▁complete ▁thor ac ic ▁arm or ▁ ▁Ent el ogn ath us , ▁a ▁pla cod erm ▁inc ert ae ▁sed is ▁that ▁comb ines ▁features ▁of ▁primitive ▁ar th rod ires ▁with ▁j aw ▁an atom y ▁otherwise ▁only ▁seen ▁in ▁b ony ▁fish ▁and ▁t et rap ods . ▁ ▁Q il iny u , ▁a ▁close ▁relative ▁of ▁Ent el ogn ath us ▁that ▁further ▁links ▁Ent el ogn ath us ▁as ▁a ▁trans itional ▁form ▁between ▁pla cod erm s ▁and ▁other ▁stem - gn ath ost om es ▁and ▁crown - group ▁g n ath ost om es . ▁ ▁The ▁first ▁officially ▁described ▁and ▁oldest ▁Sil ur ian ▁pla cod erm ▁is ▁an ▁anti arch , ▁Sh imen ole pis , ▁which ▁is ▁known ▁from ▁distinct ively ▁orn ament ed ▁pl ates ▁from ▁the ▁late ▁L land overy ▁of ▁Hun an , ▁China . ▁Sh imen ole pis ▁pl ates ▁are ▁very ▁similar ▁to ▁the ▁early ▁Dev onian ▁y unn ano le pid ▁Z han j ile pis , ▁also ▁known ▁from ▁distinct ively ▁orn ament ed ▁pl ates . ▁ ▁Pale ont olog ists ▁and ▁pla cod erm ▁special ists ▁suspect ▁that ▁the ▁scarc ity ▁of ▁pla cod erm s ▁in ▁the ▁Sil ur ian ▁foss il ▁record ▁is ▁due ▁to ▁pla cod erm s ' ▁living ▁in ▁environments ▁un con du ci ve ▁to ▁foss il ▁pres ervation , ▁rather ▁than ▁a ▁genu ine ▁scarc ity . ▁This ▁hypothesis ▁helps ▁to ▁explain ▁the ▁pla cod