Ordovician (Llandielo) – Upper Devonian (Frasnian).
Atrypida is an extinct order of stationary, epifaunal suspension feeders.
Characteristics of the Order
- Impunctate, rarely fenestratev shell wall
- Biconvex or dorsibiconvex profile, usually rounded and astrophic
- Apical to transapical foramen
- Ventral adductors small with rounded dorsal adductors
- Small, bushy cardinal process
- Crura on inner socket ridges, laterally to dorsovartically directed
- Weak median septum
Treatise on Invertebrate Paleontology, Part H, Vol. 3 (2000):
- Shell wall impunctate, rarely fenestrate; macroornamentation ranging from smooth to ribbed to divaricate, lamellose, spinose, or fenestrate; microornamentation of primary layer usually absent, or with filae, capillae. Shell usually rounded, biconvex to dorsibiconvex, usually astrophic, less commonly flat, strophic; ventral valve rarely partly or wholly cemented; small, flat or rounded ventral area; foramen apical to transpical, dividing 2 triangular deltidial plates; pedicle callist or collar common, perhaps fused with overlying deltidial plates; ventral diductor muscle field large, usually incised, less commonly raised; ventral adductors small; dorsal adductors rounded, separated by weak median septum; small, bushy cardinal process normally located in cardinal pit; weak to prominent ventral vasculae branched around muscle field, saccate to lemniscate, weaker on dorsal valve; faint to strong gonadal pits flanking all muscle fields; crura arising from inner socket ridges, laterally to dorsovertically directed, merging into primary lamellae at sides of shell, may be feathered; medially to dorsally directed, conical spiralia, arising laterally, connected by simple jugum or jugal process, rarely ajugate.