Subclymeniidae Shimansky, 1962

Family Subclymeniidae Shimansky, 1962

Fig. 16 View Fig

Diagnosis

Family of the superfamily Subclymenioidea with a discoidal, subinvolute to evolute conch; whorl overlap usually very small, coiling rate ranging from moderately high to extremely high. Whorl profile often with a distinct ventrolateral shoulder, a weakly convex, flat or concave venter and a rounded or subangular umbilical margin. Ornament in the early species with spiral lines in the juvenile and preadult stage; advanced species show the reduction of spiral lines. Siphuncle close to the venter. Suture line with a shallow to deep, broadly rounded or V-shaped external lobe that is produced by a ventral inflexion of the septum.

Included genera

Subclymenia d’Orbigny, 1849 (Viséan to Serpukhovian; 7 species).

Aphelaeceras Hyatt, 1884 (Tournaisian to Serpukhovian; 10 species).

Mesochasmoceras Foord, 1900 (Tournaisian; 1 species).

Maccoyoceras Miller, Dunbar & Condra, 1933 (Tournaisian to Viséan; 11 species).

Epidomatoceras Turner, 1954 (Tournaisian to Serpukhovian; 17 species).

Catastroboceras Turner, 1965 (Viséan to Bashkirian; 13 species).

Pseudocatastroboceras Turner, 1965 [synonym of Catastroboceras ].

Remarks

Subclymenia is a remarkable genus because it has a deeply V-shaped external lobe in the suture line ( Fig. 16 View Fig ). Unlike many nautiloids, this lobe is not caused by the geometry of the whorl profile alone, but by a ventral inflexion of the septum. Shimansky (1962) based the family Subclymeniidae on this single genus. Kummel (1964) saw Subclymenia as a genus belonging to the family Trigonoceratidae and did not accept the family Subclymeniidae .

Turner (1965) downgraded the Subclymeniidae to a subfamily and expanded it to include the genera Maccoyoceras , Epidomatoceras , Catastroboceras and Pseudocatastroboceras . He showed that several species of the latter three of these genera also have a slightly angular external lobe. Shimansky (1967: 156) then withdrew the family Subclymeniidae and placed Subclymenia in the subfamily Aphelaeceratinae , together with the genera Aphelaeceras , Mesochasmoceras , Catastroboceras , Epidomatoceras and Maccoyoceras . Dzik (1984: 174) did not accept the systematic independence of a family Subclymeniidae and interpreted Subclymenia as a basal representative of the family Grypoceratidae .

In the Osnovy, Shimansky (1962) named the subfamily Aphelaeceratinae for the genera Aphelaeceras and Mesochasmoceras in addition to the family Subclymeniidae . According to the original description by Meek & Worthen (1873: 522), the type species of Aphelaeceras has a V-shaped external lobe, indicating that it belongs to the Subclymeniidae . However, it is not clear whether the internal lobe in Aphelaeceras is caused by the shape of the whorl profile with a deeply concave venter or by a septal inflexion.

Both Turner (1965) and Shimansky (1967) included Maccoyoceras in the subfamily Subclymeniinae . However, typical specimens of Maccoyoceras have a broadly rounded external lobe and an almost central siphuncle ( Histon 1999); the genus is therefore included here in the Subclymeniidae with reservations.

The Subclymeniidae are treated here as a family because the ventral inflexion of the septum is a very unusual feature. In addition, the position of the siphuncle rather close to the venter is a distinguishing feature from other families in the superfamily Subclymenioidea . The name Subclymeniidae is preferred to Aphelaeceratidae because Subclymenia characterises the family much better than Aphelaeceras .

In addition to the shape of the septum and the presence of a ventral lobe, a second important feature of the subfamily is the presence of a usually subangular ventrolateral shoulder separating the usually flattened flanks from an equally flattened or concave venter. A third feature is the more or less distinct umbilical margin, which is not present in this form in the other subfamilies of the Trigonoceratidae .

The family Subclymeniidae is not included here in the superfamily Trigonoceratoidea , because its morphological characteristics with a subangular umbilical margin suggests a transitional phylogenetic position leading to the Grypoceratoidea . The close morphological relationship favours placement in the latter superfamily.