Temporal range: Lower Ordovician
Ellesmeroceras is the type genus for the Ellesmeroceratidae, a family of primitive nautiloid cephalopods, that is characterized by its small, generally compressed, gradually expanded, orthoconic shell, found in Lower Ordovician marine sediments. The septa are close spaced and the siphuncle is ventral, about 0.2 the diameter of the shell. Septal necks are typically orthochoanitic (short, straight) but may slant inwardly (loxochoanitic) or reach halfway to the previous septum (hemichoanitic). Connecting rings are thick. As common for the Ellesmerocerida, Ellesmeroceras has diaphragms within the siphuncle tube.
Ellesmeroceras is one of three straight shelled Ellesmeroceratids, the other two being Ectenolites and Eremoceras. It differs from Ectenolites, from which it is probably derived, in being stouter and proportionally wider, and from Eremoceras in being more straight overall.
- Rousseu H Flower, (1964). "The Nautiloid Order Ellesmeroceratida (Cephalopoda)" New Mexico Bureau of Mines and Mineral Resources Memoir 12.
- W.M Furnish & Brian F. Glenister (1964). Nautiloidea - Ellesmerocerida. Treatise on Invertebrate Paleontology, Part L. (Nautiloidea). Geological Society of America and Univ. Kansas Press.