Superfamily Trigonoceratoidea Hyatt, 1884

Figs 6–10

Diagnosis

Superfamily of the suborder Temnocheilina, in which a ventrolateral shoulder is formed early in ontogeny. Conch gyroconic or advolute in some species, but more or less tightly coiled in most of the species; general conch form usually discoidal, subevolute or evolute. Whorl profile depressed elliptical or bicarinate in the initial growth stage of the early species and circular in the advanced species. Conch ontogeny with the widest area of the initial growth stage transforming into a pronounced ventrolateral shoulder. Adult whorl profile often rounded triangular but also rounded trapezoidal, polygonal or circular. Juvenile stage with longitudinal ridges or lines, which are particularly pronounced in the area of the venter and the ventrolateral shoulder of the adult stage. Septa simply domed. Suture line with shallow lobes and saddles; internal lobe absent or very shallow and rarely triangular.

Included families

Trigonoceratidae Hyatt, 1884 (Early Carboniferous to Early Permian; 13 genera, 72 species).

Triboloceratidae Hyatt, 1884 [synonym of Trigonoceratidae].

Rineceratidae Hyatt, 1893 [synonym of Trigonoceratidae].

Rhineceratidae Hyatt, 1900 [nomen nullum, synonym of Trigonoceratidae].

Temnocheilidae Mojsisovics, 1902 (Early Carboniferous to Early Permian; 11 genera, 59 species).

Thrincoceratidae Ruzhencev & Shimansky, 1954 (Early Carboniferous to Early Permian; 4 genera, 17 species).

Knightoceratidae Shimansky, 1962 [synonym of Temnocheilidae].

Neothrincoceratidae Shimansky, 1962 [synonym of Thrincoceratidae].

Dasbergoceratidae fam. nov. (Late Devonian; 1 genus, 1 species).

Remarks

The superfamily Trigonoceratoidea probably originated in a genus similar to Dasbergoceras in the latest Famennian (Late Devonian). Along with the superfamily Aipoceratoidea, it is the second evolutionary lineage of coiled nautiloids to cross the Devonian-Carboniferous boundary (Dzik & Korn 1992). There are as yet no records from the earliest Carboniferous period, the Gattendorfia Stufe in the traditional ammonoid stratigraphy. On the basis of current knowledge, it is likely that coiled nautiloids underwent rapid morphological evolution during the Middle Tournaisian, which may be the reason for the great conch and sculpture diversity in the Late Tournaisian.

The adult conch shapes of the Trigonoceratoidea are diverse, ranging from cyrtoconic to subevolute; the latter being the dominant ones (Fig. 6). This is the reason why it is difficult to characterise the superfamily on the basis of the adult conch shape alone. The same is true for the ornamentation. In many species, there are more or less pronounced spiral ridges or spiral lines developed, but this can also be reduced in many advanced species. The course of the suture line is usually strongly dependent on the shape of the whorl profile. This is because the septum is usually uniformly concave in shape. For this reason, the ontogeny of conch shape and ornamentation is used here as a key character to distinguish families and subfamilies.

Four families within the Trigonoceratoidea can be separated on the base of the following principal characters:

Dasbergoceratidae fam. nov. – Ancestral taxa with a triangular whorl profile and a large umbilical foramen. Sculpture with short radial ribs ending in conical ventrolateral nodes. Siphuncle subcentral, close to the venter (Fig. 7).

Trigonoceratidae. – Ancestral taxa with a wide variety of conch shapes ranging from cyrtoconic to more or less tightly coiled. Early juvenile whorl profile depressed oval. Internal lobe shallow, broadly rounded (Fig. 8).

Thrincoceratidae. – Derived taxa with a usually evolute conch shape. Early juvenile whorl profile nearly circular. Internal lobe shallow, broadly rounded (Fig. 9).

Temnocheilidae. – Derived taxa with a subevolute or evolute conch shape. Early juvenile whorl profile strongly depressed oval. Internal lobe deep, funnel-shaped or subtriangular (Fig. 10).

The superfamily Trigonoceratoidea differs from the superfamily Koninckioceratoidea in the shape of the whorl profile, which is triangular or trapezoidal with a flat venter in the ancestral Trigonoceratoidea, but simply oval in the Koninckioceratoidea. In addition, the shell of the Trigonoceratoidea is decorated with coarse, long ridges, whereas that of the Koninckioceratoidea is almost smooth.