arks ▁and ▁b ony ▁fish ▁those ▁b ones ▁are ▁incorpor ated ▁into ▁the ▁bra inc ase . ▁ ▁P lac od erm s ▁also ▁share ▁certain ▁an atom ical ▁features ▁only ▁with ▁the ▁j aw less ▁o ste ost rac ans ; ▁because ▁of ▁this , ▁the ▁theory ▁that ▁pla cod erm s ▁are ▁the ▁sister ▁group ▁of ▁ch ond rich thy ians ▁has ▁been ▁replaced ▁by ▁the ▁theory ▁that ▁pla cod erm s ▁are ▁a ▁group ▁of ▁stem ▁g n ath ost om es . ▁ ▁Tax onomy ▁and ▁ph y log eny ▁Currently , ▁P lac od erm i ▁are ▁divided ▁into ▁eight ▁recognized ▁orders . ▁There ▁are ▁two ▁further ▁controvers ial ▁orders : ▁One ▁is ▁the ▁monot yp ic ▁St ens io ell ida , ▁containing ▁the ▁en ig m atic ▁St ens io ella ; ▁the ▁other ▁is ▁the ▁equally ▁en ig m atic ▁Pseud op etal ich thy ida . ▁These ▁orders ▁are ▁considered ▁to ▁be ▁bas al ▁or ▁primitive ▁groups ▁within ▁P lac od erm i , ▁though ▁their ▁precise ▁pla cement ▁within ▁the ▁class ▁remains ▁uns ure . ▁F oss ils ▁of ▁both ▁are ▁currently ▁known ▁only ▁from ▁the ▁H uns ruck ▁l ager stat ten . ▁ ▁Cl ad ogram ▁The ▁following ▁cl ad ogram ▁shows ▁the ▁inter rel ations hips ▁of ▁pla cod erm s ▁according ▁to ▁Car r ▁et ▁al . ▁( 2 0 0 9 ): ▁ ▁[[ File : Hom ost ius 2 DB . jpg | th umb | right