Helicolithus blairiae


Classification: Mesozoic -> Eiffellithales -> Eiffellithaceae -> Helicolithus -> Helicolithus blairiae
Sister taxa: H. anceps, H. trabeculatus, H. leckiei, H. tectufissus, H. turonicus, H. varolii, H. compactus, H. blairiae, H. sp.,

Distinguishing features: A small to medium-sized; axial cross nearly fills central area; sinuous-shaped longitudinal bar separates two very short, disjunct segments along the short axis.


Taxonomy:

Citation: Helicolithus blairiae Kita, Watkins & Bergen 2016
Rank: Species

Farinacci & Howe catalog pages: Helicolithus blairiae *

Distinguishing features: A small to medium-sized; axial cross nearly fills central area; sinuous-shaped longitudinal bar separates two very short, disjunct segments along the short axis.


Morphology remarks: This species is a small to medium-sized (4.3 – 5.3 μm), normally elliptical murolith. The broad inner rim cycle displays pronounced birefringence while the narrow outer rim cycle is dull in cross-polarized light. A broad, lozenge-shaped bar aligned with the major axis nearly fills the central area and is constructed of numerous lath-shaped elements. When aligned with the polarizers, this bar exhibits a distinct ‘s’-shaped extinction pattern as the two sets of elements that form the bar on each side do not exhibit simultaneous extinction. When aligned at 45° to the polarizers, the opposite elements appear birefringent. This longitudinal bar bisects two short, disjunct bar segments aligned with the minor axis which, in cross-polarized light, resemble two bright, birefringent dots when aligned with the polarizers. These disjunct segments are not orthogonal to the longitudinal bar. The central area elements display birefringence equal to the broad inner rim cycle. [Kita et al. 2016]

Differentiation: Helicolithus blairiae differs from most Helicolithus species by having an axial cross instead of a diagonal cross. Helicolithus compactus contains a near-axial cross that is offset from both the major and minor axis. Helicolithus blairiae differs from H. turonicus by being smaller in size and having a lozenge-shaped bar along the major axis. In less well-preserved specimens, the central area of H. blairiae can resemble that of Broinsonia signata, but with a very different rim structure. Helicolithus blairiae differs from H. varolii by being smaller in size (H. varolii averages 6.24μm) and by not having triangular-shaped crossbar elements with medial sutures in the central area (Blair & Watkins, 2009). [Kita et al. 2016]

Size: length 4.3 – 5.3 μm; width 3.1-3.8 µm (Kita et al. 2016)

Geological Range:
Last occurrence (top): within Late Santonian Substage (83.64-85.23Ma, top in Santonian stage). Data source: Kita et al. 2016
First occurrence (base): within Late Santonian Substage (83.64-85.23Ma, base in Santonian stage). Data source: Kita et al. 2016

Plot of occurrence data:

  • (NB There is no histogram as there are no occurrence records for the taxon in the Neptune database) Parent: Helicolithus

    References:

    Kita, Z. A., Watkins, D. K. & Bergen, J. A. (2016). A new calcareous nannofossil species of the genus Helicolithus from the Santonian and its biostratigraphic significance in the Cretaceous Western Interior Seaway. Journal of Nannoplankton Research. 36(1): 77-82. gs


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    Helicolithus blairiae compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 20-9-2019

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    Short stable page link: http://mikrotax.org/Nannotax3/index.php?id=10507 Go to Archive.is to create a permanent copy of this page - citation notes



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