y so zo a ▁have ▁been ▁grouped ▁as ▁Cy clone ural ia ▁but ▁they ▁are ▁more ▁usually ▁considered ▁par ap hy let ic ▁in ▁as ▁representing ▁the ▁primitive ▁condition ▁from ▁which ▁the ▁Pan arth rop oda ▁evol ved . ▁ ▁A ▁modern ▁cons ensus ▁ph y log en etic ▁tree ▁for ▁the ▁proto st om es ▁is ▁shown ▁below . ▁It ▁is ▁indicated ▁when ▁approximately ▁cl ades ▁radi ated ▁into ▁newer ▁cl ades ▁in ▁millions ▁of ▁years ▁ago ▁( My a ); ▁dashed ▁lines ▁show ▁especially ▁uncertain ▁pla c ements . ▁ ▁The ▁ph y log en ic ▁tree ▁is ▁based ▁on ▁N iel sen ▁et . al . ▁with ▁prov is ional ▁pla cement ▁of ▁Lor ic if era ▁from ▁H iro sh i ▁et ▁al . ▁ ▁Old er ▁alternative ▁group ings ▁ ▁Art icul ata ▁hypothesis ▁ ▁The ▁grouping ▁proposed ▁by ▁Agu inal do ▁et ▁al . ▁is ▁almost ▁univers ally ▁accepted , ▁replacing ▁an ▁older ▁hypothesis ▁that ▁Pan arth rop oda ▁should ▁be ▁class ified ▁with ▁Ann el ida ▁in ▁a ▁group ▁called ▁the ▁Art icul ata , ▁and ▁that ▁E cd y so zo a ▁are ▁poly phy let ic . ▁N iel sen ▁has ▁suggested ▁that ▁a ▁possible ▁solution ▁is ▁to ▁regard ▁E cd y so zo a ▁as ▁a ▁sister - group ▁of ▁Ann el ida , ▁though ▁later ▁considered ▁them ▁un related . ▁In clusion ▁of ▁the ▁round w orm s ▁within ▁the ▁E cd y so zo a ▁was ▁initially ▁cont ested ▁but ▁since