Orthonychia parva (Swallow, 1858 in Shumard and Swallow)

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

publication ID	https://dx.doi.org/10.3897/zitteliana.97.115688
publication LSID	lsid:zoobank.org:pub:BA2DA079-4906-4AC8-AE11-05E21BBF12B9
persistent identifier	https://treatment.plazi.org/id/63E480A4-87FC-513E-B6B0-F7E8ED9C5D97
treatment provided by	Zitteliana by Pensoft
scientific name	Orthonychia parva (Swallow, 1858 in Shumard and Swallow)
status

Treatment

Figs 9 View Figure 9 , 10 View Figure 10 , 11 View Figure 11 , 12 View Figure 12

*1858 Capulus parvus sp. nov. - Swallow (in Shumard and Swallow): 205.

1967 Platyceras (Orthonychia) parvum (Swallow, 1858) - Yochelson and Saunders: 173 [additional synonymy and chresonymy therein].

1999 Orthonychia parva (Swallow, 1858) - Bandel and Frýda: text-fig. 1, pl. 2, figs 5-8, pl. 3 figs 1-2.

2005 Orthonychia parva - Frýda: 382, fig. 3D.

2008a Orthonychia parva - Frýda et al.: 254, fig. 10.8E, G.

2009 Orthonychia parva (Shumard & Swallow, 1858) - Frýda et al.: 112, 116, fig. 5C-D.

2012 Orthonychia parva - Frýda et al.: 417, fig. 14E, M.

2014 Pseudorthonychia - Nützel: 491, fig. 14E, H.

Material.

6 specimens (SNSB-BSPG 2020 XCI 117-122) from the Finis Shale Member of the Graham Formation (late Pennsylvanian , Virgillian , North Central Texas, USA) at the locality TXV-200: Lost Creek Lake emergency spillway at dam, approximately 4 km northeast of Jacksboro , Jack County, Texas on Texas Highway 59. AMNH Locality 5562, 33°14'11.17"N, 98°07'11.33"W, 33.230, -98.136 GoogleMaps .

Description.

Shell limpet-shaped; protoconch (description largely based on SNSB-BSPG 2020 XCI 117, Figs 9A View Figure 9 , 12 View Figure 12 ) with an initial bulb (Fig. 12 I View Figure 12 ) slightly elongated, 100-120 µm long, 70-75 µm wide, terminating in a slight constriction; width of initial bulb at 100 µm shell length 70-80 µm; shell after initial bulb forming a straight tube slowly increasing in width (Fig. 12 View Figure 12 II); dextral coiling starts at 270-360 µm shell length and coiling comprises 100-180° (Fig. 12 View Figure 12 III) until strong co-marginal ribs develop on the shell; the smooth initial shell has the form of a hook that encloses a central gap i. e., it is openly coiled; terminal width of whorl of initial smooth shell 0.5-0.6 mm; smooth initial whorl followed by rapidly expanding shell portion with strong collabral ribs (Fig. 12 View Figure 12 IV); ribs separated by wider interspaces, prosocyrt in apical view and prosocline in lateral view; initial smooth shell part resting on axially ribbed shell; shell part with strong axial ribs has width of 0.8-1 mm at termination; shell distinctly and abruptly widening after initial shell with axial ribs resulting in a cap-shaped morphology (Fig. 12 V View Figure 12 ), transition at an angle; following shell cap-shaped with variable co-marginal ornament; one specimens shows fine longitudinal lirae on the teleoconch (Fig. 10B6 View Figure 10 ).

Discussion.

Orthonychia parva is widely distributed throughout the Carboniferous of the US ( Yochelson and Saunders 1967). It has also been reported from the Pennsylvanian of North Central Texas (Mineral Wells Fm.) ( Plummer and Moore 1921) although not from the Finis Shale. To our knowledge, Knight (1934, figs 1d-g) reported the only specimens (four) with preserved protoconchs. Two of these specimens were also illustrated in detail (SEM) by Bandel and Frýda (1999, pl. 2 figs 5-8): (reproduced by Frýda 2005, fig. 3D; Frýda et al. 2008a, fig. 10.8 E, G; Frýda 2012, fig. 14E,M; Nützel 2014, fig. 14E, H, erroneously as Pseudorthonychia ). The same type of protoconch with stretched initial part and open coiling has been reported for the Devonian Praenatica cheloti by Frýda et al. (2009).

The present shell displays considerable ontogenetic change. There can be little doubt that the initial bulb represents the embryonic shell as was also concluded by Bandel and Frýda (1999), Frýda et al. (2008b), and Frýda (2012). Its small size of 100-120 µm × 70 µm suggests indirect, planktotrophic larval development. The question is at which stage the larval shell terminates and the teleoconch starts. Knight (1934) for the first time reported and described this type of protoconch and noticed its morphology which is unusual for gastropods ('curious vermiform nuclei’). He described the ‘nucleus’ as a narrow vermiform hook of one smooth revolution. The abrupt expansion of the shell which then bears co-marginal ribs was called by him ‘neanic’ i. e., as representing the early juvenile teleoconch. By contrast, based on the restudy of Knight’s (1934) material from the Labette Shale, Bandel and Frýda (1999) and Frýda (2012) assumed that the axially ribbed part following the smooth hook-like shells belongs to the larval shell. Here, we follow Knight’s (1934) interpretation because the smooth shell terminates abruptly and the size of the following axially ribbed portion of the shell (here Fig. 12 View Figure 12 IV) has a width of ca. 2 mm at its termination which is too large for a larval shell. Hence the larval shell of O. parva comprises portions I-III as given in Fig. 12 View Figure 12 .

The change from a straight narrow tube to a coiled tube (Fig. 12 View Figure 12 II to III) is readily explained: it would be highly disadvantageous for the pediveliger to settle with an entirely straight narrow tube (stretched, uncoiled shell tube would probably be vulnerable and hinder locomotion) and therefore coiling in the last period of larval life does make sense as a preparation for benthic life. The early axially ribbed teleoconch was probably formed by the crawling juvenile and the sudden widening of the shell could indicate the start of the sedentary life on crinoids.