{{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 cephalopods of the family Baltoceratidae that lived in what would be North America during the Early Ordovician from 480—472 mya, 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'' 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") or gently curved. In life, these animals may have been similar to the modern squid, except for the long shell.
==References== {{Reflist}}
{{Portal|Paleontology}} {{Taxonbar|from=Q7335393}}
Category:Prehistoric nautiloid genera Category:Ordovician cephalopods of North America