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Article: Polymorphism in Late Cretaceous phylloceratid ammonoids: evidence from ontogenetic trajectories of septal spacing

Papers in Palaeontology - Volume 6 Issue 1 - Cover
Publication: Palaeontology
Volume: 6
Part: 1
Publication Date: Febuary 2020
Page(s): 155 172
Author(s): Tetsuro Iwasaki, Yui Iwasaki, and Ryoji Wani
DOI: 10.1002/spp2.1266
Addition Information

How to Cite

IWASAKI, T., IWASAKI, Y., WANI, R. 2020. . Papers in Palaeontology, 6, 1, 155-172. DOI: /doi/10.1002/spp2.1266

Author Information

  • Tetsuro Iwasaki - Graduate School of Environment & Information Sciences Yokohama National University Yokohama 240‐8501 Japan
  • Yui Iwasaki - College of Education & Human Sciences Yokohama National University Yokohama 240‐8501 Japan
  • Ryoji Wani - Faculty of Environment & Information Sciences Yokohama National University Yokohama 240‐8501 Japan

Publication History

  • Issue published online: 28 January 2020
  • Manuscript Accepted: 13 December 2018
  • Manuscript Received: 27 August 2018

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Abstract

The ontogenetic trajectories of septal spacing between succeeding chambers of two phylloceratid ammonoids, Hypophylloceras subramosum and Phyllopachyceras ezoense, from the Haboro and Kotanbetsu areas, north‐western Hokkaido, Japan, were analysed. The ontogenetic trajectories of septal spacing of H. subramosum demonstrate a general trend with large intraspecific variation: two cycles of increasing to decreasing spacing followed by almost constant spacing. The large intraspecific variation can be subdivided into three types, making this species polymorphic. The ontogenetic trajectories of septal spacing of P. ezoense also show intraspecific variation, but with a different trend: one cycle of increasing to decreasing spacing and an increasing trend after that. The intraspecific variation can be subdivided into two types, which suggests that this species is dimorphic, possibly sexually dimorphic. Dimorphism is supported by two observations: (1) the difference in the ontogenetic trajectories of septal spacing is only seen in the later ontogenetic stages; and (2) the two types co‐occur with comparable abundance throughout all stratigraphic horizons. Detailed analyses of ontogenetic trajectories of septal spacing may reveal polymorphism in other ammonoid clades.

Acknowledgements

We are grateful to D. Aiba for his donations of some of the examined specimens and for his help to access the specimens studied by Ikeda & Wani (2012) deposited in the Mikasa City Museum. We sincerely thank M.M. Yacobucci for her linguistic corrections and constructive suggestions on the early draft and C. Klug and D. Ware for their valuable and thoughtful comments on an earlier draft of the manuscript.

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