Cyrtocerinida is an order of

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...

cephalopods. The order includes the families

Cyrtocerinidae Cyrtocerinidae is a family of nautiloid cephalopods in the Order Cyrtocerinida Cyrtocerinida is an order of Ordovician nautiloid cephalopods. The order includes the families Cyrtocerinidae and Eothinoceratidae, as well as the genera ''Bathmocer ...

and Eothinoceratidae, as well as the genera '' Bathmoceras'' and '' Rummoceras''. Cyrtocerinids can be characterized by a broad siphuncle encased by very thick connecting rings with concave outer surfaces. Despite their thickness, the connecting rings are nautilosiphonate, meaning that their inner layer is a poorly-mineralized organic sheath (similar to modern nautiluses) rather than a porous calcified structure. There are various calcified endosiphuncular deposits such as collars and longitudinal ridges on the inner layer of the connecting ring. These deposits are discontinuous, reforming after each chamber. This differentiates them from the continuous endosiphuncular ridges found in other nautiloids, such as the related oncocerids. The overall shell shape is typically orthoconic (straight, pointed) or breviconic (stout). The body chamber is oncomyarian, meaning that it is ringed by numerous small, undifferentiated muscle scars. Cyrtocerinida was previously known under the name Cyrtocerinina, which was considered a suborder of the order

Ellesmerocerida 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. Morphology The Ellesmerocerida are characterized by shells that ...

. Current studies generally consider Cyrtocerinida to be the earliest-branching monophyletic clade within the subclass (or superorder)

Multiceratoidea Multiceratoidea is a major subclass or superorder of Paleozoic nautiloid cephalopods. Members of this group can be characterized by nautilosiphonate connecting rings, with an organic inner layer and outer layer of calcitic spherules and blades, ...

. This is justified by the combination of oncomyarian muscle scars, nautilosiphonate connecting rings, and endosiphuncular deposits.

References

{{reflist Prehistoric nautiloids Prehistoric cephalopod orders Middle Ordovician first appearances