gen es ▁and ▁were ▁structures ▁that ▁developed ▁further ▁down ▁the ▁body ▁as ▁an ▁extra ▁and ▁independent ▁pair ▁of ▁append ages , ▁but ▁which ▁during ▁development ▁turned ▁into ▁body ▁parts ▁used ▁for ▁reprodu ction ▁only . ▁Because ▁they ▁were ▁not ▁attached ▁to ▁the ▁pel vic ▁fins , ▁as ▁are ▁the ▁cl asp ers ▁in ▁fish ▁like ▁sh arks , ▁they ▁were ▁much ▁more ▁flexible ▁and ▁could ▁probably ▁be ▁rot ated ▁forward . ▁ ▁E volution ▁and ▁ext inction ▁ ▁It ▁was ▁thought ▁for ▁a ▁time ▁that ▁pla cod erm s ▁became ▁ext inct ▁due ▁to ▁competition ▁from ▁the ▁first ▁b ony ▁fish ▁and ▁early ▁sh arks , ▁given ▁a ▁combination ▁of ▁the ▁supposed ▁inher ent ▁superior ity ▁of ▁b ony ▁fish ▁and ▁the ▁pres umed ▁sl ugg ish ness ▁of ▁pla cod erm s . ▁With ▁more ▁accurate ▁summar ies ▁of ▁pre histor ic ▁organ isms , ▁it ▁is ▁now ▁thought ▁that ▁they ▁system atically ▁died ▁out ▁as ▁marine ▁and ▁fresh water ▁e colog ies ▁suffered ▁from ▁the ▁environmental ▁cat ast roph es ▁of ▁the ▁Dev onian / Car bon ifer ous ▁ext inction ▁events . ▁ ▁F oss il ▁record ▁The ▁earliest ▁ident ifiable ▁pla cod erm ▁foss ils ▁are ▁from ▁China ▁and ▁date ▁to ▁the ▁early ▁Sil ur