Riocerida
Extinct order of nautiloids
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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]
| Riocerida Temporal range: | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | †Orthoceratoidea |
| Order: | †Riocerida King & Evans, 2019 |
| Families | |
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