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Did the silicoflagellate genus Octactis evolve from Bachmannocena?

Kevin McCartney, Richard W. Jordan and Kenta Abe
Journal of Micropalaeontology, 35, 190-194, 24 June 2016, https://doi.org/10.1144/jmpaleo2015-028
Kevin McCartney
1Department of Environmental Studies and Sustainability, University of Maine at Presque Isle, Presque Isle, ME 04769, USA
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  • For correspondence: kevin.mccartney@maine.edu
Richard W. Jordan
2Department of Earth & Environmental Sciences, Faculty of Science, Yamagata University, 1–4–12 Kojirakawa-machi, Yamagata 990–8560, Japan
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Kenta Abe
3Graduate School of Science & Engineering, Yamagata University, 1–4–12 Kojirakawa-machi, Yamagata 990–8560, Japan
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Abstract

It has long been assumed that Octactis and Stephanocha (previously known as Distephanus) are closely related. Both have apical rings, but with Octactis possessing an apical ring of larger diameter relative to the basal ring and made of thinner elements. However, closer examination of the basal rings and double skeletons (doublets) shows other differences that have received little attention. The location of the struts and pikes on each side of the Neogene Stephanocha basal ring are consistently rotated towards the basal corners. This establishes a distinct basal element between the struts and pikes that is tilted to form a zig-zag basal design that creates an interlocking of the doublet skeletons. The pikes project into the region of the paired skeleton and the apical ring is also rotated with respect to the basal ring. In contrast, Octactis has a flatter basal ring without pikes, and possesses strut attachments located midway between the corners with the doublet held together only by organic material.

Recent examination of living Octactis from the Seto Inland Sea of southern Japan provides a better understanding about the variability and structure of Octactis skeletons than previously available. Among these are double skeletons that have an incompletely formed daughter that consists only of the basal ring. The older literature includes line drawings that illustrate similar double skeletons, but these have not been previously photographed or adequately understood. Such skeletons are also found singly after disarticulation, and have a strong resemblance to those of fossil Bachmannocena (formerly known as Mesocena), which also has a flat basal ring lacking apical structures and pikes. The hypothesis that Octactis evolved from Bachmannocena rather than from Stephanocha is presented, with the possible evolution of an apical ring in Bachmannocena accounting for the differences between the skeletal morphologies of Octactis and Stephanocha.

  • Bachmannocena
  • Octactis
  • silicoflagellates
  • Stephanocha (Distephanus)
  • © 2016 The Author(s)
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Journal of Micropalaeontology: 35 (2)
Journal of Micropalaeontology
Volume 35, Issue 2
July 2016
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Did the silicoflagellate genus Octactis evolve from Bachmannocena?

Kevin McCartney, Richard W. Jordan and Kenta Abe
Journal of Micropalaeontology, 35, 190-194, 24 June 2016, https://doi.org/10.1144/jmpaleo2015-028
Kevin McCartney
1Department of Environmental Studies and Sustainability, University of Maine at Presque Isle, Presque Isle, ME 04769, USA
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
  • For correspondence: kevin.mccartney@maine.edu
Richard W. Jordan
2Department of Earth & Environmental Sciences, Faculty of Science, Yamagata University, 1–4–12 Kojirakawa-machi, Yamagata 990–8560, Japan
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
Kenta Abe
3Graduate School of Science & Engineering, Yamagata University, 1–4–12 Kojirakawa-machi, Yamagata 990–8560, Japan
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site

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Did the silicoflagellate genus Octactis evolve from Bachmannocena?

Kevin McCartney, Richard W. Jordan and Kenta Abe
Journal of Micropalaeontology, 35, 190-194, 24 June 2016, https://doi.org/10.1144/jmpaleo2015-028
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