Multiceratoidea is a major subclass or superorder of

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

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 species ...

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

. Members of this group can be characterized by nautilosiphonate connecting rings, with an organic inner layer and outer layer of calcitic spherules and blades, similar to the modern

nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in t ...

. The earliest-diverging multiceratoids have oncomyarian muscle scars (with numerous small muscle attachments ringing the

body chamber The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk. In gastropods In gastropods, the ...

), though several orders trend towards a ventromyarian condition (with muscle scar area concentrated at the bottom of the body chamber). Multiceratoid shells are generally short and curled, with a relatively small

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

(opening). Cameral deposits are never found among the multiceratoids, though several orders are known to bear endosiphuncular deposits within their

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

. When originally defined in 2013, Multiceratoidea included four nautiloid

orders Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

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 ...

Oncocerida The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous; one possible member is known from the Early Permian), in which the connecting rings are t ...

Discosorida Discosorida are an order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through an ...

, and

Ascocerida The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longiconic juvenile portion and an inflate ...

. The order

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 ...

was considered a potential member of the subclass, though their larger body chamber and specialized muscle attachments lent uncertainty to this idea. A later analysis added the early-diverging order

Cyrtocerinida Cyrtocerinida is an order of Ordovician nautiloid cephalopods. The order includes the families Cyrtocerinidae and Eothinoceratidae, as well as the genera '' Bathmoceras'' and '' Rummoceras''. Cyrtocerinids can be characterized by a broad si ...

, which was previously considered a suborder of Ellesmerocerida.

Nautilida The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, ''Nautilus'' and ''Allonauti ...

(the order containing the modern nautilus) was allied with multiceratoids in a broader group termed "Nautilosiphonata", defined by its namesake connecting ring structure. Solely on the basis of morphological traits, Tarphycerida and Nautilida appear to be well-nested within Multiceratoidea. Multiceratoidea would be an equivalent term to Nautiloidea ''sensu stricto'' if nautilids are confirmed to be within the group, though molecular divergence timing disagrees with this assessment. The recently-named order

Bisonocerida Bisonocerida is an order of Ordovician to Silurian nautiloid cephalopods. Members of this order were originally placed in the order Endocerida, but later investigation argued that this broad usage of Endocerida was a polyphyletic assemblage en ...

shares traits with ellesmerocerids and endocerids, and some studies have placed it as a member of Multiceratoidea. A 2022 bayesian phylogenetic analysis supports the existence of Multiceratoidea as a valid

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

(discounting the question of nautilus origins), with a membership that includes Tarphycerida and excludes most ellesmerocerids. This is because Ellesmerocerida is a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group, an assemblage of early nautiloids ancestral to various later groups, more than just Multiceratoidea. The order Oncocerida was found to be paraphyletic and ancestral to ascocerids and discosorids, though it was an unambiguous member of Multiceratoidea regardless. Bisonocerids were excluded from Multiceratoidea and instead allied with Endocerida, similar to older perspectives on their relationships.

References

{{reflist Mollusc subclasses Paleozoic cephalopods Ordovician first appearances