TERMINOLOGY

9. Nannolith shapes


Nannoliths display a wide range of shapes, including the following types which all occur independently in more than one group. These shape terms are independent of structure, e.g. tetraradiate nannoliths may be formed of one, four or many crystal units.

Dibrachiate consisting of two sub-parallel arms joined at one end. Includes horseshoe, arrow-head, and arcuate shapes (e.g. Ceratolithus, Amaurolithus, Ceratolithina, Ceratolithoides - except C. verbeekii).
Compact more or less equidimensional nannoliths. Includes conical (e.g. Sphenolithus), obconical (i.e. inverted cone-shaped, e.g. Conusphaera), cylindrical (e.g. Fasciculithus) and cubic (e.g. Micula) shapes.
Rod-shaped elongate, and apparently without a basal disc. Includes bladed (e.g. Lithraphidites quadratus, Triquetrorhabdulus carinatus) and (sub-)cylindrical (e.g. Microrhabdulus) shapes.
Radiate with radial symmetry. N.B. the number of crystal-units may be larger or smaller than the number of rays.
Triradiate threefold radial symmetry (e.g. Marthasterites, Trochasterites).
Tetraradiate fourfold radial symmetry (e.g. Micula, Quadrum, Nannotetrina).
Pentaradiate fivefold radial symmetry (e.g. Goniolithus, Braarudosphaera).
Multiradiate more than fivefold radial symmetry (e.g. many Discoaster spp.).
Central body core part of radiate nannolith where elements are in contact.
Free rays parts of radiate nannolith extending beyond central body.
Short free rays length of free rays is less than radius of central body, resulting in a rosette-shaped outline.
Long free rays length of free rays is greater than radius of central body, resulting in a star-shaped outline.
Convex outline without free rays (e.g. Braarudosphaera). Including e.g. triangular, square, and pentagonal shapes.
Stellate with free rays (e.g. Micrantholithus, Discoaster). Including rosette and star-shaped.

nannolith shapes
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