▁art ic ulated ▁arm oured ▁pl ates ▁and ▁the ▁rest ▁of ▁the ▁body ▁was ▁scale d ▁or ▁n aked , ▁depending ▁on ▁the ▁species . ▁P lac od erm s ▁were ▁among ▁the ▁first ▁j aw ed ▁fish ; ▁their ▁j aws ▁likely ▁evol ved ▁from ▁the ▁first ▁of ▁their ▁g ill ▁arch es . ▁P lac od erm s ▁are ▁par ap hy let ic , ▁and ▁consist ▁of ▁several ▁distinct ▁out groups ▁or ▁sister ▁tax a ▁to ▁all ▁living ▁j aw ed ▁verte br ates , ▁which ▁origin ated ▁among ▁their ▁ranks . ▁This ▁is ▁illustrated ▁by ▁a ▁ 4 1 9 - mill ion - year - old ▁foss il , ▁Ent el ogn ath us , ▁from ▁China , ▁which ▁is ▁the ▁only ▁known ▁pla cod erm ▁with ▁a ▁type ▁of ▁b ony ▁j aw ▁like ▁that ▁found ▁in ▁modern ▁b ony ▁fish es . ▁This ▁includes ▁a ▁dent ary ▁b one , ▁which ▁is ▁found ▁in ▁humans ▁and ▁other ▁t et rap ods . ▁The ▁j aws ▁in ▁other ▁pla cod erm s ▁were ▁simplified ▁and ▁consisted ▁of ▁a ▁single ▁b one . ▁P lac od erm s ▁were ▁also ▁the ▁first ▁fish ▁to ▁develop ▁pel vic ▁fins , ▁the ▁prec ursor ▁to ▁h ind lim bs ▁in ▁t et rap ods , ▁as ▁well ▁as ▁true ▁teeth . ▁Par ap hy let ic ▁group ings ▁are ▁problem atic , ▁as ▁one ▁can ▁not ▁talk ▁precisely ▁about ▁their ▁ph y log en ic ▁relationships , ▁their ▁characteristic ▁tra