Imaging the internal structure of Borelis schlumbergeri Reichel (1937): Advances by high-resolution hard X-ray microtomography

We explored reconstructing the internal structure of an individual of the extant alveolinid larger benthic foraminifer Borelis schlumbergeri Reichel (1937) using highresolution hard X-ray microtomography (μCT). Chamber segmentation analysis revealed a complex void architecture once filled by the living protoplasm beginning with a subsphaerical embryo (proloculus), surrounded by a streptospiral nepionic structure. It begins with Saturn-ring like, curved tubes perpendicular to each other. In late nepionic stage, they split up into an increasing number of chamberlets. Adult growth begins when streptospiral winding changes to planispiral coiling and the test starts stretching into the typical alveolinid fusiform shape. In the investigated individual the terminal chamber #48 is subdivided in 71 chamberlets. In the terminal stage, chamber volume is reduced and chamber growth becomes irregular. The cell volume increases 7,584-fold from a prolocular size of 13,592 μm3 to reach a cell volume of 103,077,248 μm3, at a length of the protoplasmic body of 1,995 μm. μCT impressively illustrates the adaptive advantage of planispiral fusiform winding for squeezing a giant protoplasmic sheet into a compact and mechanically robust body without losing cellular connection from the interior to its periphery and still allowing endosymbionts to maintain photosynthetical function under high-energy shallow marine environments. The μCT data are compared with historical drawings and plasticine models of early alveolinid growth stages fabricated by micropaleontological pioneer Manfred Reichel in the 1930s, highlighting the unprecedented quality of structural analysis during those early days. Siri K. Kellner. Museum für Naturkunde, Leibniz-Institut für Evolutionsund Biodiversitätsforschung, Invalidenstrasse 43, 10115-Berlin, Deutschland, Siri.Kellner@mfn.berlin Michael W. Knappertsbusch. Natural History Museum Basel, Augustinergasse 2, 4001-Basel, Switzerland, Michael.knappertsbusch@unibas.ch Loïc Costeur. Natural History Museum Basel, Augustinergasse 2, 4001-Basel, Switzerland, Loic.costeur@bs.ch Kellner, Siri K. , Knappertsbusch, Michael W. , Costeur, Loïc, Müller, Bert, and Schulz, Georg. 2019. Imaging the internal structure of Borelis schlumbergeri Reichel (1937): Advances by high-resolution hard X-ray microtomography Palaeontologia Electronica 22.1. 17A 1-19. https://doi.org/10.26879/854 palaeo-electronica.org/content/2019/2471-borelis-micro-ct Copyright: April 2019 Paleontological Society. This is an open access article distributed under the terms of Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0), which permits users to copy and redistribute the material in any medium or format, provided it is not used for commercial purposes and the original author and source are credited, with indications if any changes are made. creativecommons.org/licenses/by-nc-sa/4.0/ KELLNER ET AL.: BORELIS MICRO-CT Bert Müller. University of Basel, Biomaterials Science Center, Gewerbestrasse 14, 4123 Allschwil, Switzerland, bert.mueller@unibas.ch Georg Schulz, University of Basel, Biomaterials Science Center, Department of Biomedical Engineering, Gewerbestrasse 14, 4123 Allschwil, Switzerland, georg.schulz@unibas.ch