Nanogyra (Palaeogyra) reniformis (Goldfuss, 1833)

Koppka, Jens, 2015, Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland — Ostreoidea, Zootaxa 3927 (1), pp. 1-117 : 29-32

Nanogyra (Palaeogyra) reniformis (Goldfuss, 1833)

Fig. 13.1–2; Pl. 7.2–4; Pl. 8.1–2

* 1833b Exogyra reniformis nobis.—Goldfuss: p. 34, pl. 86, figs. 6a–c (not fig. 7 = N. nana ). 1835 Exogyra reniformis Goldf. —Roemer: p. 65.

1845 Exogyra reniformis .—(Goldfuss.)—d´Orbigny: p. 479, pl. 42, fig. 9, 10.

v 1859 Ostrea (Exogyra) auriformis Goldf. sp.—Contejean: p. 322. 1866 Exogyra reniformis Gf. —Giebel: p. 44.

1874 Exogyra reniformis Goldfuss—Brauns : p. 355.

n 1882 Exogyra reniformis Goldfuss 1836 —Roeder: p. 36, pl. 1, figs. 3a–e, pl. 2, figs. 1a,b.

? 1888 Exogyra reniformis Goldfuss. 1838 —Schlippe: p. 115, pl. 1, fig. 13a,b. 1905 Exogyra reniformis Goldf. —Schmidt: p. 14.

1913 Exogyra reniformis Goldfuss—Wójcik : p. 31.

1924 Exogyra reniformis Goldfuss—Jourdy : p. 71.

p 1924 Exogyra nana Sow. —Jourdy: pl. 2, fig. P.s., left fig. (right = N. nana ), pl. 2, fig. R.s., left fig. (right fig. = N.

nana ).

1971 Exogyra reniformis Goldfuss, 1834 -40—Pugaczewska: p. 291, pl. 31, figs. 1–5, pl. 32, figs. 1–7.

Type series (not seen). Two syntypes— Exogyra reniformis Goldfuss, 1833 (b) (p. 34, pl. 86, fig. 6a–c). Buxweiler (= Bouxwiller, Department Bas-Rhin, Alsace), France; from "Fuller’s Earth" equivalent, Middle Jurassic, Bathonian.

The types were not studied, and the original composition of the syntype series is presently unknown. However, Goldfuss (1833b, p. 34) gave a good description and figures (pl. 86, fig. 6a–c) allowing a taxonomic identification. The capacious variety, Exogyra reniformis var. gibbosa figured by Goldfuss (1833b, pl. 86, fig. 7) from the Corallian facies of the Kimmeridgian of Nattheim, Bavaria, Germany, is more likely Nanogyra nana .

Goldfuss’ reference to Neuenburg (Tithonian?) could not be confirmed, but Roemer’s detailed description and comparison with " Exogyra " auriformis Goldfuss suggests that N. (P.) reniformis also occurs in Osterkappeln in the Weser Mountains of Northwest Germany ( Roemer 1835, p. 65).

Material. More than 20 LVs from the Banné Marls of Vâ Tche Tchâ ( VTT) near Courtedoux (Canton Jura), Lower Kimmeridgian. Two additional specimens from the Contejean collection (MC-27E-38, MC-27E-40) labelled as Ostrea auriformis Goldf. by Contejean (1859) from the Lower Kimmeridgian (cymodoce Zone, Rang Marls or “Marnes à Ptérocères”) of Beauregard near Montbéliard ( France) (for measurements see Appendix, Table 3 ).

Description. Shape, size—LV nearly entirely attached except for anterior shell margin which bends steeply upwards, posterior margin flat (Pl. 7.2–4), slightly larger than RV (Fig. 13.2); outline round to oval (ear-shaped), usually higher than long; umbo small, moderately opisthogyrate. RV weakly convex, almost flat; not of the typical convex RV type of Malchus (1990, p. 94). Examined specimens 0.6–1.75 cm in height (Appendix, Table 3 ). Sculpture—LV free margin with fine concentric growth lines and no or few weak, irregular growth crests (Pl. 7.3c); RV nearly smooth, with fine concentric growth lines and few inconspicuous growth ridges at its ventral third. Few thin antimarginal riblets (Fig. 13.2; Pl. 7.2b) restricted to ventral half of the shell; xenomorphic structures absent in present material because of smooth settling grounds.

Ligament area—Short and moderately prosodetic; resilifer only weakly sunken, anterior bourrelet relatively broad, posterior bourrelet reduced to a thin lamella, becoming partially overgrown by a lobe-like projection of the posterodorsal hinge margin.

Internal shell characters—Umbonal cavity missing (Pl. 7.2a) or weak (Pl. 7.3a–b); posterior adductor scar (PAM) of LV small, only shallowly inserted, slightly posterocentral (Pl. 7.3b), outline round to crescentic (Pl. 7.2a, 3b), dorsally truncated, and dorsocentrally slightly concave (Fig. 13.1), scar inconspicuous in some specimens (Pl. 7.4b).

Commissural shelf thin, well developed anterodorsally and fading towards the ventral margin; posterodorsally weak and posteroventrally absent (Pl. 7.3b, 4b). Numerous, well developed straight chomata (Pl. 7.2a, 3b, 4b) along anterodorsal part of the commissural shelf, vanishing ventrally; posterodorsal shelf of LV with few but well developed vermiculate chomata (Pl. 7.2–4; Fig. 13.1); posteroventral margin without chomata; LV posterior chomata (length 0.2–0.6 mm, width 0.09–0.18 mm) larger than anterodorsal chomata (length 0.07–0.16 mm, width 0.07–0.12 mm). Rare observations of thin antimarginal furrows at the inner side of the anteroventral margin (Pl. 8.2) could represent relict chomata.

Microstructure—Not examined, but see Nanogyra (P.) virgula for probably comparable features.

Prodissoconch, juvenile—One specimen (VTT009-33b) shows a relatively small internal calcitic mould underlying the prodissoconch and nepioconch (Pl. 7.2b,c) but was not examined under SEM. The length of the dorsally broken prodissoconch mould is ca. 250 µm.

The LV of the juvenile specimen VTT009-33a (Fig. 13.1) shows a narrow, weakly recurved ligament area, with a short (only 500 µm long) and thin, weakly elevated posterior bourrelet (dorsal of the projection of the posterodorsal hinge-margin), which disappears in later growth stages; the juvenile resilifer is initially very narrow, but becomes broader during growth and as wide as the anterior bourrelet.

Paleoecology. Nanogyra (P.) reniformis preferentially settled on the interior of dead, moderately large bivalve shells (Banné Marls, Appendix, Table 3 ). It was also found in the Lower Callovian of Liesberg (Herznach Member, Ifenthal Formation; Switzerland) in a shallow depression on a plain hardground surface, together with several attached specimens of Pernostrea luciensis (d’Orbigny, 1850) . However, it was absent on the more exposed and eroded cavernous lateral continuation of this hardground. In contrast to its congeners, the present species seemed to have preferred protected habitats with low water energy.

Occurrence. The species is known from the Bathonian of France, Bathonian and Kimmeridgian of Germany, Callovian and Kimmeridgian of Switzerland, and the Oxfordian of Poland ( Pugaczewska 1971, p. 294). In NW Germany, the species likely occurs in Osterkappeln (Weser Mountains), Galgenberg, Hoheneggelsen, Goslar and Hannover, all from the "Coral rag" (Oxfordian) ( Roemer 1835, p. 65).

Comparisons. The species was rarely mentioned in the past, probably because it was frequently confounded with the similar Nanogyra (N.) nana such as Goldfuss’ Exogyra reniformis var. gibbosa (mentioned above) and “ Exogyra nana ” of Jourdy (1924, pl. 2, fig. R.s., fig. P.s). All N. (Palaeogyra) spp. possess chomata [see also N. (N.) nana and Nanogyra (P.) virgula ].

Nanogyra (P.) catalaunica (de Loriol in de Loriol, Royer & Tombeck, 1872) (p. 401, pl. 23, fig. 15, 15a)—This species was described from the Upper Kimmeridgian of the Department Haute-Marne, France; the figured syntype is from the “Virgulien” eudoxus Zone of Blaise. Further specimens are from the Portlandian of Bure and Vaux in the same area.

The species differs from N. (P.) reniformis by its stronger inflation, the virguliform, strongly curved outline , a larger adult size (1.2–4.2 cm) and the presence of some well-developed antimarginal ribs on the anterior side of the LV. The chomata are more pronounced in N. catalaunica and cover nearly the whole anterior side and two-thirds of the posterior margin.

Nanogyra (P.) welschi ( Jourdy, 1924) (p. 70, pl. 2, fig. V.s., pl. 6, fig. 7)—This species was described from the Virgulian (Upper Kimmeridgian) of Tout-y-Faut, close to Niort and Saint-Jean-d’Angely, region Poitou-Charentes (W France).

The species has a more triangular shape, a larger size and inflation and shows numerous fine ribs on the outer surface of the LV. Furthermore, it has a more pointed umbo, the posterior lobate protrusion is more pronounced, and chomata are better developed and are also present on the ventral margin ( Pugaczewska 1971, pl. 33, 34).

Nanogyra ? (P.) fourtaui ( Stefanini, 1925) —Taxonomic details are discussed under N. nana . This species has a more pointed umbo, a much smaller attachment area and finer antimarginal riblets close to the ventral margin of the RV.