▁m amm ill ary ▁layer ▁on ▁the ▁inside . ▁The ▁m amm illa e ▁are ▁com pressed ▁into ▁groups ▁of ▁two ▁or ▁three , ▁and ▁the ▁p ore ▁can als ▁are ▁well - develop ed . ▁ ▁History Ph ac el ool ith us ▁was ▁first ▁described ▁as ▁the ▁sole ▁species ▁of ▁the ▁o of amil y ▁Ph ac el ool ith idae ▁in ▁ 1 9 7 9 ▁by ▁Chinese ▁pale ont olog ists ▁Z eng ▁Dem in ▁and ▁Z hang ▁Jin j ian , ▁following ▁its ▁discovery ▁in ▁the ▁D ong ting ▁bas in ▁of ▁Hun an . ▁ ▁Par ata x onomy ▁While ▁Ph ac el ool ith us ▁has ▁always ▁been ▁considered ▁an ▁o os pecies ▁of ▁Ph ac el ool ith idae , ▁there ▁is ▁dispute ▁about ▁whether ▁or ▁not ▁this ▁o of amil y ▁is ▁syn onymous ▁with ▁the ▁later - named ▁D end ro ol ith idae . ▁The ▁o of amil ies ▁were ▁first ▁syn onym ized ▁by ▁F ang ▁et ▁al . ▁in ▁ 2 0 0 3 . ▁This ▁position ▁was ▁supported ▁by ▁R ibe iro ▁et ▁al . ▁in ▁ 2 0 1 4 , ▁but ▁other ▁authors ▁have ▁considered ▁the ▁two ▁o of amil ies ▁distinct , B arta , ▁D . ▁E ., ▁Bru nd ridge , ▁K . ▁M ., ▁Cro ghan , ▁J . ▁A ., ▁Jackson , ▁F . ▁D ., ▁Var ric chio , ▁D . ▁J ., ▁Jin , ▁X ., ▁& ▁P oust ,