Rioceratidae Kröger & Evans, 2011

Kröger, Björn & Pohle, Alexander, 2021, Early-Middle Ordovician cephalopods from Ny Friesland, Spitsbergen - a pelagic fauna with Laurentian affinities, European Journal of Taxonomy 783 (1), pp. 1-102 : 19

Family Rioceratidae Kröger & Evans, 2011

Diagnosis

Slender, orthocones with essentially simple, straight, transverse sutures and thin marginal, or nearly so siphuncle; sutures may form ventral lobe; septal necks achoanitic through loxochoanitic to nearly orthochoanitic; siphuncular segments concave, with thick connecting rings; endosiphuncular and cameral deposits unknown; diaphragms may be present in some forms (from Kröger & Evans 2011).

Remarks

The Rioceratidae were erected to include Rioceras Flower, 1964 , Felinoceras Kröger & Evans, 2011 , Microbaltoceras Flower, 1964 , and Pachendoceras Ulrich & Foerste, 1936 . Rioceras and Felinoceras , but not Pachendoceras (which is better interpreted as an ellesmerocerid of different affinity, see discussion in Evans & King 2012, and below), are longitudinally faintly curved with the siphuncle at the concave side of the curvature, and are hence slightly endogastrically curved. The direction of the curvature of the conch is – as well as their loxochoanitic septal necks – the rationale behind classifying the newly erected, slightly endogastrically curved, partly annulated longiconic genera of the Olendisletta Member within the Rioceratidae and not within the closely similar Rudolfoceratidae Ulrich, Foerste, Miller & Unklesbay, 1944 (see also below). Kröger & Landing (2009) revived the Rudolfoceratidae for a group of annulated, breviconic to longiconic ellesmerocerids with slightly exogastrically curved conchs, with thin connecting rings with concave segments (see also Evans 2011). The group with high probability comprises a paraphyletic set of genera which can be phylogenetically positioned basal to the Orthocerida and Dissidocerida Zhuravleva, 1964 (see below).The paraphyletic grouping is also evident by the presence of a spherical protoconch in Ethanoceras gen. nov. and a cup-shaped apex in Svalbardoceras gen. nov. (see below). Further studies and new finds with early ontogenies preserved are needed in order to better resolve the phylogenetic relationships of the genera which are now placed within the Rioceratidae .