| mhorn | Дата: Sunday, 14.03.2010, 18:54 | Сообщение # 1 |
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| Nakada K. (2008) The first occurrence of Petranoceras, Harpoceratinae from the Lower Jurassic Toyora Group in Yamaguchi Prefecture , Southwest Japan // Bulletin of the Firefly Museum of Toyota Town. no.1. P.85-91. [in Japanese]
pdf: http://60.43.213.120/pamphlet/images/bul%202008/2008nak.pdf
The Toyora Group, Lower Jurassic continental shelf sediments exposed in the western part of Yamaguchi Prefecture, is known as the major source of Early Jurassic ammonoid fossils. This article reports the first occurrence of Petranoceras from the Toyora Group and discusses its significances. The Toyora Group is divided into the Higashinagano, the Nishinakayama, and the Utano Formations in ascending order, and the ammonoid fossils mainly occur within the Nm member, lower part of the Nishinakayama Formation. The specimen examined in this study was obtained from the lowermost part of the Nm Member, exposed around the entrance of the Sakuraguchi-dani Valley. It is characterized by somewhat involute whorls, by wide interspaces of ribs, and by some lirae between the ribs. These characters are similar to those of Petranoceras rinaldinii VENTURI, reported from the lower Toarcian in central Italy, but the whorl of this specimen is more involute. Because of the difference, the specimen is treated as Petranoceras sp. aff. P. rinaldinii VENTURI. This is the first occurrence of this genus in Japan, and the horizon (the lowermost part of the Nm Member) seems consistent with the lower Toarcian. Moreover, the occurrence of this genus supports the faunal similarity between the Toyora Group and the Mediterranean province which was pointed out by previous study.
Middle Jurassic - Lower Cretaceous ammonites & aptychi
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