<国立科博専報>四国の三畳紀オウム貝化石

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Although many kinds of nautiloids have been discovered in the Mesozoic rocks of all parts of the world, only several species of nautiloids were reported from Japan. The present paper deals with the result of the study of a Triassic nautiloid recently collected from the Zohoin Group of Sakawa, Shikoku. The Zohoin Group is subdivided into five formations, i.e. green tuffaceous mudstone, black muddy sandstone, black mudstone, greenish grey fine-grained sandstone, black sandy mudstone, in ascending order. The nautiloid fossil, associated with many individuals of Daonella spp., was found in the upper sandy part of the black mudstone formation. The locality of the specimen obtained is shown in the index map of Figure 1. Figure 2 shows the geological route map and the geological section of the area. The stratigraphic position of the fossil nautiloid is given in the geological columnar section of Figure 3. The Zohoin Group is assigned to the upper Ladinian, because it yields Protrachyceras aff. archelaus and P. cf. pseudoarchelaus, the upper Ladinian indices. The specimen is photographed in Figure 4. The restored figure is demonstrated in Figure 5. From the septated shell preserved, the following morphologic characters are concluded. The individual seems to have grown up to about 60 mm. or more in diameter. The shell has a fairly wide umbilicus, D=60 and U=30 in mm., showing very evolute involution. The umbilicus is rather deep, surrounded by a high wall which is slightly convex and subrounded to subangular at rim. The whorl is subrectangular in section, rather compressed in the proportion of breadth to height, i.e. H=18.7 and B=17.0 in mm. The maximum breadth is at about middle of the flank. Flanks are nearly parallel, gently inflated, and rather abruptly pass into the flattened venter. The ventrolateral edge is rather sharp, forming the subangular to subrounded shoulder. There are numerous spiral lirae on the surface of the shell. The numbers of lirae are counted about forty-five at the height of 18.7mm. The suture is simple, gently concave on the flank, and is slightly prorsiradiate in the ventro-lateral area. It shows a rather convex curvature on the umbilical wall, but on the ventral side it shows a fairly wide and shallow lobe. Thus, in the shell form and the sutural pattern, the present specimen bears some resemblance to the species belonging to the genus Grypoceras. Grypoceras seems to be a cosmopolitan, having been reported from Europe, Himalayas, California, Nevada and Idaho. The fact that this specimen was found in the Zohoin Group is interesting. The occurrence of Grypoceras might be expected in other areas of Japan.
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<国立科博専報>四国の三畳紀オウム貝化石

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