Multiceratoidea

Multiceratoidea Temporal range:Possible descendant taxonNautilidasurvives to present
Cyrtoceras,a member of the orderOncocerida
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Superorder:	†Multiceratoidea Mutvei, 2013
Orders
Ellesmerocerida(in part) Cyrtocerinida Tarphycerida Oncocerida(paraphyletic) Ascocerida Discosorida

Multiceratoideais a major subclass or superorder ofPaleozoicnautiloidcephalopods.Members of this group can be characterized by nautilosiphonateconnecting rings,with an organic inner layer and outer layer ofcalciticspherules and blades, similar to the modernnautilus.The earliest-diverging multiceratoids have oncomyarian muscle scars (with numerous small muscle attachments ringing thebody chamber),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 smallaperture(opening).Cameral depositsare never found among the multiceratoids, though several orders are known to bearendosiphuncular depositswithin theirsiphuncles.

When originally defined in 2013, Multiceratoidea included four nautiloidorders:Ellesmerocerida,Oncocerida,Discosorida,andAscocerida.The orderTarphyceridawas 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 orderCyrtocerinida,which was previously considered a suborder of Ellesmerocerida.Nautilida(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 ofmorphologicaltraits, Tarphycerida and Nautilida appear to be well-nested within Multiceratoidea.Multiceratoidea would be an equivalent term to Nautiloideasensu strictoif nautilids are confirmed to be within the group, thoughmolecular divergence timingdisagrees with this assessment.The recently-named orderBisonoceridashares traits with ellesmerocerids andendocerids,and some studies have placed it as a member of Multiceratoidea.

A 2022bayesian phylogenetic analysissupports the existence of Multiceratoidea as a validmonophyleticclade(discounting the question of nautilus origins), with a membership that includes Tarphycerida and excludes most ellesmerocerids. This is because Ellesmerocerida is aparaphyleticgroup, 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.