Riocerida

{{Automatic taxobox | fossil_range = {{Fossil range|Tremadocian|Katian}} | image = | image_caption = | taxon = Riocerida | authority = King & Evans, 2019 | subdivision_ranks = Families | subdivision = *Bactroceratidae *Rioceratidae }}

'''Riocerida''',<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 |issn=1741-7007 |doi-access=free }}</ref> originally named as '''Rioceratida''',<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 |bibcode=2019SwJP..138...65K }}</ref> is an extinct order of Ordovician nautiloid cephalopods. They were the earliest-diverging members of the major group Orthoceratoidea, with which they share dorsomyarian muscle scars (a few large muscle scars concentrated at the top of the body chamber).<ref name=":1" /><ref name=":2" /> However, they also possess several plesiomorphic ("primitive") traits which are absent in other orthoceratoids: Cameral and endosiphuncular deposits are absent, and the siphuncle has a marginal position along the lower edge of the shell. The conch (external shell) shape is slender, usually orthoconic (straight) or weakly cyrtoconic (slightly curved).<ref name=":2" /> The monophyly of Riocerida as a unique group is plausible but difficult to confirm, and they may instead be a paraphyletic grade or polyphyletic assemblage ancestral to all later orthoceratoids.<ref name=":1" />

== References == {{Reflist}}

Category:Prehistoric nautiloids Category:Prehistoric cephalopod orders Category:Orthoceratoidea Category:Early Ordovician first appearances Category:Late Ordovician extinctions