Middle Ordovician – Late Carboniferous (Pennsylvanian)
Actinocerida is an extinct order of nektobenthic carnivores
Characteristics of the Order
- Straight and slender conchs
- Apertures are unmodified in shape by constrictions and are entire
- Body chambers are largest in diameter in the middle
Treatise on Invertebrate Paleontology, Part K, Vol. 3 (1964):
- Characters of subclass: Includes nautiloids having medium-sized to large conchs which generally are straight and moderately slender; some are slightly curved. Body chambers tend to become contracted in the position of the aperture, reaching greatest diameter in the middle, but the apertures are uniformly entire and unmodified in shape by constrictions. The most diagonistic characters of the actinoceratoids are found in structures of the siphuncle, which are described and illustrated in the subsequent chapter devoted to this subclass and order. A systematic diagnosis of the assemblage is introduced here in company with those just given for Nautiloidea and Endoceratoidea.
- Medium-sized to very large, generally straight conchs, but weakly cyrtoconic in some genera. Siphuncle with cyrtochoanitic to recumbent septal necks, with segments inflated between tips of septal necks and preceding septal foramina, segments typically wider than long but in some forms longer than wide or subspherical; large siphuncles generally with broad contact of segments with ventral wall of shell, small siphuncles mostly subventral to subcentral; annular deposits within siphuncle, at adult growth stage filling all of it except central canal, radial canals, and perispatia. Episeptal and hyposeptal types of cameral deposits in many genera, circulus in some.
Flower and Kummel (1950):
- Shell dominantly straight, siphuncle broadly expanded, usually large, occupied by annular deposits which may fill the entire siphonal cavity except for a vascular system of radial and central canals terminating in a thin cavity against the connecting ring, the perispatium. The classification is essentially that of Teichert (1933) with the removal of a few genera to the Discosoroidea and a few subsequently described forms.