ian . ▁At ▁that ▁time , ▁they ▁were ▁already ▁differenti ated ▁into ▁anti arch s ▁and ▁ar th rod ires , ▁as ▁well ▁as ▁other , ▁more ▁primitive , ▁groups . ▁Ear lier ▁foss ils ▁of ▁bas al ▁P lac od erm i ▁have ▁not ▁yet ▁been ▁discovered . ▁ ▁The ▁Sil ur ian ▁foss il ▁record ▁of ▁the ▁pla cod erm s ▁is ▁both ▁literally ▁and ▁figur atively ▁fragment ed . ▁Until ▁the ▁discovery ▁of ▁Sil uro le pis ▁( and ▁then , ▁the ▁discover ies ▁of ▁Ent el ogn ath us ▁and ▁Q il iny u ), ▁Sil ur ian - aged ▁pla cod erm ▁spec im ens ▁consisted ▁of ▁fragments . ▁Some ▁of ▁them ▁have ▁been ▁tent atively ▁identified ▁as ▁anti arch ▁or ▁ar th rod ire ▁due ▁to ▁hist ological ▁similar ities ; ▁and ▁many ▁of ▁them ▁have ▁not ▁yet ▁been ▁formally ▁described ▁or ▁even ▁named . ▁The ▁most ▁commonly ▁c ited ▁example ▁of ▁a ▁Sil ur ian ▁pla cod erm , ▁Wang ole pis ▁of ▁Sil ur ian ▁China ▁and ▁possibly ▁Vietnam , ▁is ▁known ▁only ▁from ▁a ▁few ▁fragments ▁that ▁currently ▁def y ▁attempts ▁to ▁place ▁them ▁in ▁any ▁of ▁the ▁recognized ▁pla cod erm ▁orders . ▁So ▁far , ▁only ▁three ▁officially ▁described ▁Sil ur ian ▁pla cod erm s ▁are ▁known ▁from ▁more ▁than ▁scra ps : ▁ ▁the ▁bas al ▁anti arch ▁Sil uro le pis , ▁from ▁the ▁Lud low ▁epo ch ▁of ▁Y unn an , ▁China , ▁known ▁from ▁an ▁almost