Oxytoma octavia (d’Orbigny, 1850)

Hryniewicz, Krzysztof, Little, Crispin T. S. & Nakrem, Hans Arne, 2014, Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard, Zootaxa 3859 (1), pp. 1-66 : 23-25

Oxytoma octavia (d’Orbigny, 1850)

( Figure 10 C–G )

1850 Avicula Octavia sp. nov.—d’Orbigny, p. 61.

1984 Oxytoma (Oxytoma) octavia (d’Orbigny) —Kelly, p. 61, pl. 5, figs. 1–9 and references therein.

2011 Oxytoma sp. — Hammer et al. 2011, fig. 7q, tab. 2.

Material. 37 specimens, all single valves with completely or partially preserved shells. Four right valves, 33 left valves. See Appendix 1 for the list of specimens.

Dimensions. 7–24.5 mm in length, 7–23.5 mm in height. See Figure 11 A–C and Appendix 2F for details.

Description. Adult shell more than 24 mm long, inequivalved with left valve significantly larger than right. Left valve oval and weakly posteroventrally elongated; thin, inequilateral, moderately inflated, ornament of 15–20 radial primary ribs, intercalated with secondary riblets and occasionally very weak tertiary riblets. Primary radial ribs dense, but thin, on early shell growth stages, progressively thicker during ontogeny. On internal moulds ribs visible only on later growth stages. Inter-rib spaces around five times wider than primary ribs. Secondary ribs appear on shell in late growth stages, but on internal moulds are present only on ventral margins of larger specimens. Ribs occasionally show disturbances and deflections. Shell growth lines weak; no nodes present at radial ribs intersections. Beak weakly prosogyrate, hinge line straight. Posterior auricle present; in investigated specimens not projecting beyond posterior valve margin, demarcated from the rest of valve by weak, oblique sulcus. Anterior auricle absent, groove in inner shell surface below anterior part of hinge line visible on outer shell surface as an oblique, elongated ridge. Right valve thin, circular to weakly oval in outline; inequilateral, flat to weakly inflated. Radial ribs present on external shell surface, absent on internal moulds. Beak weakly prosogyrate, hinge line straight, posterior auricle moderately long, projecting beyond posterior valve margin; indistinct sulcus demarcating auricle from rest of valve. Anterior auricle straight, triangular, acute, around a fifth to a sixth of hinge line length. Triangular and moderately deep byssal notch developed below anterior auricle, with main axis slightly oblique to the hinge line. Ctenolium not observed.

Remarks. Our specimens are synonymized with Oxytoma octavia (d’Orbigny, 1850) due to their strong secondary ribs on the right valve, which is markedly smaller than the left valve. Oxytoma octavia is inequivalved and characterized by having 12 to 20 primary ribs on the left valve (12 to 19 in Kelly 1984, p. 62), separated by distinct secondary and tertiary riblets (e.g. Kelly 1984, text-fig. 40). The rather similar O. inequivalve (J. Sowerby, 1819) , widespread from the Lower Jurassic up to the Kimmeridgian (e.g. Gerasimov 1955; Ichikawa 1958; Cox 1965; Duff 1978; Wierzbowski et al. 1981; Pugaczewska 1986; Clausen & Wignall 1990) has a left valve with a similar number of radial ribs to O. octavia and with inter-rib spaces containing variably strong secondaries and tertiaries (e.g. Duff 1978, p. 55). The difference between O. octavia and O. inequivalve lies in the presence of primary and secondary ribbing on the right valve in O. octavia contrasting with the smooth or weakly striated right valve of O. inequivalve ( Duff 1978; Kelly 1984). The right valves of our specimens are small and circular, which suggests an inequivalve condition, separating our species from O. expansa ( Phillips, 1829) which is approximately equivalved (e.g. Arkell 1931, p. 191). Our specimens are much less inequilateral and have a very different style of ribbing than O. (Boreioxytoma) aucta Zakharov, 1966 , from the Volgian of Siberia ( Zakharov 1966), which has seven to nine widely spaced primary ribs ( Zakharov 1966, pl. I: 6, pl. II:1).

Occurrence. Seeps 3, 5, 8 and 9 (Upper Volgian–uppermost Ryazanian), Slottsmøya Member, Svalbard ( Tab. 1 ). Occurs in the Volgian–Ryazanian of Europe, Greenland, Russia and Siberia (de Loriol & Pellat 1867; Lewinski 1922; Spath 1936; 1947; Gerasimov 1955; Zakharov 1966; Fürsich 1982; Birkenmajer et al. 1982; Kelly 1984).

Palaeoecology. Oxytoma octavia was a byssally attached filter feeder. This mode of life is shared by many other pteriomorphs ( Stanley 1970). The long posterior auricle likely sheltered the posterior exhalant current from being swept back into the shell by water currents, as in pteriids ( Stanley 1970). Because of the relatively thin valves, O. octavia was more likely preserved in less agitated environments, where it was living attached to loose shells and local carbonate hardgrounds. A pseudoplanktonic mode of life and attachment to drifting algal fronds is also possible for O. octavia , an interpretation supported by the lack of articulated specimens in the studied material.

Figures

FIGURE 10. (A–B) Pectinida gen. et. sp. indet. (A) Cross section of an articulated specimen. (B) Internal mould of the upper valve. Note possible adductor muscle scar and a line close to the dorsal margin, representing a possible dorsal furrow. (C–G) Oxytoma octavia (d’Orbigny, 1850). (C) Lateral view of a small left valve internal mould with fragments of the shell preserved showing weak radial ornament. (D) Enlarged view of anterodorsal portion area showing weak byssal notch. (E) Lateral view of a large left valve showing well developed radial ornament composed of primary and secondary ribs. (F) Lateral view of a right valve; lack of ornament is caused by shell delamination. Note very weak posterior sulcus and flattening of the valve. Probable shape of auricle is shown with dashed line (evidence from other partial specimens). (G) Plasticine cast of right valve external mould. Note radial ornament composed of primary and secondary ribs. (H–K) Camptonectes (Costicamptonectes) aff. milnelandensis Fürsich, 1982. (H–I) Right valve internal mould with shell partially preserved showing some divaricating ornament preserved close to the posterior shell margin. (J–K) External mould of right valve of a specimen from the Dorsoplanites bed, showing radial ribs close to the anterior shell margin and divaricating ornament preserved close to the posterior shell margin. (L–O) Camptonectes (Camptochlamys) clathratus (Roemer, 1836). (L–M) Lateral view of poorly preserved left valve showing cancellate external ornament in ventral margin. (N–O) Lateral view of a disarticulated and compressed shell from the Dorsoplanites bed showing cancellate ornament. Note distinct anterior auricle in the right valve. (P–R) Pseudolimea arctica (Zakharov, 1966). (P) Lateral view of left valve internal mould with some shell preserved around the ventral margin, with V-shaped ribs on the outer shell surface and corresponding rounded ribs on the internal mould. (Q) Lateral view of right valve, with anterior and posterior auricles and lack of secondary ribs caused by shell delamination. (R) Silicone rubber cast of small area of shell surface showing detail of external ornament, with large primary ribs and smaller secondary riblets in the interrib spaces. (A) PMO 226.609, (B) PMO 226.610, (C–D) PMO 217.562, (E) PMO 217.555, (F) PMO 217.214, (G) PMO 217.570, (H–I) PMO 226.606, (J–K) PMO 226.604, (L–M) PMO 226.605, (N–O) PMO 226.603, (P) PMO 217.194, (Q) PMO 217.353, (R) PMO 217.370. Scale bars 5 mm (A–I), 10 mm (J–R).

FIGURE 11. Measurements of Oxytoma octavia (d’Orbigny, 1850) (in mm). (A) Height to length. (B) Length of hinge line to length. (C) Length of the posterior part of the hinge line to total length of the hinge line.