{{Short description|Extinct order of nautiloids}} {{Automatic taxobox | taxon= Cyrtocerinida | fossil_range= {{Fossil range|Ordovician}} | image = | image_caption = | authority = Flower, 1964 | subdivision_ranks = Subgroups | subdivision = *Cyrtocerinidae *Eothinoceratidae *''Bathmoceras'' *''Rummoceras'' }}
'''Cyrtocerinida''' is an order of Ordovician nautiloid cephalopods.<ref name=":0">{{Cite journal |last=Mutvei |first=Harry |date=2015-07-03 |title=Characterization of two new superorders Nautilosiphonata and Calciosiphonata and a new order Cyrtocerinida of the subclass Nautiloidea; siphuncular structure in the Ordovician nautiloid ''Bathmoceras'' (Cephalopoda) |url=http://www.tandfonline.com/doi/full/10.1080/11035897.2015.1061592 |journal=GFF |language=en |volume=137 |issue=3 |pages=164–174 |doi=10.1080/11035897.2015.1061592 |s2cid=41162554 |issn=1103-5897}}</ref> The order includes the families Cyrtocerinidae and Eothinoceratidae, as well as the genera ''Bathmoceras'' and ''Rummoceras''.<ref name=":1">{{Cite journal |last1=Pohle |first1=Alexander |last2=Kröger |first2=Björn |last3=Warnock |first3=Rachel C. M. |last4=King |first4=Andy H. |last5=Evans |first5=David H. |last6=Aubrechtová |first6=Martina |last7=Cichowolski |first7=Marcela |last8=Fang |first8=Xiang |last9=Klug |first9=Christian |date=2022-04-14 |title=Early cephalopod evolution clarified through Bayesian phylogenetic inference |journal=BMC Biology |volume=20 |issue=1 |pages=88 |doi=10.1186/s12915-022-01284-5 |pmid=35421982 |pmc=9008929 |s2cid=248158390 |issn=1741-7007|doi-access=free }}</ref>
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.<ref name=":0" /><ref name=":1" /> 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).<ref name=":0" /> The body chamber is oncomyarian, meaning that it is ringed by numerous small, undifferentiated muscle scars.<ref name=":0" /><ref name=":2">{{Cite journal |last1=King |first1=Andy H. |last2=Evans |first2=David H. |date=2019 |title=High-level classification of the nautiloid cephalopods: a proposal for the revision of the Treatise Part K |journal=Swiss Journal of Palaeontology |language=en |volume=138 |issue=1 |pages=65–85 |doi=10.1007/s13358-019-00186-4 |s2cid=133647555 |issn=1664-2384|doi-access=free }}</ref>
Cyrtocerinida was previously known under the name '''Cyrtocerinina''', which was considered a suborder of the order Ellesmerocerida.<ref>{{Cite journal |last=Flower |first=Rousseau H. |date=1964 |title=The nautiloid order Ellesmeroceratida (Cephalopoda) |url=https://geoinfo.nmt.edu/publications/monographs/memoirs/downloads/12/Memoir-12.pdf |journal=New Mexico Bureau of Mines and Mineral Resources Memoir |volume=12 |pages=1–164}}</ref> Current studies generally consider Cyrtocerinida to be the earliest-branching monophyletic clade within the subclass (or superorder) Multiceratoidea. This is justified by the combination of oncomyarian muscle scars, nautilosiphonate connecting rings, and endosiphuncular deposits.<ref name=":2" /><ref name=":1" />
== References == {{reflist}}
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Category:Prehistoric nautiloids Category:Prehistoric cephalopod orders Category:Middle Ordovician first appearances