Family Ephippioceratidae Miller & Youngquist, 1949

Fig. 36

Diagnosis

Family of the superfamily Ephippioceratoidea with a pachyconic or globular, involute to subinvolute conch. Whorl profile in the adult stage usually more or less strongly depressed; flanks and venter form a continuous arch. Ornament usually consisting of fine growth lines; some species have spiral lines or fine ribs. Septum strikingly bilobate; suture line with high external saddle.

Included genera

Ephippioceras Hyatt, 1894 (Viséan to Roadian; 16 species).

Megaglossoceras Miller, Dunbar & Condra, 1933 (Bashkirian to Asselian; 9 species).

Arthuroceras Shimansky, 1962 (Bashkirian; 1 species).

Remarks

The composition of the family Ephippioceratidae proposed here agrees with that outlined by Shimansky (1962) and Kummel (1964), while Dzik (1984) included the genera Ephippioceras and Megaglossoceras, together with Styrionautilus and others, in the family Liroceratidae.

The members of the family Ephippioceratidae cannot be confused with other nautiloids if the shape of the septa is preserved. Ephippioceras and Megaglossoceras are characterised by a very conspicuous bilobate septal surface, the peculiar shape of which is produced by a high ventrodorsal ridge dividing the entire septum (Fig. 36). There are apparently no known species that could be considered as intermediates between Bistrialites or Liroceras and Ephippioceras.

Dzik (1984: 169) discussed the origin of Ephippioceras and suggested Stearoceras as a possible ancestor. The reason for this suggestion was that the suture line of Stearoceras has a ventral undulation, which may have developed into the conspicuous external saddle. According to Dzik (1984), Ephippioceras gave rise to Megaglossoceras, which is the ancestor of the Triassic genus Styrionautilus. This assumption is based on the superficially similar sutures with a ventral saddle. However, it overlooks the fact that the ventral saddle in Ephippioceras and Megaglossoceras was produced by the bilobate deformation of the entire septum by a high ridge, which is not present in Clydonautilus Mojsisovics, 1882 and related genera. Therefore, a phylogenetic lineage from the Liroceratidae to the Clydonautilidae is preferred here.