Memoir 9 — The Phragmocone of Ecdyceras

By Rousseau H. Flower, 1961, 29 pp., 5 figs., 4 plates.

Ecdyceras proves to have a rather long phragmocone, at the base of which a rather flat, thick septum suggests loss of earlier growth stages by truncation. This shallow basal septum is followed abruptly by very deeply curved septa, their edges extended far forward. Episeptal deposits, thin centrally and thick peripherally, result in a peripheral zone in which the shell material was essentially solid and an axial zone in which cup-shaped cavities remain in the camerae; with growth, the axial zone widens adorally at the expense of the peripheral zone. The siphuncle, beyond a basal arcuate stage, is tubular and thick-walled, with diaphragms, later becoming thin-walled and showing slightly fusiform segments. Septa are thin, the necks ambiguous from the present material, but evidently vestigial.

Chazyan material studied is indecisive at the specific level, but indicates that the extended septal margins of young stages are responsible for the apparent thickening of the base of the living chamber on the basis of which Pachecdyceras was distinguished. A new species, E. expansum is described from the Viola Limestone of Oklahoma.

Ecdyceras is remote in structure from other cephalopods, requiring its separation into a family and an order by itself. Origin is obscure, but only derivation from the Ellesmerocertaida seems possible.