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Did the silicoflagellate genus Octactis evolve from Bachmannocena?

Kevin McCartney, Richard W. Jordan and Kenta Abe
Did the silicoflagellate genus Octactis evolve from Bachmannocena?
Journal of Micropalaeontology (July 2016) 35 (2): 190-194


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

ISSN: 0262-821X
EISSN: 2041-4978
Serial Title: Journal of Micropalaeontology
Serial Volume: 35
Serial Issue: 2
Title: Did the silicoflagellate genus Octactis evolve from Bachmannocena?
Affiliation: University of Maine at Presque Isle, Department of Environmental Studies and Sustainability, Presque Isle, ME, United States
Pages: 190-194
Published: 201607
Text Language: English
Publisher: British Micropalaeontological Society, London, United Kingdom
References: 42
Accession Number: 2016-070827
Categories: Invertebrate paleontology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
N37°07'00" - N37°07'00", W05°27'00" - W05°27'00"
Secondary Affiliation: Yamagata University, JPN, Japan
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from The Geological Society, London, London, United Kingdom
Update Code: 201634
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