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Volume 37, issue 1 | Copyright
J. Micropalaeontol., 37, 283-293, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Feb 2018

Research article | 07 Feb 2018

Morphometric analysis of early Eocene Corbisema skeletons (Silicoflagellata) in Mors, Denmark

Hideto Tsutsui1,a, Richard W. Jordan1, Niichi Nishiwaki2, and Shiro Nishida3 Hideto Tsutsui et al.
  • 1Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan
  • 2Faculty of Social Science, Nara University, 1500 Misasagi-cho, Nara 631-8502, Japan
  • 3Earth Science, Faculty of Education, Nara University of Education, Takabatake, Nara 630-8528, Japan
  • acurrent address: Marine Microorganism Ecology, Division of Marine Biology & Dynamics, Department of Fisheries, Faculty of Fisheries, Nagasaki University, Bunkyo-cho 1-14, Nagasaki 852-8521, Japan

Abstract. A two-dimensional morphometric programme, recently designed to measure fossil skeletons of the silicoflagellate genus Corbisema, was used to investigate specimens of the C. apiculataC. triacantha complex found in a sample from the Fur Formation on the island of Mors, Jutland, Denmark. The semi-automated programme measured the lengths of the basal sides and radial spines, the basal side curvature, and the location of the pikes (if present) from a photographic database (N = 469). As a result, two distinct morphological groups were revealed based on their radial spine length:basal side length ratio and the presence or absence of pikes: group A (ratio of 1:1.3, no pikes) and group B, with the latter subdivided into B1 (ratio of 1:7, with pikes) and B2 (ratio of 1:6, no pikes). Group A (C. triacantha sensu lato) possesses a small basal ring with relatively straight basal sides and long radial spines, while group B has a large basal ring with curved basal sides and short radial spines. In B1 specimens (C. apiculata sensu stricto) the pikes are positioned 0 to 1µm away from the junction point of the strut and basal ring. This would suggest that B1 double skeletons are likely to be in the Star-of-David configuration, while A and B2 double skeletons (which lack pikes) are likely to be in the corner-to-corner configuration. Compared with the previously published biometric studies of extant Stephanocha (Stephanocha speculum complex in the Southern Ocean and S. medianoctisol in the Arctic Ocean), the results are somewhat different: although C. triacantha sensu lato (group A) is similar to the modern species of Stephanocha, the latter have smaller basal ring diameters, whereas specimens of C. apiculata sensu lato (types B1 and B2) have large basal rings. If their cell diameters are calculated, B1 is the largest, with S. speculum being the smallest – about half the size of B1. This could suggest that the relationship between radial spine length and mean basal ring size has shifted over geological time.

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Short summary
Skeletal morphology is the traditional way to classify living and fossil silicoflagellates (siliceous phytoplankton); however, recent studies have demonstrated that measurements of skeletal parts can also provide important information about evolution and skeletal development. In the present study, a 2-D morphometric programme was modified in order to study the taxonomic confusion within a fossil species complex of the genus Corbisema. The results show that two species can be clearly separated.
Skeletal morphology is the traditional way to classify living and fossil silicoflagellates...