# Rioceras

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{{Short description|Extinct genus of molluscs}}
{{Automatic taxobox
| taxon = Rioceras
| fossil_range = Lower Ordovician
| image = 
| image_caption = 
| authority= Flower (1964)
}}

'''''Rioceras''''' is an extinct genus of [orthocerid](/source/Orthocerida) cephalopods of the family [Baltoceratidae](/source/Baltoceratidae) that lived in what would be North America during the Early [Ordovician](/source/Ordovician) from 480—472 [mya](/source/Annum), existing for approximately 8 million years.<ref>[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=12415 PaleoBiology Database: ''Rioceras'', basic info]</ref><ref name = Flower>R. H. Flower. 1964. The Nautiloid Order Ellesmeroceratida (Cephalopoda). State Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, Memoir 12:234</ref>

==Taxonomy==
''Rioceras'' was named by Flower (1964). Its type is ''Rioceras nondescriptum''. It was assigned to ''[Baltoceratidae](/source/Baltoceratidae)'' by Flower (1964) and Hook and Flower (1977),<ref name = Flower/><ref>S. C. Hook and R. H. Flower. 1977. Late Canadian (Zones J, K) cephalopod faunas from Southwestern United States. New Mexico Bureau of Mines and Mineral Resources 32:1-56</ref> but reassigned to the Ellesmeroceratidae by Kröger et al. (2007)<ref name = Kroger>B. Kröger, M. S. Beresi, and E. Landing. 2007. Early orthoceratoid cephalopods from the Argentine Precordillera (Lower-Middle Ordovician). Journal of Paleontology 81(6):1266-1283</ref>
 
Whether Rioceras is assigned to the Baltoceratidae and therefore to the Orthocerida or to the Ellesmeroceratidae depends on the definition of each. Flower<ref name = Flower/> placed Rioceras in the Baltoceratidae when he proposed the genus since it lacks diaphragms in the siphuncle which for him was a defining character of the family. Kröger<ref name = Kroger/> removed Rioceras to the Ellesmeroceratidae on the basis of its inwardly thicked connecting rings which produce concave segments, a defining character of that family as emended.  Flower did point out in his original diagnosis<ref name = Flower/> that Rioceras has moderately thick connecting rings "of ellesmeroceratoid aspect"

==Morphology==
The shell is usually long, and may be straight ("[orthoconic](/source/Orthocone)") or gently curved.  In life, these animals may have been similar to the modern [squid](/source/squid), except for the long shell.

==References==
{{Reflist}}

{{Portal|Paleontology}}
{{Taxonbar|from=Q7335393}}

Category:Prehistoric nautiloid genera
Category:Ordovician cephalopods of North America

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