Blackites praeinflatus Steurbaut & Nolf, 2021, n. sp.

Blackites praeinflatus n. sp.

(Fig. 20 A-G)

urn:lsid:zoobank.org:act: 1B1CD3C8-D67B-4DF3-B110-1F390FBA6203

Rhabdosphaera inflata – Aubry 1983: pl. 4, figs 18-19; 1986: pl. IV, figs. 18-19 (non R. inflata Bramlette & Sullivan, 1961).

Blackites aff. inflatus – Herman et al. 2001: 238.

HOLOTYPE. — Fig. 20A (IRSNB b7141) (negatives stored in the collections of the RBINS).

PARATYPES. — 6 figured specimens from the Brussel Sand Formation, 3 from he Mont-des-Récollets (lower part of Unit B3) (Fig. 20B, C, E) (IRSNB b7142, b7143, b7145), 1 from Zaventem, sample 17 (middle part of Unit B3) (Fig. 20D) (IRSNB b7144) and 2 from Vossem, otolith sample Nolf 1974 (Fig. 20F, G) (IRSNB b7146, b7147).

DIAGNOSIS. — Slightly and somewhat asymmetrically inflated robust rhabdoliths, with faintly rugose surface but without a basal enlargement or collar, of which the maximum width of the stem is much less than the width of the more or less complexly structured basal plate.

DERIVATIO NOMINIS. — Refers to the outline of these rhabdoliths, which is inflated, although only slightly and asymmetrically compared to that of B. inflatus, to which it bears a certain resemblance.

LOCUS TYPICUS. — Mont-des-Récollets, ‘ Grande Carrière’, N France; 50°48’02.74”N, 2°30’23.06”E.

STRATUM TYPICUM. — Brussel Sand Formation, lower part of Unit B3 (122.26- 122.21 m); lower middle part of NP14.

DIMENSIONS. — Length = 22 to 29 µm (top is generally broken off); Maximum width of stem = 2.5 to 3.8 µm; Width of basal plate = 3.3 to 5.0 µm (holotype: l = 23.0 µm, wm = 3.0 µm, wbp = 4.0 µm).

DISTRIBUTION. — At the Mont-des-Récollets Blackites praeinflatus n. sp. occurs in very low numbers in the middle and upper part of the Brussel Sand Formation (Units B2, B3 and B4). It seems to recur at the base of the Lede Sand Formation and the base of the Wemmel Sand Member, but this may be due to reworking. This species has been recorded slightly earlier (top of Unit B1) at Sint-Stevens-Woluwe, and occurs regularly in Units B2 and B3 of many outcrop and borehole sections in Belgium (Vossem, Neerijse, Zaventem; Mol borehole, from 336 to 338 m depth). The earliest records are from the upper part of the Aalter Sand Formation (base of Unit A4) in the Vlakte van de Raan BH (85.55 m) and from the ‘Chaumont-en-Vexin sands’ at Chaumont-en-Vexin in the Paris Basin, both attributable to the base of NP14. It is also known from the base of the ‘Glauconie Grossière s.s.’ at Prémontré and at the top of the ‘Glauconie Grossière s.s.’ at Gisors (Steurbaut unpublished). Thus, its range appears to be restricted to NP14, if the spotty occurrences in lower NP15 (base Lede Sand Formation and base Wemmel Sand Member) are considered to be due to reworking.

DESCRIPTION

This species is marked by weakly inflated half-long rhabdoliths with slightly rugose surface and which distally gradually taper to a sharp tip (length = 22 to 29 µm, about 6 times greater than the diameter of the base, which is about 3.3 to 5 µm). The stem starts thickening distally, although only slightly and somewhat asymmetrically, at about 1/6 of its length (not at the base, where the width is only 80% of the maximum width), to reach its greatest diameter at about 1/3 of its length. The maximum width of the stem (c. 2.8 µm) measures about 14% of the total length of the rhabdolith, and is always substantially smaller than the diameter of the basal plate. The latter is complexly structured and seems to consist of two cycles of elements (Fig. 20G), each with alternating different optical behaviour (clearly seen when inserting the gypsum or quartz plate).

DISCUSSION

This form is identical to what has been figured and erroneously attributed to Rhabdosphaera inflata by Aubry (1983: pl. 4, figs 18-19; 1986: pl. IV, figs 18-19). By the distally slightly inflating stem and the rugose surface it bears a vague resemblance to Blackites inflatus. However, the latter is clearly much more inflated, undoubtedly pointing to a different taxon: the maximum width measures about c. 20 to 25% (mean of 24%) of the length of the rhabdolith and is equal or larger than the width of the basal plate (Fig. 20H: l = 17.5 µm, tip is missing; wm = 3.4 µm; wbp = 2.0 µm, incomplete?). Blackites praeinflatus n. sp. differs from B. spinosus by the general outline of the stem, reaching its maximum thickness at about 1/3 of its length, while in B. spinosus the stem is widest at its base (see Bown 2005: pl. 24, figs 21-25). It differs also from B. scabrosus (see Deflandre & Fert 1954: pl. XII, fig. 30, text-fig. 85) by its more elongated, less compact stem.