Riocerida

Riocerida, originally named as Rioceratida, 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. However, they also possess several plesiomorphic 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 shape is slender, usually orthoconic or weakly cyrtoconic. 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.