Nannotax3 - ntax_cenozoic - Hornibrookina indistincta

Citation: Hornibrookina indistincta Self-Trail et al. 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 (3-5 µm), narrowly elliptical to lenticular. Like H. a arca but smaller and flatter.

Diagnosis: A small to medium-sized pointed elliptical species of Hornibrookina having two plates that are only slightly arched and a central area that contains narrow but thick horizontal/transverse laths that meet in the middle to form a distinct longitudinal bar. [Self-Trail et al. 2022]

Description: Hornibrookina indistincta has 19–25 slightly overlapping elements in the distal shield and a central collar of steeply dipping imbricated elements that surround the central area and form a bright ring when viewed with the LM. The proximal shield is slightly smaller than the distal shield. The central area contains robust horizontal/transverse laths, typically 13–24, that meet in the central area to form a thick longitudinal bar that is faintly visible with the LM. A small element present in the apex of the coccolith of H. indistincta often appears as a small white dot in the LM (Pl. 5, fig. 21, 24–25), but is not visible in the SEM. The overall shape of H. indistincta is pointed elliptical to slightly rounded. [Self-Trail et al. 2022]

Differentiation: Hornibrookina indistincta most closely resembles H. australis arca and was included with this species in the original description (Bybell and Self-Trail 1995). Close examination of LM and SEM images shows that H. indistincta is less pointed on its ends, is only slightly, rather than highly, arched and lacks the diagnostic “bulge” that is present in H. australis arca. The width for H. indistincta is thinner (averages 2.7 μm) than those recorded for H. australis arca (averages 3.1 μm). Additionally, the longitudinal bar in H. indistincta is more robust than in H. australis arca and is more easily identified in the LM (see Pl. 5, figs. 22, 23). Specimens of H. indistincta are identified from the Atlantic Coastal Plain of North America and from El Kef, Tunisia (text-fig. 11). [Self-Trail et al. 2022]

References:

Bybell, L. M. & Self-Trail, J. (1995). Evolutionary, biostratigraphic and taxonomic study of calcareous nanofossils from a continuous Palaeocene-Eocene boundary section in New Jersey. U.S. Geological Survey, Professional Paper. 1554: 1-36. 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

Self-Trail, J. M., Parker, M., Govoni, D. L., Bybell, L. M., Gardner, K. F. & Gohn, G. S. (2023). Designation of a composite-stratotype section for the lower Paleocene (Danian) Brightseat Formation in Prince George’s County, Maryland, USA. Stratigraphy. 20(4): 237-258. gs

Hornibrookina indistincta

References:

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