Orthonychia Hall, 1843

Nuetzel, Alexander, Ebbestad, Jan Ove, Seuss, Barbara, Munnecke, Axel, Mapes, Royal H. & Cook, Alex G., 2023, On Paleozoic platycerate gastropods, Zitteliana 97, pp. 29-51 : 29

Genus Orthonychia Hall, 1843

Type species.

Platyceras subrectum Hall, 1859; by subsequent monotypy by Hall (1859).

Remarks.

Orthonychia is a cap-shaped or elongate tube-like mollusk which has traditionally been placed in the in the Paleozoic gastropod family Platyceratidae and was also considered a subgenus of Platyceras ( Knight et al. 1960). The Devonian type species of Orthonychia , O. subrecta (Hall, 1859), is known from steinkerns only ( Knight 1941) and therefore, the status of shell ornamentation and protoconch morphology cannot be assessed. The general habitus (elongated slightly curved shell with only the early teleoconch being coiled) is close to that of Orthonychia enorme ( Lindström, 1884) and Orthonychia yutaroi Ebbestad, sp. nov. as described below, especially regarding the tube-shaped, stretched late teleoconch. Based on an openly coiled fish-hook-like protoconch, present in the Pennsylvanian species O. parva , Bandel and Frýda (1999) erected the family Orthonychiidae and placed it in a new order Cyrtoneritimorpha , forming the fossil sister group of modern Neritimorpha ( Cycloneritimorpha ).

The tall shell morphology of some Orthonychia species including O. yutaroi and O. enorme as reported herein superficially resembles that of the Ordovician archaeogastropod Pollicina , described by Peel (2020a, b). This genus can reach a height of at least 3 cm, has a thick shell, with even co-marginal ribs, and an apex truncated by septa. The shell is bilaterally symmetrical, in contrast to that of Orthonychia .