Page "Agnostida" Paragraph 2

The systematic position of the order Agnostida within the class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a stem group.

The challenge to the status has focused on the Agnostina partly because juveniles of one genus have been found with legs greatly different from those of adult trilobites, suggesting they are not members of the lamellipedian clade, of which trilobites are a part.

Instead, the limbs of agnostids closely resemble those of stem group crustaceans, although they lack the proximal endite, which defines that group.

They are likely the sister taxon to the crustacean stem lineage, and, as such, part of the clade Crustaceomorpha.

Other researchers have suggested, based on a cladistic analyses of dorsal exoskeletal features, that Eodiscina and Agnostida are closely united, and that the Eodiscina descended from the trilobite order Ptychopariida.