Early Cambrian – Early Carboniferous (Touranisian)
Craniopsida is an extinct order of stationary epifaunal suspension feeders
Characteristics of the Order
- Biconvex shell which is elongated oval to circular
- The visceral area of both of the valves extends anteriorly toward the midvalve
- Dorsal oblique muscle scars are situated posterolaterally
- Impunctate, calcitic shell
Treatise on Invertebrate Paleontology, Part H (2) (2000):
- Shell biconvex, elongate oval to subcircular; visceral area of both valves extending anterior to midvalve; scars of dorsal oblique internal muscles situated posterolaterally; cemented at apical region of ventral valve, or free lying; shell calcitic, impunctate.
- Simple morphology and taxa within the order can be defined only on characters such as the presence or absence of pseudointerareas, visceral platforms, median ridges, position of the umbo, and presence of attachement scar (cicatrix). The shell is usually equally biconvex, with the umbo situated marginally or posteriorly. Only some craniopsids have well-developed pseudointerareas.