Orthonychia sp.

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

Orthonychia sp.

Fig. 13A, B

Material.

Two specimens from the Upper Mississippian (Chesterian) Imo Formation, Arkansas (SNSB-BSPG 2023 I 89, SNSB-BSPG 2023 I 90). The material was sampled by R. H. Mapes (ARC-02, samples Imo-6 and Imo-8.) at the Location #1 as given by Jeffery et al. (1994), figs 1 and 2). This location, the Peyton Creek Roadcut, was also described in detail by Manger (1977) who measured a section in the outcrop ( Manger 1977, fig. 1). The sample Imo-6 sample came from position 20 in this section, approximately 110 feet above the base of his measured section and the Imo-8 sample came from position 24, which is about 135 feet above the base of his measured section. NE1/4 sec. 11 and NW1/4 sec. 12, T. 13 N., R. 15 W. Van Buren County, Arkansas

Description.

Juvenile shell 0.66 mm wide, 0.52 mm high; shell starts with a slightly elongated initial bulb with a length of 87 µm and a width of 63 µm; width of shell at shell length 100 µm is 66-68 µm; initial bulb is bent slightly adapically in relation to the following straight shell-tube; following shell is an entirely straight, smooth tube that increases slowly in diameter. At a total shell length of 320 µm the tube starts a sharp coiling in a dextral direction and at the same time, it is strongly expanding in diameter; shell is smooth except of a faint micro-striation visible on the coiled part of the first whorl.

Remarks.

We suppose that approximately the first openly coiled whorl represents the larval shell and that the faint micro-striation is the larval shell ornament. The protoconch of this species resembles the above described Orthonychia parva by having an initial bulb, representing most likely the embryonic shell (protoconch 1) and a following straight tube. However, the early shell of Orthonychia sp. lacks any strong co-marginal ribs or frills. Among the gastropod species described from the Imo Formation by Jeffery et al. (1994), Platyceras ( Orthonychia ?) sp. is the most likely candidate to be conspecific with the two juvenile specimens studied herein. They reported a single specimen from their rich collections and thus the species is very rare in this fauna.

Figures

Figure 13. Orthonychia sp., with openly coiled larval shell; Mississippian (Chesterian) Imo Formation, Arkansas, USA. (A) SNSB-BSPG 2023 I 89. (A 1) Apertural view. (A 2) Lateral view. (A 3) Oblique lateral view on openly coiled protoconch. (B) SNSB-BSPG 2023 I 90. (B 1) Lateral view. (B 2) Detail initial bulb of protoconch. (B 3) Apertural view. (B 4) Apical view. (B 5) Oblique lateral view.