This family was revised in admirable detail by Kier (1967). It has the primitive form of perignathic girdle, like that of juvenile cassidulids and distinct from that of Clypeasterina, where the auricles are entirely ambulacral in origin. The ambulacral structure beneath the petals is unique in that the demiplates do not extend through to the interior of the test but rest in shallow pits in the outer surface of the primary plates. Often, in weathered specimens these demiplates are lost and the adradial pits are clearly evident. This arrangement means that the primary plates are pierced by two pores, one leading to their own tube-foot and the other to the tube-foot of the superficial demiplate. Thus, like clypeasteroids, oligopygids have more than one pore/tube-foot to each ambulacral plate.
Cladistic analysis identifies Oligopygidae as advanced stem group Clypeasteroida (Smith 2001).
Kier, P. M. 1967. Revision of the oligopygoid echinoids. Smithsonian Miscellaneous Collections, 152 (2), 149 pp., 36 pls.
Smith, A. B. 2001. Optimizing phylogenetic analysis by the inclusion of fossils: Cassiduloid paraphyly and the origin of clypeasteroid echinoids. Paleobiology 27(1), 392-404.