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| Titel |
Distinguishing between biologically induced and biologically controlled mineralization in fossil organisms using electron backscatter diffraction (EBSD) |
| VerfasserIn |
Jan-Filip Päßler, Emilia Jarochowska, Michel Bestmann, Axel Munnecke |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250137979
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| Publikation (Nr.) |
 EGU/EGU2017-861.pdf |
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| Zusammenfassung |
| Although carbonate-precipitating cyanobacteria are ubiquitous in aquatic ecosystems today,
the criteria used to identify them in the geological record are subjective and rarely testable.
Differences in the mode of biomineralization between cyanobacteria and metazoans,
i.e. biologically induced calcification (BIM) vs. biologically controlled calcification (BCM)
might be possible to discern through different crystallographic structures in which they result.
We employed electron backscatter diffraction (EBSD) to investigate the structure of
calcareous skeletons in two microproblematica widespread in Paleozoic marine ecosystems:
Rothpletzella Wood 1945, considered to be a cyanobacterium, and Allonema Ulrich & Bassler
1904. We used a calcareous trilobite shell as a reference.
The shell of Allonema has a simple single-layered structure of acicular crystals
perpendicular to the surface of the organism. The c-axes of these crystals are parallel to the
elongation and thereby normal to the surface of the organism. The pole figures
and misorientation axis distribution reveal a fiber texture around the c-axis with
a small degree of variation (up to 30∘), indicating a well-organized structure. A
comparable pattern was found in the trilobite shell. This structure allows excluding
biologically induced mineralization as the mechanism of shell formation in Allonema. In
Rothpletzella the c-axes of the microcrystalline sheath show a broader clustering
compared to Allonema, but still reveal crystals tending to be perpendicular to the
surface of the organism. The misorientation axes of adjacent crystals show a random
distribution. However, Rothpletzella also shares other morphological similarities with
fossil and extant cyanobacteria. We propose that the strict limitation of rotations
(misorientations) between adjacent crystals around a specific axis of the crystal system can
be used as a criterion to distinguish shells formed through biologically controlled
biomineralization. |
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