Gigantocapulus sp.

Kaim, Andrzej, Jenkins, Robert G. & Hikida, Yoshinori, 2009, Gastropods from Late Cretaceous Omagari and Yasukawa hydrocarbon seep deposits in the Nakagawa area, Hokkaido, Japan, Acta Palaeontologica Polonica 54 (3), pp. 463-490 : 484-485

Gigantocapulus sp.

Fig. 15 .

Material.—Single juvenile shell from Yasukawa.

Dimensions.—The shell UMUT MM30196 ( Fig. 15 ) is 4.4 mm high, 5.9 mm long, and 4 mm wide.

Description.—The small shell is limpet−shaped with high elevated apex. The apex is located anteriorly (or posteriorly depending on interpretation of the animal). The majority of the shell is ornamented by thick concentric undulations. The shell becomes irregular in its anteriormost part displaying irregular shell growth expressed by meandering undulations. Aperture elliptical. Shell composed of three layers: outermost layer very thin and most probably fibrous prismatic. Medial layer also thin and crossed foliate recollecting irregular crossed foliate microstructure of Fuchigami and Sasaki (2005). The microstructure of the thick inner layer remains unrecognized.

Discussion.—The taxonomic position of Gigantocapulus has been recently reviewed by Beu (2007), who stated that this genus may represent a tergomyan monoplacophoran, a vanicoroid gastropod, or a separate, extinct gastropod group. It is beyond the scope of this paper to argue on high level classification of this mollusk, however, the presence of crossed foliated layer may suggest patellogastropod affinity.

Figures

Fig. 15. A juvenile specimen (UMUT MM30196) of Gigantocapulus sp., a mollusk of uncertain taxonomic affinity in lateral (A, B), apical (C), and posterior (D) views. E. Cross section through the shell displaying three layers: an outer, possibly fibrous prismatic (fp), a middle cross foliate (cf), and an inner layer of uncertain microstructure. F. Another section showing the same layers. G. Close up of the foliated structure, uppermost part of the layer consist of polygonal plates resembling broken semifoliated structure or detached tablets of nacreous structure.