Ellesmerocerida
Ellesmerocerida are the ancestral nautiloid stock which comprise all Late Cambrian and lowest of the Ordovician nautiloids from which the other orders evolved. Ellesmerocerids continued through the Early Ordovician but became an increasingly minor part of cephalopod faunas during the rest of the period. Ellesmerocerid families evolved from the ancestral Plectronoceratidae, one of two families found in the Upper Cambrian. (See Ectenolites  further on)
The Ellesmerocerida have been divided into three sub orders, the Plectronoceratina comprising the straight to endogastric Plectronoceratidae and the exogastric Balkoceratidae, the Ellesmeroceratina which comprise the bulk of the order, and the Cyrocerinina which have specially developed internal structures in the siphuncle.
Plectronoceratina are discussed under Plectronocerida.
The principal group, the Ellesmeroceratina have ventral siphuncles with thick, well calcified and layered connecting rings ,especially in primitive forms in which diaphragms are rather common. The predominant Ellesmeroceratidae, consists of laterally compressed ,straight or endogastric shells with short phragmocones and rapidly expanding apices. Some were compressed, others more circular in cross section. Some were straight while others were cyrtoconic. Species of Ectenolites, a sort of lean Ellesmeroceras, are found in the upper most Cambrian. Most are confined to the Gasconadian (Tremadocian) at the beginning of the Ordovician. Pachendoceras of the lower Canadian,likely gave rise to Proendoceras, or a similar form in the Endocerida through the loss of diaphragms and development of endocones instead.
One group, the Bassleroceratidae, developed an exogastric curvature, opposite from normal for the Order. The Bassleroceratidae gave rise to the Tarphycerida and are sometimes included in that Order. In another group, the Baltoceratidae, the connecting rings became thin while the siphuncle, freed of diaphragms became more central in position, giving rise to the Orthocerida (Michelinocerida).
The Cytocerinina unite three very different genera, representing three different families, in which the connecting ring in extended as a lobe or process into the siphuncle cavity. The first, Eothinoceras (Eothinoceratidae) of the Middle Canadian is slender and faintly exogastric, with a ventral siphuncle with vestigial necks and connecting rings that project inward as triangular lobes toward the middle of the respective segment. The second, Bathmoceras,(Bathmoceratidae), late Lower Ord. (late Upper Canadian) to late Middle Ordovician (Chazyan), contains rather large, slightly exogastric and slightly depressed shells, with a large ventral siphuncle with rather long septal necks in which the rings are thickened as lobes that project forward for as much a the length of two or three segments. The Bathmoceratidae is the most likely ancestor of the Actincocerida.  The third, Cyrtocerina, (Cyrtocerinidae), Upper Ordovician, from which the suborder derives its name, contains generally small, rapidly expanding, compressed, endogastric brevicones with a ventral siphuncle with short necks and connecting rings that thicken as inward pointing projections.
While certain families of Ellesmerocerida continued on to the end the Ordovician, the order pretty much ceased to dominate cephalopod faunas after the Gasconadian (early Canadian), having given rise to the Endocerida and Tarphycerida in the middle Canadian and to the Michelinocerida, just before, and the Actinocerida in the Whiterockian (early Middle Ord.). Through the more primitive Plectronoceratidae, the Ellesmerocerida also gave rise to the internally unique, Discosorida.