Revision of Eothinoceras and the status of the Eothinoceratidae (Cyrtocerinida, Multiceratoidea, Cephalopoda)

Abstract. The type specimens of Eothinoceras americanum Ulrich et al., 1944, type species of the genus Eothinoceras Ulrich et al., 1944, are revised based on new photographic material. The resulting interpretation of the conch shape of Eothinoceras shows that the type species is an endogastric cyrtocone, necessitating the restriction of Eothinoceras to the type species and requiring the revision of the family Eothinoceratidae and the order Cyrtocerinida. A new scheme for classifying the genera of the order Cyrtocerinida into its three families is proposed: (1) Eothinoceratidae Ulrich et al., 1944, containing the genera Eothinoceras; Protothinoceras Chen and Teichert, 1987; Mesothinoceras Chen and Teichert, 1987; and Conothinoceras Chen and Teichert, 1987; (2) Cyrtocerinidae Flower, 1946, including Cyrtocerina Billings, 1865; Tangshanoceras Chen, 1976; and Centrocyrtocerina Stait, 1983; (3) Bathmoceratidae Gill, 1871, containing Bathmoceras Barrande, 1865; Saloceras Evans, 2005; Sacerdosoceras Evans, 2005; Margaritoceras Cecioni and Flower, 1985; and Mutveiceras Cichowolski et al., 2014.