Ephippioceratidae Miller & Youngquist, 1949

Family Ephippioceratidae Miller & Youngquist, 1949 View in CoL

Fig. 36 View Fig

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 View Fig ). 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.