Nannotax3 - ntax_cenozoic - Hornibrookina gracilis Nannotax3 - ntax_cenozoic - Hornibrookina gracilis

Hornibrookina gracilis

Classification: ntax_cenozoic -> Isochrysidales -> Prinsiaceae -> Hornibrookina -> Hornibrookina gracilis
Sister taxa: H. australis, H. elegans, H. indistincta, H. teuriensis ⟩⟨ H. weimerae, H. gracilis, H. sp.


Citation: Hornibrookina gracilis Self-Trail et al 2022
taxonomic rank: species
Taxonomic discussion: Orthography: The species was named H. gracila in Self-Trail et al. 2022, however the Latain adjective gracilis on which the name is based does not change form in the feminine and so the correct name has to be gracilis. [my interpretation - JRY 2022]

Distinguishing features:
Parent taxon (Hornibrookina): Elliptical placoliths with bicyclic distal shields with raised tube cycle and central area grill formed from robust bars.
This taxon: Small (2-4.5 µm) Toweius-like species

Farinacci & Howe catalog pages: H. gracilis [no catalog entry yet]


Diagnosis: A small species of Hornibrookina with rounded ends and having two shields that are only slightly arched. The central area contains slender horizontal/transverse laths that occasionally merge to form a “v” shape and meet in the center to form a narrow, longitudinal bar. [Self-Trail et al. 2022]

Description: Hornibrookina gracilis has 25–30 elements in the distal shield and 27–32 elements in the proximal shield. Shield elements overlap slightly on the distal surface and show an interlocking pattern with mild diagenesis. The proximal shield is slightly smaller than the distal shield. The central area is surrounded by an oval wall of steeply dipping, imbricate elements. The central area contains slender horizontal/transverse laths, usually 12–30, attached on the outer edge to a cycle of blocky elements, and they extend inward from either side and meet to form a narrow, central longitudinal bar. These laths occasionally merge to form a “v” shape (see Pl. 3, figs. 4, 6). The longitudinal bar is not visible from the proximal surface. [Self-Trail et al. 2022]

Differentiation: Hornibrookina gracilis most closely resembles H. weimerae in being small and somewhat difficult to distinguish under the light microscope. However, the outline of H. gracilis is clearly more rounded (average width of 2.6 μm) than that of H. weimerae (whose width averages 1.8 μm) and the laths in H. weimerae are very broad (Pl. 3, figs. 7–12) rather than the narrow bars of H. gracilis. In poorly to moderately preserved material, H. gracilis in the LM can be easily confused with small specimens of Biscutum and/or Toweius (see Pl. 4, fig. 12). In particular, H. gracilis closely resembles Toweius reticulum (see images of Bown 2016). However, there are some areas of clear distinction between H. gracilis and T. reticulum. Hornibrookina gracilis (1.8–4.5 μm) is bigger than T. reticulum (1.8–3.0 μm), has a strongly imbricated inner cycle of elements (see Plate 3, fig. 1) that is lacking in T. reticulum (see Plate 10, fig. 1 of Bown (2016) for comparison), and lacks the central longitudinal structure with pores (compare Plate 3, figs. 4–5 with Bown (2016), their Plate 10, figure 5 paratype). [Self-Trail et al. 2022]

Search data:
Lith size: 1.7->4.5µm; Segments: 25->32;
Data source notes: Self-Trail et al. 2022
The morphological data given here can be used on the advanced search page. See also these notes

Geological Range:

Middle Paleocene to early Eocene, Zones NP5–14. [Self-Trail et al. 2022]

Last occurrence (top): within NP14 zone (46.29-49.11Ma, top in Lutetian stage). Data source: Self-Trail et al. 2022
First occurrence (base): within NP5 zone (59.54-61.51Ma, base in Selandian stage). Data source: Self-Trail et al. 2022

Plot of occurrence data:


Bown, P. R. (2016). Paleocene calcareous nannofossils from Tanzania (TDP sites 19, 27 and 38). Journal of Nannoplankton Research. 36(1): 1-32. gs O

Self-Trail, J. M., Watkins, D. K., Pospichal, J. J. & Seefelt, E. L. (2022). Evolution and taxonomy of the Paleogene calcareous nannofossil genus Hornibrookina. Micropaleontology. 68(1): 85-113. gs


Hornibrookina gracilis compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 23-4-2024

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