Pachendoceras is a relatively large , , (Tremedoc) in age, which is thought to be the progenitor of the
Named by Ulrich and Foerste in 1936, Pachendoceras produced a large, almost tubular orthoconic shell with a slightly depressed subcircular section, straight sutures, and large ventral siphuncle. The siphincle which is composed of tubular or faintly concave segments, contains elongate, assymetric diaphragms that tend to be concentrated in the apical portion. In general aspect, Pachendoceras more closely resembles Middle and Upper endocerids than it does the contemporary ellesmeroceratids.
Flower and Kummel(1950)included Pachendoceras in the because of its straight sutures and somewhat depressed section, where it was retained in the Treatise, in spite of its early, Gasconadian, age. Meanwhile Flower in 1964 , referring to its age which predates the later Lower and Middle Ordovician baltoceratids and to its suphunclar diaphragms which are therein absent, removed Pachendoceras to its contemporary in which it shares primary characters.
Pachendoceras gave rise to the Endocerida, so it seems, through suppression of diaphrams and the subsequent developments of cone-shaped ballasting structures in the apical part of the siphuncle, endocones, as found in the otherwise similar early Middle Canadian .
- Flower 1964: The Nautiloid Order Ellesmeroceratida (Cephalopoda), Memoir 12; New Mexico Bureau of Mines and Mineral Resources, Socorro.
- Furnish and Glenister, 1964: Nautiloidea - Ellesmerocerida; Treatise on Invertebrate Paleontology, Part K, Mollusca 3, R.C. Moore (Ed)
- Flower and Kummel 1950: A Classification of the Nautiloidea; Jour Paleontology, 24(5) 604-616, Sept.