Planetoceratidae, Korn, 2025

Family Planetoceratidae fam. nov.

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Fig. 33 View Fig

Type genus

Planetoceras Hyatt, 1893 View in CoL .

Diagnosis

Family of the superfamily Liroceratoidea with a pachyconic, usually subinvolute conch; the terminal portion of the conch is detached from the preceding whorl. Whorl profile in the adult stage usually more or less strongly depressed; flanks and venter form a continuous arch, umbilical margin pronounced, umbilical all flat and steep. Ornament consisting of fine growth lines. Septum simple in shape, concavely domed; suture line very simple, almost straight across flanks and venter or with small lobes and saddles.

Etymology

The family name refers to the type genus.

Included genus

Planetoceras Hyatt, 1893 (Tournaisian to Kasimovian; 9 species).

Remarks

The placement of Planetoceras was problematic. Planetoceras retardatum Hyatt, 1893 and P. globatum (Sowerby, 1824) from early Late Tournaisian limestones of Belgium and Ireland have a peculiar conch morphology with a terminal whorl, which is depressed and oval in profile, and is detached from the preceding whorl. It is not possible to conclude whether the detached final volution in Planetoceras is due to increased coiling or secondary uncoiling.

Contrary to previous authors, a separate family is established for the genus Planetoceras and it is placed in the superfamily Liroceratoidea . Kummel (1964) placed Planetoceras in the then very heterogeneous family Koninckioceratidae . Shimansky (1967) also placed it together with the genera Millkoninckioceras and Lophoceras in the family Koninckioceratidae , which consisted of only three genera.

The shape of the inner volutions and their sculpture argue against placement in the Koninckioceratidae . Planetoceras has a distinct umbilical margin, with some coarse longitudinal ridges, giving it a morphology that is much closer to that of early liroceratids. Another common feature with the liroceratids is the expansion of the flank and venter area. In contrast to liroceratids, the umbilical wall of Planetoceras is flattened and steep. However, the most important distinguishing feature is the detachment of the adult whorl spiral.