The Ellesmerocerida is an extinct order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.

Connecting rings which may appear layered are thick and typically wedge shaped with their maximum width at or near where they join the previous septum.

The Ellesmerocerida have been revised to include only primitive nautiloid cephalopods with thick connecting rings and siphuncle segments that are concave in outline.

The Apocrinoceratidae, once included, is now assigned to the Discosorida[12] The Ellesmerocerida are derived from the Plectronocerida,[5][8] having first appeared early in the Trempealeauan Stage of the Late Cambrian and quickly diversifying into four families, only one of which, the Ellesmeroceratidae, continued by means of the genera Ectenolites and Clarkoceras into the Gasconadian in the Lower Ordovician.

[9] The dominant family of Ordovician Ellesmerocerida is the Ellesmeroceriatidae which are distinguished from the generally similar Protocycloceratidae by the presence of broad lateral lobes in the suture.

The Ellesmerocerida gave rise to the Endocerida through Pachendoceras and to the Tarphycerida and Oncocerida through Bassleroceras and is the source for the Orthocerida, Pseudorthocerida, Actinocerida, and Discosorida.