Discovery of the early Spathian (late Olenekian, Early Triassic) ammonoid Tirolites in the Hiraiso Formation, South Kitakami Belt, Northeast Japan

作者：Shigeta Yasunari^*; Nakajima Yasuhisa

来源：Paleontological Research, 2017, 21(1): 37-43.

DOI：10.2517/2016PR017

• ammonoid
• Hiraiso Formation
• South Kitakami Belt
• Spathian
• Tirolites
• Triassic

Until now, the Hiraiso Formation was considered to be entirely of Smithian (early Olenekian, Early Triassic) age, but the discovery of the ammonoid Tirolites cf. ussuriensis in the lower part of the formation on the rocky coast between the Maekawara and Akaushi ports, Motoyoshi Town, Kesennuma City, in the South Kitakami Belt, Northeast Japan, establishes that this portion is actually equivalent to the upper part of the lower Spathian Tirolites-Amphistephanus Zone (upper Olenekian). The fauna of the Hiraiso Formation, which includes the large bivalve Eumorphotis iwanowi and crinoid ossicles, very much resembles the Tirolites ussuriensis beds in South Primorye, Russian Far East, suggesting a wide distribution of this shallow marine fauna in the low to middle northern latitudes on the western side of the Panthalassa during the early Spathian. Because preSpathian deposits contain only small-sized species of Eumorphotis and very rare crinoid ossicles, the appearance of large species of Eumorphotis and the flourishing of crinoids in the shallow marine environment during the early Spathian suggest that the Smithian-Spathian boundary marks a major change in the marine ecosystem.

• 出版日期2017-1