Inoperna plenicostata ( Anderson, 1970 )

Hryniewicz, Krzysztof, Amano, Kazutaka, Bitner, Maria Aleksandra, Hagström, Jonas, Kiel, Steffen, Klompmaker, Adiël A., Mörs, Thomas, Robins, Cristina M. & Kaim, Andrzej, 2019, A late Paleocene fauna from shallow-water chemosynthesis-based ecosystems, Spitsbergen, Svalbard, Acta Palaeontologica Polonica 64 (1), pp. 101-141 : 114-115

Inoperna plenicostata ( Anderson, 1970)

Fig. 10 .

1970 Mytilus plenicostatus n. sp.; Anderson 1970: 93, pl. 9: 6–7. 2016 Inoperna ? sp.; Hryniewicz et al. 2016: table 2, fig. 12E.

Material.— Six specimens, all fragmentarily preserved shells, including three ( GPIBo 155–156 ) from the upper Paleocene , locality 500 m west of Trigonometric point 25, Hollendarbukta, Spitsbergen, Svalbard , and three ( NRM-PZ Mo 183947–183949) specimens from the upper Paleocene of Zachariassendalen , Spitsbergen, Svalbard .

Measurements.—L, 11.4–~ 59.8 mm; n = 2. Other measurements difficult to estimate due to the poor quality of the material.

Description.—A description of the species was provided by Anderson (1970: 94). We hereby provide a description of juvenile specimens, which were not studied previously.

Earliest growth stages unknown. Shell shorter than 1.75 mm ornamented with regularly spaced commarginal corrugations with similarly wide depressions separating them. Commarginal growth lines superimposed on fine radial ornament visible in commarginal depressions but diminishing on commarginal corrugations. Possible growth halt visible ~ 0.75 mm from umbones; second growth halt at ~ 1.75 mm, roughly coincides with change of ornamentation from regular and strong commarginal ornament superimposed on fine radial ornament to less regular and weaker commarginal currugations and no radial ornament. Latter ornamentation pattern present on remainder of shell; commarginal ornament of largest specimen ( GPIBo 155; L ~ 59.8 mm) grouped in bundles of 3–5, whereas commarginal corrugations on shorter specimens show no obvious grouping. Anterior adductor muscle scar minute, rounded, located ventrally to umbo and separated from pallial line. Pallial line visible only in anteroventral part of shell; remainder unknown.

Remarks.—Some morphological features of this mytilid from Spitsbergen suggest its inclusion in the poorly known genus Inoperna Conrad, 1875 . The specimens from locality 500 m west of Trigonometric point 25 are partial shells with a slightly different shape from the specimens from Zachariassendalen. The holotype ( GPIBo 155; L ~ 59.8 mm) is an incomplete mytiliform shell with the posterior lobe broken off; therefore, it was originally larger than the measurement given above. The shell has a terminal umbo and no well-defined carina, whereas the paratype ( GPIBo 156; L ≥ 11.4 mm) has a more complete, modioliform shell and a subterminal umbo. Shell shape changes during mytiloid ontogeny (e.g., Fuller and Lutz 1989; Cosel and Olu 1998; Saether et al. 2010); therefore, the observed shape differences among specimens of different size are likely an ontogenetic feature. In general, the shell shapes observed in the material from Svalbard are similar to those known from other species of Inoperna (reviewed by Stenzel et al. 1957: 74–75). The curved mytiloid shell of I. plenicostata is somewhat reminiscent of some specimens of I. flagellifera Forbes, 1845 (e.g., Forbes 1845: pl. 16: 9; Woods 1900: pl. 17: 1), especially of the specimens from the Santonian (Late Cretaceous) Hochmoos Formation of the Gosau Group in Austria ( Dhondt 1987: pl. 1: 13). Most of the species of Inoperna are known mostly from partially preserved specimens (e.g., Stephenson 1923; Wade 1926; Popenoe 1937); therefore, it is difficult to determine their external shapes, and to estimate the degree to which the shell shape of Inoperna is taxonomically or phenotypically controlled.

The holotype ( GPIBo 155) of Inoperna plenicostata has a characteristic external ornament composed of commarginal corrugations arranged in groups of three to five, separated by broader furrows devoid of any visible ornament. The paratype ( GPIBo 156) has commarginal corrugations not grouped into bundles. We infer that this difference in ornamentation between two type specimens is likely an ontogenetic feature, as the two are of very different size. An ornamentation composed of commarginal corrugations is typical for Inoperna ( Woods 1900; Stephenson 1923; Wade 1926; Popenoe 1937). The ornament of I. plenicostata differs from that known from other species of Inoperna because the corrugations do not merge close to the dorsal margin to form broader commarginal folds as seen in I. carolinensis ( Stephenson 1923: pl. 62: 14; Wade 1926: pl. 13: 10) and I. flagellifera (e.g., Dhondt 1987: pl. 1: 11–13), and, to some extent, I. bellarugosa ( Popenoe 1937: pl. 45: 7 [not fig. 6]). The commarginal corrugations of most of the species are more regular than those of I. plenicostata , but we are unable to determine to what extent this feature is of taxonomic significance.

Stratigraphic and geographic range.—Upper Paleocene cold seep carbonate and associated sunken driftwood from the Basilika Formation, Zachariassendalen and locality

500 m west from Trigonometric point 25, Hollendarbukta, Spitsbergen, Svalbard.

Figures

Fig.10.Mytilid bivalve Inoperna plenicostata (Anderson, 1970) from the upper Paleocene,Basilika Formation,Zachariassendalen, Spitsbergen,Svalbard. A. NRM-PZ Mo 183949, shell, dorsal view of both valves (A1), right valve (A2), enlarged dorsal view of both valves, showing ornamentation of the early growth stages (A3), oblique anterodorsal view (A4). B. NRM-PZ Mo 183947, partial shell, left valve (B1), ventral view of both valves (B2). C. NRM-PZ Mo 183948, partial shell, left valve (C1), ventral view of partially preserved valves (C2), oblique anterior view of left valve, showing anterior adductor muscle scar (C3).