Introduction

Recently, Tomida and Kadota (2012) described Turbo (Marmarostoma) matsuzakiensis from the Shikura Limestone, which is intercalated with the pale green, tuffaceous, fine-grained sandstone of the middle Miocene Yugashima Group, at Shikura, Matsuzaki-cho, Kamo-gun, Shizuoka Prefecture (Figures 1, 2). The Izu Peninsula is known to have drifted northward as an island on the Philippine Sea Plate, and was located in the tropical West Pacific Ocean around 15°N during the middle Miocene (Hirooka et al., 1985). Concerning the geological age of the Yugashima Group in this area, Koyama (1986), Okada et al. (1986) and Okada (1987) reported that the Sakurada Formation, the middle part of the Yugashima Group, can be assigned to the calcareous nannofossil zone CN4 (14.9–13.5 Ma) (Okada and Bukry, 1980).

The occurrence of a gastropod fossil with an in situ operculum is not so common, but there are some reports from geological formations all over the world. For example, the following records are from Paleozoic deposits: Gordon and Yochelson, 1982 (Carboniferous Naticopsis); Linsley et al., 1989 (Carboniferous Naticopsis); Guvanov and Yochelson, 1994 (Silurian Australonema); Horny and Peel, 1995 (Silurian Tophicola); Peel and Horny, 1996 (Devonian Carcassonnella); Rohr et al., 2008 (Silurian Australonema), whereas such records from Japan are less common than outside Japan: Iwasaki, 1980 (Cretaceous Pila); Kase and Maeda, 1980 (Cretaceous Hayamia); Noda and Kikuchi, 1980 (Miocene Phanerolepida); Matsuoka and Nakamura, 1981 (Pleistocene Cipangopaludina); Majima, 1984 (Neogene naticid); Majima and Fukuta, 1986. Majima and Fukuta (1986) reported 29 fossil shells of Naticidae with an in situ operculum from Japanese Neogene formations, and divided them into 5 types depending on the shell morphology and operculum position. All of the above-mentioned fossil gastropods from the Japanese formations were sandy or muddy bottom dwellers. In almost all cases the operculum is missing from the shell, because the shells were mostly transported far from their habitat. It is rarer to find a rocky shore gastropod with an in situ operculum than a sandy or muddy shore gastropod. A fossil of the genus Turbo with in situ operculum has been found in a Japanese formation for the first time.

The purpose of this paper is to describe two specimens of T. (M.) matsuzakiensis that still retain their operculum, and to discuss the habitat of this species from the view-point of paleoecology.

Systematics

Repository.—The materials used in this study are housed at the Nagoya University Museum (abbreviated as NUM).

Figure 1.

Map showing the fossil locality (×) (cited from Tomida and Kadota, 2012). Topographic map adopted from 1:50,000 scale “Shimoda”, published by Geospatial Information Authority of Japan (Scale bar represents 1 km).

Figure 2.

Upper Neogene stratigraphy in the western part of Izu Peninsula showing the fossil horizon of Turbo (★) (redrawn after Tomida and Kadota, 2012). Calibration of calcareous foraminiferal zones and nannofossil zones modified after Saito (1999), and the time scale after Gradstein et al. (2004).

Materials.—Two shells with an in situ operculum (NUM-Fa183 and NUM-Fa184) were examined.

Locality.—The specimens used for the description were obtained from the Shikura Limestone, which is intercalated with the pale green, tuffaceous, fine-grained sandstone of the middle part of the Yugashima Group, at Shikura (34°44′27″N; 138°47′18″E), Matsuzaki-cho, Kamo-gun, Shizuoka Prefecture (Figures 1, 2).

Figure 3.

Turbo (Marmarostoma) matsuzakiensis Tomida and Kadota, 2012; shell and operculum (NUM-Fa183). A, basal view-1 (condition at time of collection); B, apertural view-1; C, dorsal view; D, basal view-2 (discovery of an in situ operculum during cleaning); E, apertural view-2, F, basal view-3 (condition of an in situ operculum after cleaning). Scale bar = 10 mm.

Figure 4.

Turbo (Marmarostoma) matsuzakiensis Tomida and Kadota, 2012; shell and operculum (NUM-Fa184). A, apertural view; B, dorsal view; C, sectional view of anterior part (discovery of an in situ operculum after cutting); D, aperturo-basal view; E, basal view (condition of an in situ operculum after cleaning). Scale bar = 10 mm.

Figure 5.

Turbo (Marmarostoma) matsuzakiensis Tomida and Kadota, 2012; operculum 1 (NUM-Fa183). A, outer view; B, inner view; C, antero-lateral view (the outer-lip side of the operculum was partly lost during cleaning, because the calcite recrystallized). Scale bar = 10 mm.

Discussion

The Shikura Limestone yields abundant Turbo (Marmarostoma) matsuzakiensis Tomida and Kadota (at present 23 specimens), accompanied by a few shallow-water molluscan species, e.g. Cerithium aff. nodulosum Bruguière, 1792 (3 specimens), Clypeomorus sp. (1 specimen), Cypraea sp. (1 specimen), Virgiconus sp. (2 specimens) and Chlamys sp. (2 specimens), and also by some other tropical elements such as the semi-autochthonous, large, limestone blocks of reef-building corals, large calcareous algae and a large calcareous foraminifer, Nephrolepidina japonica (Yabe, 1906). Judging from these fossils, the fauna of the Shikura Limestone is composed of tropical elements, and it seems that the molluscan species, e.g. Cerithium aff. nodulosum Bruguière was a sandy bottom dweller between the reef-building corals in the intertidal zone and the subtidal zone; Clypeomorus sp., Cypraea sp., Virgiconus sp. and Chlamys sp. are small-sized forms that dwelt on the shallow rocky bottom or directly on the reef-building coral in the intertidal zone. Moreover, there were very few molluscan fossils from the Shikura Limestone other than T. (M.) matsuzakiensis. Therefore, this fact shows the scarceness of large-sized mollusks in the reef-building coral colony. T. (M.) matsuzakiensis probably inhabited the edge of the coral reefs and rocky bottom, like T. (M.) argyrostomus Linnaeus, 1758. T. (M.) argyrostomus is an extant turban distributed in the adjacent seas of the Tanegashima-Yakushima and Ogasawara islands and southward, in the intertidal zone to 30 m water depth, on rocks (Sasaki, 2000).

T. (M.) matsuzakiensis includes individuals with heights ranging from 52.5 mm to 124.7 mm. For example, living T. (Batillus) cornutus [Lightfoot, 1786], including specimens as large as 130–150 mm in height, begin life as small juvenile shells inhabiting the intertidal zone to about 2 m water depth, but after 1–2 years as the mollusks (height more than 30± mm) grow older, they begin to transfer to deeper rocky sites of about 20 m water depth (Uno, 1962). No juvenile turbinid fossils have been found in the Shikura Limestone. Almost all the shells of T. (M.) matsuzakiensis have a well preserved top of the spire and aperture, and the blocks of the reef-building corals are large-sized. These facts indicate their autochthonous occurrence.