Discosorida

Systematic Paleontology
 * Phylum Mollusca
 * Class Cephalopoda
 * Subclass Nautiloidea
 * Order  Discosorida

Discosorida is an order of nautiloid cephalopods from the lower Paleozoic with a unique siphuncle structure. Shells (conchs) are generally short, breviconic, either straight or curved, and may be medium to large in size. Primitively their shells are endogastric, the under or ventral size concave, tucked in, in longitudinal section. Derived exogastric forms, with the ventral side convex, outwardly arched, as well as those that are straight are included. Apertures are either open or constricted.

The siphuncle in most discosorids is longitudinally zoned as opposed to or in addition to being concentrically layered as in the Ellesmerocerida or Endocerida. Septal necks are generally short, strongly recumbant, folded against the under or apical side of the relevant septa and are overlain by connecting rings than form gromet-like structures, bullettes and which fassen to the under or backside of the subsequent septum, next to the septal neck. Typically, siphuncle segments expand into the adjacent chambers.

Overall, nine families are included, forming three evolutionary lineages; the ancestral Reudemannoceratidae along with the Cyrtogomphoceratidae and Westonoceratidae from the Ordovician and the Discosoridae, Lowoceratidae, Greenlandoceratidae, Phragmoceratidae, Mandoloceratidae and Mesoceratidae from the Silurian. The Discosoridae continue into the Devonian. The Reudemannoceratidae may have its origin the the Plectronoceratidae of the Ellesmerocerida. The three lineages are those of the Cyrtogomphoceratidae in which forms are endogastric, of the Westonoceratidae in which forms are exoastric, and of the Mandaloceratidae in which they are generally straight.

References
 * Curt Teichert, 1964. Nautiloidea -Discosorida. Treatise on Invertebrate Paleontolgy Part K

pp K320-K342