Orthonychia enorme ( Lindstroem , 1884)

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 enorme ( Lindstroem, 1884)

Figs 3 , 4 , 5 , 6 , 7 , 8

1884 Platyceras enorme sp. nov. - Lindström: 69, pl. 2, figs 59-72. cf. 1976 Platyceras (Orthonychia) cf. P. (Orthonychia) enorme Lindström, 1884 - Peel and Yochelson: 17.

Lectotype.

Selected here, NRM Mo 38219 from the Eke Formation at Rikvide, Gotland, illustrated by Lindström (1884, pl. 2, figs 60, 61, re-illustrated here in Fig. 3A . The specimen is laterally compressed and incomplete when it comes to height but shows the early ontogeny, the tube-like expansion of the shell and the distinct ornamentation.

Other type material.

Lindström (1884) illustrated an additional six specimens from the same locality as the lectotype, and these are here considered paralectotypes (NRM Mo 38218, Mo 38220-Mo 38324); see Figs 3B - 5B . The identity of specimens listed by Lindström (1884) from other localities cannot be established, with the exception of the specimen from Slite which is not conspecific with O. enorme (see below). The other localities mentioned by Lindström (1884) span older strata (Djupvik and Klinteberget = Slite beds of Sheinwoodian/Homerian age; Havdhem and Hammar in Kräklingbo = Hemse beds of Gorstian/Ludfordian age) and younger strata (Burgsvik = the Burgsvik Sandstone just overlying the Eke Formation).

Additional material.

Ca. 240 juvenile shells (illustrated material SNSB-BSPG 2023 X 1-15). from the Eke Formation at Bodudd (S Gotland, Sweden, 57°04'05.8"N, 18° 11'34.7"E, compare Calner et al. 2004, fig. 2) which is Ludfordian (Ludlow, upper Silurian) in age .

Description of type material

(Figs 3 - 5 , Table 1 ). Largest shell 16.5 mm tall and 7.5 mm in apertural length (in lateral view). On average the shell is twice as tall as long (in lateral view). NRM Mo 38219 (Fig. 3A ) deviates by length of aperture being 75% of height, but the shell is laterally compressed. NRM Mo 38222 (Fig. 5B ) has an aperture length that is only about 40% of shell height. Rate of expansion high when viewed in a lateral perspective, but less when viewed anteriorly/posteriorly, making aperture sub-oval. Growth lines irregularly crenulated, but overall attitude is straight across posterior section of shell (Figs 3A3 , 4A4 ), gently arched on lateral surfaces and curved into broad median sinus anteriorly. Here the growth lines are sinusoid with two asymmetrical crenulations of which the left has greater amplitude (Figs 3A4 , 4A3 ). Although the intensity and irregularity of the crenulations vary, there are no major deviations from the overall co-marginal alignment. The co-marginal ribs or frills seen in early ontogeny widens into distinct growth increments, spaced less than 1 mm apart and ending in seemingly short lamellae. Fine radial striae may be present within each growth increment. Specimen NRM 38220 appears to have a mostly smooth shell with fine growth lines only, but increments with lamellae are visible on the initial part of the shell (Figs 3B3, B4 ). Space between increments get narrower in late ontogeny (Fig. 4A, B ). Radial ornamentation is absent.

Description of additional material from sieved bulk samples

(juvenile and encrusted shells, Figs 6 - 8 ). Shell with tightly coiled ca. 1.5-2.5 initial whorls including protoconch and early teleoconch; largest shell studied with SEM 4.1 mm long, 1.9 mm wide; encrusted specimens (forming cores of oncoids) probably belonging to the present species up to 20 mm long; protoconch with round, convex whorls and slightly elevated spire, lacking ornament (but re-crystallized); encrusted (oncoid) specimens possibly belong to this species up to 20 mm long; diameter of first whorl 0.17-0.25 mm, diameter of protoconch 0.37-0.68 mm (two measurements); protoconch/teleoconch-transition indistinct due to preservation (re-crystallization) but apparent by sudden onset of co-marginal ribs (may also appear as frills or lamellae) and rapid increase of expansion rate of whorl diameter; later teleoconch detaches and becomes uncoiled, bent to slightly elongated, slowly increasing in diameter, forming a cap-shaped tube; teleoconch ornamented by sharp, narrow co-marginal ribs separated by wide interspaces; strength of ribs and density of ribbing variable; ribs can also form lamellae; ribs appear much longer (lamellae) in specimens studied in thin sections than ribs in specimens from bulk samples suggesting that such lamellar ribs broke off during processing of sample; teleoconch also ornamented by thin spiral lirae, much weaker than co-marginal ribs; shell having a calcitic layer as suggested by thin sections.

Remarks.

Lindström (1884, p. 69) mentioned several specimens from various localities on Gotland, but figured only specimens from Rikvide. In the NRM collection only specimens from Rikvide and Burgsvik are registered, but specimens from the latter locality were not available for study. The specimen from Slite in the PMU collections, mentioned by Lindström (1884), is illustrated here in Fig. 5B . It comes from the much older Slite Group sediments and is here treated as Orthonychia sp. The species was also listed from the Slite Beds by Manten (1971, table 11). However, it differs markedly from O. enorme in the initial shell having higher rate of coiling, possibly more coiled whorls before extending into the tubular shell, the shell is proportionally broader at the aperture, the W/H ratio is higher (0.65), it lacks clear growth increments and lamellae, and have a distinct radial ornamentation crossing the fine co-marginal growth lines thus creating a fine reticulate pattern. Furthermore, the shell reverses curvature during growth.

Lindström (1884, p. 69, pl. 2, fig. 71) described and illustrated a fine radial ornamentation. Some faint striations may be discerned in NRM 38218 (Fig. 4B2 ) but similar ornamentation is not seen in other macro-specimens. Fine and widely spaced lirae are present in early ontogeny (Gotland Ortho 1, C2). The two-element illustration in Lindström (1884) cannot readily be linked to any of the available specimens.

Peel and Yochelson (1976, p. 17) listed Platyceras (Orthonychia) cf. P. (Orthonychia) enorme Lindström, 1884 from the Telychian of Norway.

The present material from the bulk samples closely resembles the type material of Platyceras enorme Lindström, 1884 as described and illustrated above. The type specimens are much larger (more than 16 mm long) than the specimens from the washed residue reported here. However, strongly encrusted specimens studied by Schugmann (2015) and herein (Fig. 6 ) are also in this larger size range suggesting that the specimens from the residues are juveniles. Platyceras cornutum from the upper Silurian of Gotland as illustrated by Lindström (1884) is similar but lacks ornamentation of axial ribs/frills and spiral lirae; instead it has co-marginal irregular bulges. Moreover, the teleoconch of Platyceras cornutum is generally more curved.

The Devonian Crossoceras Boucot & Yochelson, 1966 ( Platyceratidae ) is ornamented with co-marginal sharp ribs (frills) and a fine spiral lineation similar to the ornament of Platyceras enorme . However, Crossoceras has a stronger curvature of the teleoconch and is thus close to Platyceras in this respect. A close phylogenetic relationship of Crossoceras and Orthonychia enorme seems to be likely. The protoconch of Crossoceras is unknown; judging from the illustration given by Boucot and Yochelson (1966, pl. 3, fig. 11) it could be tightly coiled as in Orthonychia enorme but the preservation is insufficient to be sure.

Pragoserpulina tomasi Frýda, 1998a, type species of Pragoserpulina Frýda, 1998a, family Pragoserpulinidae Frýda, 1998a, from the Early Devonian of the Czech Republic (Dvorce-Prokop Limestone, Praha Formation, Pragian, Czech Republic) closely resembles O. enorme in teleoconch morphology and in having the same type of tightly coiled protoconch. However, the uncoiled teleoconch of P. tomasi is more bent, its ribbing is denser, the co-marginal ribs are stronger; the ribs are rounded and not as sharp (no frills). Bouchet et al. (2005, 2017) classified Pragoserpulinidae as a Paleozoic taxon that certainly belongs to gastropods, unassigned to superfamily - it is possible that Pragoserpulina is platycerate.

The tightly coiled protoconchs shown here for Orthonychia enorme and that illustrated by Frýda (1998a) for Pragoserpulina tomasi are clearly not of the vetigastropod type (see e. g., Bandel 1982; Haszprunar et al. 1995; Nützel 2014). They probably were produced by larvae with a short-termed planktotrophic phase. Similar protoconchs were reported for the Paleozoic neritimorph Naticopsis ( Nützel and Mapes 2001; Nützel et al. 2007b) and Platyceras ( Frýda et al. 2009). The same type of protoconch was found in Devonian and early Carboniferous gastropods with a cap-shaped teleoconch placed in the family Pragoscutulidae Frýda, 1998a ( Cook et al. 2008). They were classified as Caenogastropoda but a neritimorph relation seems also possible. Pragoscutulidae are much stouter i.e., they have a much wider apical angle.

Figures

Figure 3. Orthonychia enorme (Lindstroem, 1884) (A) Lectotype NRM 38219. (A 1 - A 5) Right lateral, left lateral, posterior, anterior, and posterior oblique views. (A 6, A 7) Details of apex; illustrated by Lindstroem (1884 pl. 2, figs 60, 61). (B) Paralectotype NRM 38220. (B 1) Detail of apex. (B 2 - B 3) Left lateral, posterior oblique and right lateral views; illustrated by Lindstroem (1884, pl. 2, figs 62, 63). Scale bar: 0.5 mm (A 1 - A 5, B 2 - B 4).

Figure 4. (A, B) Orthonychia enorme Lindstroem (1884). (A) Paralectotype NRM 38221. (A 1 - A 4) Right lateral, left lateral, anterior, and posterior views; iIllustrated by Lindstroem (1884, pl. 2, figs 64 - 66). (B) Paralectotype NRM 38218. (B 1) Left lateral view. (B 2) Detail of ornamentation near the aperture. The museum label indicates that this is the specimen illustrated by Lindstroem (1884, pl. 2, fig. 59), but the specimen does not match the illustration particularly well. Scale bar: 0.5 cm (A, B 1).

Figure 5. (A, B) Orthonychia enorme Lindstroem (1884). (A) Paralectotype NRM 38223. (A 1, A 2) Right lateral and anterior oblique views; illustrated by Lindstroem (1884, pl. 2, figs 69, 70). (B) Paralectotype NRM 38222. Left lateral view; illustrated by Lindstroem (1884), pl. 2, fig. 68. (C) Orthonychia sp., PMU 21576. (C 1) Detail of apex. (C 2 - C 4) Right lateral, left lateral and posterior views. Specimen collected by Cleve and mentioned by Lindstroem (1884, p. 69). Scale bar: 0.5 cm (A, B, C 2 - C 4).

Figure 6. Orthonychia enorme (Lindstroem, 1884) in thin sections, Silurian Eke Formation, Gotland; the type of preservation of the shells suggests that they were primarily calcitic with foliated micro-structures; some of the shells have very long frills; these frills are much longer than those on specimens from the type series (Figs 3 - 5) and washed samples (Figs 7 - 8) suggesting they broke off during weathering and the processing of the samples. Thin sections reposited Friedrich-Alexander-Universitaet Erlangen-Nuernberg, GeoZentrum Nordbayern, Erlangen, Germany. (A) Shell in longitudinal section, specimen with pronounced frills. (B, C) Shell in longitudinal section, specimen with pronounced frills in apical portion (C). (D) Shell in longitudinal section, specimen shows coiling in its apical portion. (E) Upper portion of image, shell in transverse section of one whorl (large oval) and tangential to earlier whorl (round). (F) Middle left portion of image, shell in transverse section of one whorl (left large oval) and earlier whorl (right circular). (G) Shell in longitudinal section, specimen shows pronounced frills covered by a thick microbial crust. (H) Shell in transverse section, covered by a thick microbial crust. (I) Shell in longitudinal section, covered by a thick microbial crust.

Figure 7. Orthonychia enorme (Lindstroem 1884), Silurian Eke Formation, Gotland. (A) SNSB-BSPG 2023 X 1. (A 1) Lateral view. (A 2) Initial whorls in oblique apical view. (B) SNSB-BSPG 2023 X 2, juvenile specimen in lateral view. (C) SNSB-BSPG 2023 X 3. (C 1) Lateral view. (C 2) Early teleoconch with distinct axial ribs / frill and faint spiral lirae. (D) SNSB-BSPG 2023 X 4. (D 1) Juvenile specimen in lateral view. (D 2) Initial whorls in oblique apical view including tightly coiled protoconch without visible ornament. (D 3, D 4) Dorsal views including last smooth whorl of protoconch. (E) SNSB-BSPG 2023 X 10, juvenile specimen with strong frills in lateral view. (F) SNSB-BSPG 2023 X 6, juvenile specimen with strong frills in lateral view. (G) SNSB-BSPG 2023 X 11. (G 1) Lateral view. (G 2) Initial whorls including tightly coiled protoconch without visible ornament in oblique apical view. (G 3) Lateral view. (G 4) Initial whorls including tightly coiled protoconch without visible ornament in apical view. (H) SNSB-BSPG 2023 X 12, juvenile specimen in lateral view.

Figure 8. Orthonychia enorme (Lindstroem, 1884), Silurian Eke Formation, Gotland. (A) SNSB-BSPG 2023 X 13, specimen with distinct co-marginal ribs and longitudinal lirae. (A 1) Lateral view. (A 2) Initial whorls in oblique apical view. (A 3) Dorsal lateral view. (A 4) Dorsal view. (B) SNSB-BSPG 2023 X 15, lateral view.