ambulacra petaloid adapically, beyond petals ambulacra narrower than interambulacra;
ambulacra beneath petals composed of primary elements alternating with demiplates; demiplates superficial and lying in shallow depressions adjacent to the adradial suture. No accessory pores;
pores single beneath petals;
no expansion of pores towards peristome;
periproct marginal to oral;
lantern present in adults with broad, lamellate exterior wings, teeth wedge-shaped in cross-section;
perignathic girdle composed of paired, slender auricles that are of mixed ambulacral and interambulacral in origin;
tubercles crenulate, perforate.
Middle to Late Eocene of southeastern USA, Mexico, Caribbean, South America and western Africa.
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. Paleobiology27(1), 392-404.