Riocerida

Riocerida
Temporal range: Tremadocian–Katian
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Orthoceratoidea
Order: Riocerida
King & Evans, 2019
Families

Riocerida,[1] originally named as Rioceratida,[2] 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).[1][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).[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.[1]

  1. ^ a b c Pohle, Alexander; Kröger, Björn; Warnock, Rachel C. M.; King, Andy H.; Evans, David H.; Aubrechtová, Martina; Cichowolski, Marcela; Fang, Xiang; Klug, Christian (2022-04-14). "Early cephalopod evolution clarified through Bayesian phylogenetic inference". BMC Biology. 20 (1): 88. doi:10.1186/s12915-022-01284-5. ISSN 1741-7007. PMC 9008929. PMID 35421982.
  2. ^ a b c King, Andy H.; Evans, David H. (2019). "High-level classification of the nautiloid cephalopods: a proposal for the revision of the Treatise Part K". Swiss Journal of Palaeontology. 138 (1): 65–85. Bibcode:2019SwJP..138...65K. doi:10.1007/s13358-019-00186-4. ISSN 1664-2384. S2CID 133647555.