Praeexogyra monsbeliardensis ( Contejean, 1859 )

Koppka, Jens, 2015, Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland — Ostreoidea, Zootaxa 3927 (1), pp. 1-117 : 46-49

Praeexogyra monsbeliardensis ( Contejean, 1859)

Figs. 18–19 ; Pl. 12.3–4; Pl. 13; Pl. 14.1–4; Pl. 21 * 1859 Ostrea Monsbeliardensis Contej. —Contejean: p. 321, pl. 26, fig. 1–4. 1861 Ostrea Monsbeliardensis Ctj. —Thurmann & Etallon (in 1861–1864): p. 53. 1862 Ostrea Monsbeliardensis Contj. —Thurmann & Etallon (in 1861–1864): p. 272, pl. 38, fig. 8. 1867 Ostrea Monsbeliardensis, Contej. —Favre: p. 471.

1870 Ostrea Monsbeliardensis, Ctj. —Greppin, J.-B.: p. 104.

1875 Ostrea Monsbeliardensis Contj. —Blake: p. 207, 221.

1892 Ostrea monsbeliardensis, Contej. —Roberts: p. 76.

1897 Ostrea monsbeliardensis Contej. —Reed: p. 45.

1971 Liostrea monsbeliardensis ( Contejean, 1859) —Pugaczewska: p. 260, pl. 17, figs. 5–9.

Lectotype (designated herein). Ostrea monsbeliardensis Contejean 1859 (pl. 26, figs. 1–3), MC-27-E-141: bivalved specimen (Fig. 19.1–2). Rôce in Sainte-Suzanne, Montbéliard (Department Haute-Saône), France. Coordinates: ca. 47°30'20.33''N, 6°47'51.00''E (exact position of locality unknown). Lower Kimmeridgian, cymodoce Zone, niveau 6 sensu Contejean (1859), "Marnes à Ptérocères" (facies and time-equivalent of the Banné Member, Reuchenette Formation in Switzerland) = "Marnes du Rang", Contini & Hantzpergue 1973.

The label associated with the four figured syntypes of Contejean (1859, pl. 26, figs. 1–4) mentions Beauregard and Rôce as “ type ” localities and Marnes of the niveau 6 of Contejean (= “Marnes et Calcaires à ptérocères” [marls and limestones with Harpagodes ] as reference to the horizon. Obviously, the label mentioned two different localities and the material might have been assembled in one box after removing the specimens from the original tray. Both sites are close to each other, Beauregard being a locality ca. 3 km northeast of Montbéliard and Rôce referring to the ancient locality "Côte de Rôce" ( Contejean 1859, p. 203) in the district Sainte-Suzanne in Montbéliard ( Fig. 3 ). However, Contejean (1859, p. 72) mentioned the occurrence of Ostrea monsbeliardensis in Rôce, but not in Beauregard and the name of the species is apparently the latinized form of Montbéliard (“Monsbeliard”). It appears reasonable to assume Rôce in Montbéliard rather than Beauregard as type locality. Remains of a brown-grey marl on the lectotype specimen are typical of weathered samples of the “Marnes à Ptérocères” or “Marnes du Rang” of the cymodoce Zone.

Material. Lectotye (see above), and Paralectotypes—MC 27E-138–140, and 142; Musée Cuvier in Montbéliard. Paralectotype MC-27E-139, a LV not figured in Contejean (1859), was prepared by the author to reveal the shell interior (Fig. 18.1–3).

Others—More than 20 specimens from the Banné Marls (cymodoce Zone, Lower Kimmeridgian, Reuchenette Formation) of Vâ Tche Tchâ near Courtedoux. One juvenile LV from the Banné marls of Glovelier (Glov. 5.2.98 MB/3-60, FPJ-collection, Fig. 18.4, Pl. 13.3). For measurements see Appendix, Table 7 .

Description. Shape, size—Thin-shelled, strongly inequivalve, outline oval, subrectangular to subtrigonal or slightly reniform, usually higher than long. LV capacious, scoop-shaped and slightly opisthocline towards the posteroventral margin, attachment area usually large; posterior margin weakly concave, in some specimens well curved and with a weak to well-developed posterior sulcus (Pl. 12.4d; Pl. 14.3a). Most RVs evenly flat (Pl. 13.2a, 6a), occasionally with concave depression close to ventral margin (Pl. 14.3a) or dorsally slight convex. Examined specimens 2.2–4.0 cm high.

Sculpture—LV with squamous lamellae and swellings, interspaces with fine commarginal growth lines. Radial sculptures rare; fine divaricating furrows on ventral edge of attachment area (Fig. 19.4–5), in some specimens also a few antimarginal furrows close to the ventral margin (Pl. 13.4b,d). RV covered by narrowly spaced concentric growth lines with few growth squamae; occasionally thin plicae near ventral margin (Pl. 13.5b). RV shell surface usually heavily bioeroded by grazing regular echinoids (Pl. 12.3b, 4c); details of original shell structure thus rarely preserved.

Ligament area—Somewhat oblique, usually prosodetic, in some specimens opisthodetic (Pl. 13.5; Pl. 14.2).

Internal shell characters—LV dorsally with narrow commissural shelf; umbonal cavity small, better developed in small (“juvenile”) shells. PAM high-oval, large in relation to shell size, somewhat posterior to the centre; small Quenstedt muscle in some RVs, around 2–3 mm below the posterior bourrelet (Pl. 13.2c, 6b). No chomata.

Microstructure—Mostly regular foliated ( RF) with some complex CF, without chambers (Pl. 21.1a–h).

Prodissoconch, juvenile—Earliest shell stages not preserved.

Paleoecology. The oyster is often found in small groups (Pl. 12.3a) settling on shells of Isognomon , Eopecten , or flat parts of Trichites . Complete articulated shells in situ associated with large and complete Trichites shells are common in the soft marl intercalations of the Banné Member. All in situ RVs and many Trichites shells show gnawing traces of Gnathichnus pentax Bromley , caused by regular echinoids such as Pseudocidaris thurmanni ( Agassiz, 1840) , suggesting grazing on living animals (Pl. 12.3b, 4c).

Several regular echinoid species co-occur in this part of the section and prove fully marine conditions, whereas the highly diverse and abundant epibenthic bivalve fauna of more than 80 species indicates nutrient-rich seawater. It is therefore assumed that P. monsbeliardensis has been adapted to a fully marine, low energy, marly environment which is typical of the middle and upper section of the Banné Marls.

Occurrence. P. monsbeliardensis is known from the Oxfordian and Lower Kimmeridgian of France and Switzerland and from the Kimmeridgian of England and Poland ( Pugaczewska 1971).

Comparisons. Praeexogyra acuminata (J. Sowerby, 1816) (p. 82, pl. 135, fig. 2)—The type species has already been discussed under P. dubiensis . The differences to P. monsbeliardensis are the same: pointed umbo, usually strongly curved posterior margin, the generally more slender, comma-shaped outline , more prominent growth squamae, and occasional fine antimarginal ribs close to the umbo. Similar riblets have only been found on the margin of the attachment area of the lectotype of P. monsbeliardensis but not on the free parts of the shell (Fig. 19.4–5).

Praeexogyra dubiensis ( Contejean, 1859) (p. 320, pl. 21, figs. 4–11)—This species co-occurs in the Reuchenette Formation, but overall rare in the Kimmeridgian. Compared to P. monsbeliardensis , it is only attached by a tiny part of the umbo, and is also smaller, less opisthogyrate, and more triangular or drop-shaped. The concentric swellings on the LV are less developed. The species seems to have preferred somewhat higher energy water (e.g. oolitic/biodetritic shoals), whereas P. monsbeliardensis appears to have been restricted to the lower energy marls and marly limestones of the middle and upper parts of the Banné Member.

" Ostrea " hebridica Forbes, 1851 (p. 110, pl. 5, figs. 4a–c)—This species has been discussed and illustrated in detail by Arkell (1934, p. 9–15, pls. 3–4) from the Bathonian of England and Scotland (see also discussion under P. dubiensis ). The species differs from P. monsbeliardensis by its larger size and more elongate and posteriorly stronger recurved shape (except for the elongated, straight morphotype). The attachment area is much smaller, and the umbo more pointed and stronger opisthogyrate. The distance between the marked growth squamae is at least twice as wide as in P. monsbeliardensis . The ostreoid hinge is similar to the one known from P. monsbeliardensis (compare Arkell 1934, pl. 4, figs. 6, 13–14, 16 with pl. 8, figs. 2–3), but the ligament area is larger (in comparison more than twice as high). Both species are known to have radial furrows and ribs in the dorsal third of the LV. But in O. hebridica the radial elements are also visible (if developed) on the LV behind the attachment area, whereas they are restricted to the attached part in P. monsbeliardensis . Long antimarginal riblets are well developed on the RV of some specimens ( Arkell 1934, pl. 4, fig. 8) and much stronger than in P. monsbeliardensis . Furthermore, large chambers in the LV of P. hebridica are unknown from the thin-shelled P. monsbeliardensis .

" Ostrea " pustulosa Sharpe, 1850 (p. 188, pl. 24, 4a,b)—The species was originally described by Sharpe (1850) from the Kimmeridgian of Torres Vedras/Sobral in Portugal. Fürsich (1981), Fürsich et al. (2009), Schneider (2009) and Schneider et al. (2010) included it in Praeexogyra . However, the two figured syntypes of Sharpe (1850) do not fit in the diagnosis by Stenzel (1971) or the modified diagnosis of Praeexogyra used in this study.

In contrast to P. monsbeliardensis the LV is much thicker, the shape oval to subrectangular, somewhat longer than high and not recurved. The LV surface ( Sharpe 1850, pl. 24, fig. 4a) is dorsally covered with broad radial ribs and rough concentric growth squamae. The ligament area of the other LV ( Sharpe 1850, pl. 24, fig. 4b) is relatively high and broad (around one half of the shell length), the hinge is massive with prominent bourrelets and a deep resilifer. The juvenile part of the resilifer is curved anteriorly (prosodetic), but the resilifer is in general orthogyrate directed. This species is probably better assigned to Helvetostrea n. gen.

Liostrea ? moreana ( Buvignier, 1852 (p. 26, pl. 16, figs. 41–43) —This Oxfordian species was originally described from the Department de la Meuse and is widely distributed in France, England and Poland (Arkell 1932 in 1929–1937, p. 157–159). The generic position is uncertain (Arkell 1932 in 1929–1937, p. 158), but the majority of modern authors refer it to Liostrea .

This species is somewhat larger, around twice as long as the average adult shell of P. monsbeliardensis , but the shape, attachment size and shell inflation appear comparable. Differences are a weakly concave or straight posterior margin, the terminal and centrally situated umbo (which is not opisthogyrate), and a remarkably wide ligament area with a broad resilifer, which is at least three times as wide as in P. monsbeliardensis . The most upturned region of the shell is posteroventral in Liostrea ? moreana but anteroventral in P. monsbeliardensis . The round and dorsally convex PAM is close to the centre in L.? moreana (cf. Buvignier 1852, pl. 16, figs. 41, 43; Arkell 1932, text-fig. 27) but high-oval and situated close to the posterior margin in P. monsbeliardensis (Pl. 14.2a). Further information especially concerning its shell microstructures and chambering would be needed to clarify the generic position of L.? moreana (see also under Circunula cotyledon ).

Circunula cotyledon ( Contejean, 1859) (p. 319, pl. 24, fig. 15–17)—This species co-occurs with Praeexogyra monsbeliardensis in the Banné Marls and is occasionally attached to the same object (e.g. Trichites ) which may lead to confusion. C. cotyledon , however becomes twice as long and several millimetres thick, its attachment area is larger and the ventral margin less bent upwards. The shape is more circular and the ventral margin of the LV nearly smooth without traces of commarginal swellings which are typical of P. monsbeliardensis . The ligament area is broader; the PAM is rounder and more centrally situated, somewhat inserted and commonly ventrally buttressed in both valves, which is in strong contrast to the non-inserted and not elevated PAM of P. monsbeliardensis .

Figures

FIGURE 3. Map of locations in northwestern Switzerland and adjacent areas in France (region of Montbéliard). 1 – 9. Excavation sites of the Paléontologie A 16 on the Transjurane highway (A 16) and nearby situated outcrops (Ajoie, Canton Jura, Switzerland): 1, Courtedoux – Vâ Tche Tchâ (CTD-VTT); 2, Courtedoux – Bois de Sylleux (CTD-BSY); 3, Courtedoux – Tchâfouè (CTD-TCH); 4, Courtedoux – Sur Combe Ronde (CTD-SCR); 5, Chevenez – La Scierie (CHE-CHS); 6, Porrentruy – Tunnel du Banné (POR-TLB, same position as the Banné hill); 7, Porrentruy – Cras d’Hermont (POR-CRA); 8, Courgenay – Alombre aux Vaches (CGN-ALO); 9, Boncourt – Queue au Loup (BON-QLP). 10 – 14. Localities of Contejean (1859) in the area of Montbéliard (Franche-Comté, France): 10, Bussurel close to Héricourt (Department Haute-Saône); 11, Beauregard near Châtenois-les-Forges (Department Territoire de Belfort); 12, Rôce in Sainte-Suzanne, Montbéliard (department Doubs); 13, Arbouans (Department Doubs); 14, La Baume in Audincourt (Department Doubs). 15 – 17. Swiss localities for compared material: 15, Glovelier (Canton Jura); 16, Liesberg, clay pit Andil (Canton of Basel-Country); 17, Blauen (Canton of Basel-Country).

FIGURE 18. Praeexogyra monsbeliardensis (Contejean, 1859). 1 – 3. LV, MC- 27 E- 139, paralectotype, Contejean Collection (Musée Cuvier in Montbéliard, France): 1, interior; 2, anterior; 3, lateral, with large attachment area; scale x 1. 4. Juvenile LV, attached to Camptonectes auritus, FPJ Glov. 5.2. 98 MB / 3 - 60, Glovelier (Banné Marls, Lower Kimmeridgian).

FIGURE 19. Praeexogyra monsbeliardensis. 1 – 5. Lectotype, MC- 27 - E- 141 (designated herein): 1, refigured drawing of Contejean’s (1859) fig. 1 of pl. 26; 2, RV lateral; 3, LV anterodorsal view; 4, LV, attachment area; 5, magnified portion of ventral margin, with antimarginal furrows; “ Côte de Rôce ” in Montbéliard, France, Lower Kimmeridgian, cymodoce Zone. Scale bars 10 mm (Figs. 1 – 3); 5 mm (Figs. 4 – 5).