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1 April 2021 A New Species of the Heteromorph Ammonoid Didymoceras from the Upper Cretaceous Izumi Group in Shikoku, Southwestern Japan, and Its Evolutionary Implications
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Abstract

In this study, a new late Campanian (Late Cretaceous) heteromorph ammonoid, Didymoceras morozumii Misaki and Tsujino sp. nov., from the Hiketa Formation, Izumi Group, southwestern Japan, is described. It is characterized by a loose helical post-embryonic early whorl and relatively tight helical middle whorls, followed by a retroversal hook with a highly elevated apertural end, and two rows of irregular tubercles. Didymoceras morozumii sp. nov. occurs in the upper part of the Didymoceras sp. Zone, which is overlain by the Didymoceras awajiense Zone. Didymoceras awajiense-bearing beds of the Hiketa Formation in the study area is correlated to the lower part of that of the Toyajo Formation in the Kii Peninsula on the basis of the shell forms of D. awajiense. Didymoceras morozumii sp. nov. and D. awajiense have similar characteristics, and it is thought that D. awajiense evolved from D. morozumii sp. nov. As indicated in the descendants, some specimens of D. morozumii sp. nov. are also encrusted by anomiid bivalves.

© by the Palaeontological Society of Japan
Akihiro Misaki and Yasuyuki Tsujino "A New Species of the Heteromorph Ammonoid Didymoceras from the Upper Cretaceous Izumi Group in Shikoku, Southwestern Japan, and Its Evolutionary Implications," Paleontological Research 25(2), 127-144, (1 April 2021). https://doi.org/10.2517/2020PR010
Received: 8 December 2019; Accepted: 4 March 2020; Published: 1 April 2021
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