Late Ordovician – Permian
Isorophida is an extinct order of stationary low-level epifaunal suspension feeders
Characteristics of the Order
- Discoidal theca with well-developed peripheral rim
- Imbricate interambulacral plates
- Very narrow ambulacra with hidden uniserial floor plates
- Oral frame formed by five enlarged, proximal ambulacral floorplates
- Theca 10 to 50 mm in diameter – with domal forms 15 to 25 mm the most common
Sumrall & Gahn (2006):
- Characterized, in post-Cambrian taxa, by a discoidal theca bordered by a well-developed peripheral rim, imbricate interambulacral plates, and extremely narrow ambulacra with hidden uniserial floor plates (Bell, 1967)
- Edrioasteroidea with: domal or clavate theca, ambulacra commonly limited to upper oral surface; oral frame formed by five enlarged, proximal ambulacral floorplates, with or without intrathecal extensions from primary orals; hydropore structure in or posterior to the right posterior margin of oral area, opening along junctions between plates, elongate parallel to plate margins; ambulacra formed by biserial or cyclic sets of coverplates, with intra-ambulacral and/or intrathecal extensions, and uniserial floorplates; ambulacral coverplate passageways present in some; margin of oral surface a peripheral rim.
- The thecae of adult Isorophida range from approximately 10 to 50 mm in diameter with domal forms 15 to 25 mm the most common. The oral area is elevated above the interambulacra and forms the central oral rise which is distally continuous with the elevated ambulacral ridges.