Mytilus hauniensis ( Rosenkrantz, 1920 )

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 : 112-113

? Mytilus hauniensis ( Rosenkrantz, 1920)

Fig. 8 .

1920 Modiola hauniensis n. sp.; Rosenkrantz 1920: 37, pl. 2: 15a, b.

1970 Modiolus hauniensis ( Rosenkrantz, 1920) ; Anderson 1970: 92, pl. 9: 5a–c.

2016 Inoperna ? sp.; Hryniewicz et al. 2016: table 1 (not fig. 12E 1, E 2).

Material.— Three specimens, shells, including one from Vonderbank’s collection ( GPIBo 154 ) and two newly collected specimens, with one figured ( NRM Mo 183950), from upper Paleocene of Zachariassendalen ( NRM-PZ Mo 183950) and locality 500 m west of Trigonometric point 25 ( GPIBo 154 ), Spitsbergen, Svalbard .

Measurements.—L, 6.6–10.9 mm; H, 4.1–5.3 mm; W, 2.4– 3.7 mm; n = 3.

Remarks.—? Mytilus hauniensis was erected by Rosenkrantz (1920) on the basis of three mytiliform bivalves from the lower Paleocene Craniakalk, Copenhagen, Denmark. The only figures provided are two illustrations of a left valve ( Rosenkrantz 1920: pl. 2: 15a, b), which show a shell with commarginal ornament, a pronounced umbonal carina, a straight dorsal margin and an obtuse posterodorsal shell angle. These features are all present in the material described by Anderson (1970) from the upper Paleocene from locality 500 m west of Trigonometric point 25, Spitsbergen, Svalbard, although the specimens from Svalbard seem to a have less pronounced umbonal carina. The latter is likely caused by the smaller size of the specimens from Svalbard (Lmax 10.9 mm) as compared to those from Denmark (Lmax 36 mm). Anderson (1970) mentioned that 25 specimens were measured, with Lmax reaching 21.3 mm; however, we were able to examine only one specimen from his collection. The terminal umbones of this species and poorly preserved taxodont dentition in the umbonal area suggest placement in Mytilus Linnaeus, 1758 , rather than in Modiolus Lamarck, 1799 . However, we consider this placement tentative due to the rather poor quality of the material and possible taxodont dentition visible both anteriorly and posteriorly from the umbones instead of just anteriorly as in Mytilus . The species clearly does not belong to Inoperna (cf. Hryniewicz et al. 2016), which has external ornament of commarginal corrugations rather than commarginal growth lines.

Stratigraphic and geographic range.—Lower Paleocene of Denmark ( Rosenkrantz 1920) and upper Paleocene cold seep carbonates from the Basilika Formation, Zachariassendalen and locality 500 m west from Trigonometric point 25, Hollendarbukta, Spitsbergen, Svalbard.

Figures

Fig. 8. Mytilid bivalve?Mytilus hauniensis (Rosenkrantz, 1920) from the upper Paleocene, Basilika Formation, Zachariassendalen (A) and locality 500 m west from Trigonometric point 25, Hollendarbukta (B), Spitsbergen, Svalbard. A. NRM-PZ Mo183950, shell, left valve sculptured with fine commarginal growth lines superimposed on growth halts (A1), dorsal view of both valves (A2), oblique umbonal view showing delaminated prodissoconch, arrows point on poorly preserved taxodont teeth (A3). B. GPIBo 154, shell, right valve.