Ellesmerocerida
| Ellesmerocerida Temporal range: Upper Cambrian–Sandbian |
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| Life positions of Ellesmerocerida | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Superorder: | †Plectronoceratoidea |
| Order: |
†Ellesmerocerida Flower, 1950 |
| Subgroups | |
The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.
The Ellesmerocerida are characterized by shells that are typically small, some even tiny, with close-spaced septa and relatively large ventral siphuncles. In some genera (e.g. Palaeoceras), the septa are uniformly spaced. Shells of ellesmerocerids are typically smooth and compressed and vary in form. They may be breviconic (short) or longiconic (elongate), straight (orthoconic) or curved (cyrtoconic). Cyrtoconic forms are usually endogastric, with longitudinally convex ventral margins. The apeces of straight forms typically have an endogastric curvature. Some may have grown to as much as 15 cm.
Siphuncle segments are tubular or concave. Septal necks are short. 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 siphuncle interior is commonly crossed by irregular partitions, known as diaphragms, but are otherwise free of internal deposits
As soft parts are not prone to fossilization, little can be surmised as to their soft part anatomy. Preserved muscle attachment scars indicate that they may have had segmented muscles reminiscent of primitive monoplacophoran molluscs. As for arms or tentacles, little can be said except that eight or ten, retained in modern coleoids, seems to be the primitive or ancestral number.
Rousseau Flower defined the Ellesmerocerida as containing all archaic, ancestral cephalopods and established three suborders within: the Plectronoceratina, Ellesmeroceratina, and Cyrtocerinina. Furnish and Glenister, in the Treatise on Invertebrate Paleontology, Part K, essentially followed suit with minor differences at the family level. Mary Wade (1988) included the Ellesmerocerida in the superorder Plectonoceratoidea, which she defined as containing the Plectronocerida, Ellesmerocerida and two orders introduced by Chen and Teichert in 1983, the Cambrian Yanhecerida and Protactinocerida. The Plectronocerida, also Cambrian, includes forms once included in the suborder Plectronoceratina, now elevated in rank.
The Ellesmerocerida have been revised to include only primitive nautiloid cephalopods with thick connecting rings and siphuncle segments that are concave in outline. Accordingly, the order includes the Ellesmeroceratidae, , Cyclostomiceratidae, Bassleroceratidae, Eothinoceratidae, Bathmoceratidae, and . The Ellesmeroceratidae, Protocycloceratidae, Cyclostomiceratidae, Bassleroceratidae are found in Flower's basic Ellesmeroceratina. The Eothinoceratidae, Bathmoceratidae, and Cyrtocerinidae are combined in the Cyrtocerinina. The Schideleroceratidae, Apocrinoceratidae, Baltoceratidae and certain members of the Protocycloceratidae, all which have thin tubular or expanded siphuncles, are now excluded. The Apocrinoceratidae, once included, is now assigned to the Discosorida
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