Riocerida, originally named as Rioceratida, is an extinct 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. They were the earliest-diverging members of the major group

Orthoceratoidea Orthoceratoidea is a major subclass of nautiloid cephalopods. Members of this subclass usually have orthoconic (straight) to slightly cyrtoconic (curved) shells, and central to subcentral siphuncles which may bear internal deposits. Orthocerato ...

, with which they share dorsomyarian muscle scars (a few large muscle scars concentrated at the top of the body chamber). However, they also possess several plesiomorphic ("primitive") traits which are absent in other orthoceratoids:

Cameral Cameralism (German: ''Kameralismus'') was a German science of public administration in the 18th and early 19th centuries that aimed at strong management of a centralized economy for the benefit mainly of the state. The discipline in its most na ...

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 An orthocone is an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named '' Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloids ...

(straight) or weakly cyrtoconic (slightly curved). The

monophyly 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 ...

of Riocerida as a unique group is plausible but difficult to confirm, and they may instead be a paraphyletic

grade Grade most commonly refers to: * Grade (education), a measurement of a student's performance * Grade, the number of the year a student has reached in a given educational stage * Grade (slope), the steepness of a slope Grade or grading may also ref ...

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

assemblage ancestral to all later orthoceratoids.

References

{{reflist Prehistoric nautiloids Prehistoric cephalopod orders Orthoceratoidea Early Ordovician first appearances Late Ordovician extinctions