Praeexogyra dubiensis ( Contejean, 1859 )

Koppka, Jens, 2015, Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland — Ostreoidea, Zootaxa 3927 (1), pp. 1-117 : 44-46

Praeexogyra dubiensis ( Contejean, 1859)

Fig. 17 ; Pl. 13.5a–d; Pl. 14.6a–c

1859 Ostrea exogyroides Roem. —Contejean: p. 46, 47, 49, 210.

* 1859 Ostrea dubiensis Contj. —Contejean: p. 320, pl. 21, figs. 4–11.

1862 Ostrea dubiensis Contj. —Thurmann & Etallon: p. 272, pl. 39, fig. 6.

1866 Ostrea Dubiensis Contejean—de Loriol & Pellat: p. 115, pl. 11, figs. 2–3. n 1872 Ostrea dubiensis Contejean—de Loriol, Royer & Tombeck: p. 407, pl. 24, figs. 19–25.

1878 Ostrea Dubiensis Ctj. —Choffat: p. 70.

1882 Ostrea Dubiensis Contejean, 1859 —Roeder: p. 31, pl. 3, fig. 10a,b.

1917 Ostrea Dubisensis Contejean—Rollier : p. 590.

1923 Ostrea cf. Dubiensis Contejean—Lewinski: p. 65, pl. 3, figs. 6–8.

1927 Ostrea dubiensis Contejean—Pčelinčev : p. 78.

1935 Ostrea (Liostrea) dubiensis Contejean—Cox : p. 171, pl. 17, figs. 4–5.? 1936 Ostrea dubiensis Contejean—Arkell : p. 366, pl. 55, figs. 4a,b, 5.? 1936 Ostrea (Liostrea) cf. dubiensis Contejean—Reed : p. 6.

1939 Liostrea dubiensis (Contej.) —Stefanini: p. 194, pl. 22, fig. 3.

1952 Liostrea (Catinula) dubiensis (Contejean) —Chavan: p. 40, pl. 2, figs. 20–24.

1955 Ostrea dubiensis Contejean, 1858 —Gerasimov: p. 127, pl. 28, fig. 7.

1958 Liostrea dubiensis (Contejean) —Hudson: p. 419.

1971 Liostrea dubisensis ( Contejean, 1859) —Pugaczewska: p. 252, pl. 14, figs. 1–7.

1976 Liostrea cf. dubiensis (Contejean) —Gu Zhi-wei et al: p. 244, pl. 48, fig. 1.

Lectotype (designated herein). Ostrea dubiensis Contejean, 1859 , pl. 21, figs. 4–5, MC-27-E-112; single LV, damaged anteroventrally); Bussurel near Montbéliard (Department Haute-Saône, Franche-Comté, France), "Calcaire à Natices" ( Contejean 1859, p. 320), Upper Oxfordian (bifurcatus Zone), equivalent of the Vorbourg Member of the Vellerat Formation in Switzerland.

In the stratigraphic part of his monograph, Contejean (1859, p. 46, 47, 49, 210) mentioned the abundant occurrence of Ostrea exogyroides Roemer, 1835 , indicating that it came from a 50 cm thick blue or greyish oolitic marl-unit, which grades laterally into solid limestone ( Contejean 1859, p. 210, section Bussurel). However, the species name does not appear in the systematic part. Instead, Contejean (1859, p. 320) mentions a new species, O. dubiensis , the original label stating that it was collected from a railroad cutting at Bussurel close to Héricourt, northwest of Montbéliard ( France) which coincides with the locality given on page 210 for Ostrea exogyroides . It is also consistent with remains of a blue grey marly matrix still adherent to the here designated lectotype of “ O. ” dubiensis . It appears very likely, therefore, that Contejean changed his mind concerning the determination of this species while earlier parts of his volume were already in print. Apart from the "Calcaire à Natices", Contejean (1859, p. 320) recorded O. dubiensis from the somewhat younger "Calcaire à Térébratules" (Upper Oxfordian) but there is no mentioning of a type locality.

The species name refers to the nearby located river Doubs (ancient name with Celtic roots was Dubis). The ending “s” in Doubs or Dubis is aphonic in spoken French. There is no reason to assume that the original spelling dubiensis was an inadvertent error by Contejean (in the sense of ICZN 32.5); the emendation by Rollier (1917, p. 590) and Pugaczewska (1971, p. 255) to “ dubisensis ” appears therefore unjustified.

Material. Lectotype (see above and Fig. 17 ), and paralectotypes—MC-27-E-113 ( Contejean 1859, pl. 21, figs. 8–9), lacks the hinge; MC-27-E-114, RV ( Contejean 1859, pl. 21, fig. 10); MC-27-E-116, LV attached on a RV (Pl. 15.1a–f, 2a–b).

All former syntypes, stored in the Museum Cuvier, were available for study. Contejean (1859) did not determine a holotype, and the type series was not revised before. The figured syntypes of Contejean (1859, pl. 21, figs. 4–11) could be easily identified by comparison with his published drawings. The lectotype was chosen for its identification as one of the figured types, and its preserved hinge and attachment area, which show a characteristic attachment cast, probably of an alga (Fig. 17.1, 3, 5). However, it lacks part of the anteroventral shell margin (Fig. 17.1, 3–4).

Others—One additional single RV ( VTT 001-1171) from the Banné Marls of Vâ Tche Tchâ (cymodoce Zone, Lower Kimmeridgian) was traceable in the collections of the PAL A16. This specimen and a questionable juvenile LV ( VTT 001-3238) are figured on Pl. 14.5–6. For measurements see Appendix, Table 6 .

Description. Shape, Size—Thin-shelled, convex, inequivalve, outline elongate to ovate or drop-shaped, always higher than long; umbo pointed, posterodorsal margin weakly curved or straight. LV moderately inflated, with a small- or medium-sized attachment area and the highest inflation always posteroventrally. RV normally flat, some specimens posteroventrally concave, or xenomorphic. Small, largest dimension of examined specimens 1.3–2.6 cm.

Sculpture—LV nearly smooth, with weak, irregular spaced concentric swellings and faint growth welts, with narrow, depressed interspaces, without radial elements (Fig. 17.3, 6; Pl. 15.1d). RV nearly smooth, with fine commarginal swellings and few antimarginal threads close to the ventral margin (Pl. 15.2b).

Ligament area—Visible initial portion in small adults prosodetic, but overall uncoiled, trigonal, small, with weakly sunken, relatively broad resilifer (Fig. 17.7); bourrelets small, weakly elevated, ventral edge of ligament area slightly undulated, ligament area not supported by shell buttress (sensu Stenzel, 1971).

Internal shell characters—LV without or with a weakly developed umbonal cavity, commissural shelf narrow (LV), PAM slightly biconcave ( Malchus 1990, tab. 9) with convex dorsocentral bump (LV: Fig. 17.4; Pl. 15.1c; RV: Pl. 15.1f), attachment scar weak, ventrally not elevated, position posterocentral close to posterior margin, ventral margin of PAM directed towards posteroventral angle. Chomata absent.

Microstructure—Broken edge of thin-shelled lectotype revealing regularly foliated shell under the microscope, without chambers; RV with thin prismatic outer layer visible under light microscope (magnification x40). Because of the rarity of the available material, no thin section was prepared.

Prodissoconch, juvenile—Earliest shell stages not preserved.

Paleoecology. Most specimens from the study area are left valves, attached solitarily, and with small, often indistinct attachment scars; only one specimen was found attached to a RV of the same species. Some xenomorphic imprints indicate that the species was able to attach to marine plants, (Fig. 17.3, 5). Left valves show little if any signs of transport, and the co-occurrence of large “ Natica ” sp. and an essentially marly background sedimentation with ooids and biodetritus suggest a marine, shallow subtidal environment, influenced only by weak currents. The coarser sediment fraction of this facies is probably allochthonous.

Occurrence. The species occurs from the Lower Oxfordian to Upper Kimmeridgian in France ( Contejean 1859; de Loriol & Pellat 1866) and is abundant in the Upper Oxfordian near Montbéliard ( Contejean 1859). It is relatively rare in the Kimmeridgian of NW Switzerland. However, Thurmann & Etallon (1862, p. 272) also mentioned it from the “Marnes à Astartes” (Bure Member, Vellerat Formation).

Outside France and Switzerland, the species occurs in the Oxfordian of England (Arkell 1936; Hudson 1958; Reed 1936), Upper Oxfordian–Lower Kimmeridgian of Russia (Crimea, Caucasus) ( Gerasimov 1955; Pčelinčev 1927), and the Tithonian of Poland (Pomerania and Polish Lowlands) ( Lewinski 1923; Pugaczewska 1971). It has also been reported from the Callovian and Oxfordian of Somalia ( Cox 1935; Stefanini 1939) and the Upper Jurassic of China (Gu Zhi-wei et al. 1976).

Comparison. Praeexogyra acuminata (J. Sowerby, 1816) —The type species of Praeexogyra is widespread in Europe ( Pugaczewska 1971, p. 243) and common in the “ Acuminata Beds” of the Upper Bajocian and Lower Bathonian ( Arkell 1934, p. 7, for detailed biogeographic and stratigraphic distribution). Other than P. dubiensis , the type species is usually slender comma shaped, tapering posteroventrally, the LV is covered by prominent, evenly spaced concentric, weakly angulate swellings separated by wide interspaces and in rare cases shows faint antimarginal riblets at the umbo (ribbed variety, see Arkell 1934, p. 9).

“ Ostrea ” hebridica Forbes, 1851 (p. 110, pl. 5, figs. 4a–c)—The species originates from the Middle Jurassic of Loch Staffin (Isle of Skye, Inner Hebrides), Scotland. It differs from P. dubiensis in having a much larger and elongated shell, rather prominent and wider spaced concentric swellings, antimarginal riblets and furrows close to the umbo of LV, RV with long antimarginal riblets, and large shell chambers which are absent in the thin-shelled P. dubiensis . The long ligament area and the thick shell with many large chambers exclude “ O. ” hebridica from Praeexogyra (see remarks under that genus).

Praeexogyra monsbeliardensis ( Contejean, 1859) —The species co-occurs with P. dubiensis in the Kimmeridgian of the Reuchenette Formation. P. monsbeliardensis is usually larger, more capacious and of reniform shape. The attachment area is much larger and covered with antimarginal furrows and riblets (when exposed, Fig. 19.4–5), and the surface of the LV is ornamented with many commarginal growth swellings, separated by concentric depressions.

" Ostrea " sandalinoides de Loriol, 1901 (p. 110, pl. 6, figs. 15–16)—The species was first described from the Lower Oxfordian “Terrain à Chailles” (= Sornetan Member, Bärschwil Formation, cordatum Zone ) of the Jura Bernois (Montenay, Trembiaz) in NW Switzerland. De Loriol (1901) synonymized Oxfordian material from Ferrette (Haute-Rhin, France) that had been erroneously determined as Ostrea sandalina Goldfuss, 1833 , by Roeder (1882, p. 32, pl. 1, fig. 2). De Loriol’s species is also typical of Middle Callovian (jason Zone) glacial erratic boulders in Northern Germany (Pl. 15.3–4) and equivalent erratic boulders in NW Poland (material housed at the University of Greifswald, Germany). Although very similar to P. dubiensis , the species possesses a more pointed and stronger opisthogyrate umbo (Pl. 15.3b) and a smaller and shorter ligament area. The presence of weak antimarginal riblets of the RV distinguishes the species from Liostrea spp. Juvenile RVs tentatively attributed to this species have a Liostrea / Crassostrea -like prodissoconch (Pl. 6.5 and Pl. 10.3a–b).

Figures

FIGURE 17. Praeexogyra dubiensis (Contejean, 1859). 1 – 7. MC- 27 - E- 112, lectotype, designated herein, Oxfordian, Bussurel near Montbéliard, France. 1. LV lateral, scanned from Contejean (1859, pl. 21, fig. 4); 2, LV interior, scanned from Contejean (1859, pl. 21, fig. 5); 3, LV lateral, with cylindrical attachment scar, probably from an algal stem; 4, interior; 5, dorsal view; 6, anterior view; 7, magnified portion of dorsal region. Scale bars 5 mm.