Solenochilidae Hyatt 1893

Family Solenochilidae Hyatt, 1893 Fig. 39 Diagnosis Family of the superfamily Aipoceratoidea with a subinvolute or involute conch. Whorl profile circular or depressed. Umbilical wall with long, spine-alike outgrows in the adult stage. Included genera Solenochilus Meek & Worthen, 1870 (Tournaisian to hanghsingian; 35 species). Solenocheilus Hyatt, 1884 [synonym of Solenochilus]. Acanthonautilus Foord, 1896 (Tournaisian; 1 species). Remarks The family Solenochilidae contains only two genera, Solenochilus and Acanthonautilus, which, according to Kummel (1964: K441), differ only in the shape of the septal necks. As this character has not been studied in most species, little can be said about the clear delimitation of the two genera. It is therefore uncertain whether the very asymmetric distribution of the species described so far, one from Acanthonautilus and about 35 from Solenochilus, will be confirmed in the necessary revision. Mikesh & Glenister (1966) considered it possible that many of the Early Carboniferous species of the family might be better assigned to Acanthonautilus. However, they also admitted that a detailed study of the species, some of which are poorly known, would be necessary for a definitive clarification. The species of these two genera are characterised by a peculiar conch morphology, the combination of which is not known in any other group of cephalopods. On the one hand, the conch grows very rapidly in width and height, with a circular or depressed oval whorl profile, and on the other hand, the formation of very conspicuous, long outgrowths on the umbilical wall (Fig. 39). Interestingly, very similar umbilical processes also occur in the family Permonautilidae, a genus of the superfamily Liroceratoidea, which is not closely related to Solenochilus. Permonautilus also has a stout conch like Solenochilus, but usually differs from Solenochilus in having a much lower coiling rate (WER ~ 2.60) than Solenochilus (WER ~ 2.80–5.50). The most important difference is the position of the siphuncle, which is almost central in Permonautilus and slightly ventral in Solenochilus.