The Ellesmeroceratidae constitute a family within the

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

order

Ellesmerocerida 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. Morphology The Ellesmerocerida are characterized by shell ...

. They lived from the Upper Cambrian to the

Lower Ordovician The Early Ordovician is the first epoch of the Ordovician period, corresponding to the Lower Ordovician series of the Ordovician system. It began after the Age 10 of the Furongian epoch of the Cambrian and lasted from to million years ago, unt ...

. They are characterized by straight and endogastric shells, often laterally compressed, so the dorso-ventral dimension is slightly greater than the lateral, with close spaced sutures having shallow lateral lobes and a generally large tubular ventro-marginal

siphuncle The siphuncle is a strand of biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

with concave segments and irregularly spaced diaphragms. Connecting rings are thick and layered, externally straight but thickening inwardly with the maximum near the middle of the segment so as to leave concave depressions on internal siphuncle molds. Septal necks are typically orthochoanitic but vary in length from almost absent (achoanitic) to reaching halfway to the previous septum (hemichoanitic) and may even slope inwardly (loxochoanitic).

Evolution and phylogeny

The Ellesmeroceratidae have their derivation in the

Plectronoceratidae Plectronocerida is a primitive order from which subsequent Cephalopoda, cephalopod orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic ...

, order Plectronocerida, in Trempealeauan stage of the Late Cambrian from which time 13 genera have been described. The earliest described, assigned to the Ellesmeroceratidae, is the early Trempealeauan ''Hunuanoceras'', which comes from the lower part of the upper Yenchou Member of the Fengshan Formation in China. ''Hunuanoceras'' is a small endogastric cyrtocone resembling the ancestral ''Plectronoceras'' except for having resistant calcified connecting rings. ''Hunuanoceras'' is followed by ''Eburoceras'', which first appears in the upper part of the upper Yenchou and continues throughout the overlying Wanwankou Member of the Fengshan. The Wanwankou is middle and early upper Trempealeauan. The remaining eleven genera are restricted to the Wanwankou, except for '' Clarkoceras'' and '' Ectenolites'', which persist into the Lower Ordovician. ''Clarkoceras'' and ''Ectenolites'' provide the ancestry for the diverse ellesmerceratitds of the Early Ordovician, Gasconadian, and those that followed. The Gasconadian was dominated to virtual exclusion by the Ellesmeroceratidae, which diversified during that time into a variety of forms and genera. Some like ''

Ellesmeroceras ''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 sedime ...

'' and ''Eremoceras'' were straight shelled, following the example of ''Ectenolites''. Others like ''Dakeoceras'' and ''Burenoceras'' were endogastric in the sense of ''Clarkoceras''. Gradations are found between elongate (longiconic) and short (breviconic) forms and between straight (orthoconic) and curved (cyrtoconic) forms, and between those with simple open apertures and those with apertures that have contracted. The Ellesmeroceratidae gave rise within the Ellesmerocerida to the Protocycloceratidae, Bassleroceratidae, and possibly the Cylostomiceratidae in the early Canadian (late Gasconadian (?), Demingian) and to the Bathmoceratidae, and Cyrtocerinidae in the late Canadian (late Jeffersonian or Cassinian). The Ellesmeroceratidae also gave rise at about the close of the Gasconadian to the

Endocerida Endocerida, from Ancient Greek ἔνδον (''éndon''), meaning "inside", and κέρας (''kéras''), meaning "horn", is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. End ...

Tarphycerida The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician (middle and upper Canad) to the Middle Devonian. Some, such as '' Aphetoceras'' and '' Estonioceras'', are loosely coiled and gyroconi ...

, and to the

Orthocerida Orthocerida, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", also known as the Michelinocerida, is an order of extinct Orthoceratoidea, orthoceratoid cephalopods that lived from the ...

through the ancestral

Baltoceratidae Baltoceratidae is an extinct family of orthoconic cephalopods belonging to the subclass Nautiloidea endemic to what would be Asia, Australia, Europe, North America, and South America during the Ordovician living from about 480–460 mya, exist ...

, at which time they cease to be the dominant element in cephalopod faunas.

References

{{Taxonbar, from=Q5365269 Prehistoric nautiloid families Cambrian first appearances Early Ordovician extinctions Ellesmerocerida