Middle Ordovician – Late Ordovician
Isorophus is an extinct genus of stationary epifaunal suspension feeders
Identification in Hand Sample:
- Long and curved ambulacra, one curved clockwise
- Circular outline of theca with a diameter of 8 to 40mm
- Ambulacrum short and broad in most species
- Extra series of ambulacral commonly present between usual pairs of rows
- Anal pyramid composed of 6 to 12 plates
- Like Carneyella and Curvitriordo, Isorophus has relatively long and curved ambulacra, one of which is curved clockwise. Isorophus is distinguished by the presence of ambulacral plates that form double alternating biseries, rather than a single series as in Carneyella or a triple series as in Curvitriordo.
Fossils of Ohio (1996):
- Known from Middle and Upper Ordovician strata in Ohio and adjacent states.
- Isorophina with: domal theca; oral area including four primary orals, two pairs of laterally shared coverplates, one large hydropore oral, and secondary orals; hydropore structure having few plates, integrated with central oral rise; ambulacral coverplates forming alternating multiple biseries; interambulacrals squamose and imbricate.
Treatise on Invertebrate Paleontology, Part U, Echinodermata 3 Vol. 1 (1966):
- Theca circular in outline, diameter 8 to 40 mm., attached by entire aboral surface to Rafinesquina and other shells, disc-shaped to hemispherical, usually bordered by wide peripheral ring; interambulacrals scale-like, imbricating or nearly forming mosaic; ambulacra 5, almost straight to strongly curved, ambulacrum C solar, others contrasolar, relatively short and broad in most species; extra series of ambulacral cover plates commonly present between usual pair of rows; anal pyramid well defined, composed of 6 to 12 plates, in some specimens surrounded by zone of small plates, in species with curved rays periproct encircled by ambulacra C and D (4, 26, 30).