Trochoaster nodosus, Steurbaut, Etienne & Nolf, Dirk, 2021

Steurbaut, Etienne & Nolf, Dirk, 2021, The Mont-des-Récollets section (N France): a key site for the Ypresian-Lutetian transition at mid-latitudes - reassessment of the boundary criterion for the base- Lutetian GSSP, Geodiversitas 43 (11), pp. 311-363 : 347

Trochoaster nodosus n. sp.

( Fig. 18 W-Y)

urn:lsid:zoobank.org:act:BEE661BF-859D-4C27-AC92-BF41326F94A1

HOLOTYPE. — Fig. 18W ( IRSNB b7115) (negative stored in the collections of the RBINS).

PARATYPES. — 2 figured specimens; 1 from the Vilvoorde borehole at 5.80 m depth (d = 15 µm) ( Fig. 18X ) ( IRSNB b7116) and 1 from the middle part of the Oedelem Sand Member (Aalter Sand Formation) in the Oedelem BH (11.70 m) (d = 15 µm) ( Fig. 18Y ) ( IRSNB b7117); both base of NP14.

DIAGNOSIS. — Six-rayed body, enterily covered with bumps, and marked by strong median ridges along the rays, which are connected with a central circular ring.

DERIVATIO NOMINIS. — The species name refers to the presence of numerous small bumps popping up all over this robust six-rayed holococcolith.

LOCUS TYPICUS. — Quarry at Chaumont-en-Vexin, sample 16, Paris Basin; c. 49°15’33.59”N, 1°52’44.20”E.

STRATUM TYPICUM. — ‘ Chaumont-en-Vexin sands’; base of NP14; basal Lutetian.

DIMENSIONS. — Diameter = 15 to 16 µm (holotype: d = 16 µm). DISTRIBUTION. — Trochoaster nodosus n. sp. has been identified in many outcrop and borehole sections in the Belgian Basin and the Paris Basin. At the Mont-des-Récollets it has been recorded at the base of Unit A 4 in the upper part of the Aalter Sand Formation. It has been found in the same position (Unit A4) in the Oedelem BH and the Vlakte van de Raan BH. It has been encountered in the Vilvoorde BH at the base of the Brussel Sand Formation (Unit BA) and is present throughout the ‘Chaumont-en-Vexin sands‘ at Chaumont-en-Vexin. The LO of T. nodosus n. sp. is also part of the famous calcareous nannofossil turnover, marking the base of NP14 and the base of the historical Lutetian stratotype.

DESCRIPTION

This Trochoaster species is characterised by six symmetrically positioned protruding rays with strong median ridges along the rays. The ridges are connected with a central circular ring, which surrounds a triradiate central bridge. The entire body, also the areas between the centrally positioned ridges of the arms, is covered with numerous bumps. These flat interray areas show no furher ornamentation. Specimens of this taxon do not exhibit birefringence under crossed nicols.

DISCUSSION

This taxon differs from all known Trochoaster taxa by its rugged and knobby outline and the configuration of the central ring, where the median ridges along the rays meet. It presents a vague resemblance with T. martinii from the Early Miocene of Germany because of the presence of six protruding rays.The latter is less knobby and presents some fundamental differences in configuration of the central structure and the interray areas.

Figures

FIG. 18. — New or less known calcareous nannofossil taxa from the Eocene of the Mont-des-Récollets and other localities in Belgium;a, b, c, views of the same specimen, in transmitted light or cross-polarized light or vice versa, and in different positions with respect to the polarisation directions. Numbers (e.g. IRSNB bXXXX) refer to the collections of the Royal Belgian Institute of Natural Sciences (Brussels).Pars 1: A-D, Braarudosphaera stylifera Troelsen & Quadros,1971,Mont-des-Récollets,122.85 m (IRSNB b7093,b7094,b7096);A, wm = 13 µm;B, wm = 12 µm;D, wm = 14.5 µm;C, Mont-des-Récollets,124.10 m (IRSNB b7095),wm = 13 µm;E, Lanternithus minutus Stradner, 1962, Mont-des-Récollets, 122.85 m, l = 5.6 µm (IRSNB b7097); F, G, Martiniaster cecellanoriae n. sp.: F, Chaumont-en-Vexin, sample 11, 0.55 m above base of the Lutetian, paratype,d = 13 µm (IRSNB b7098);G, Leuven BH,23.70 m depth, holotype,d = 14 µm (IRSNB b7099);H, Discoaster praebifax Wei & Wise,1989, Mont-des-Récollets,116.45 m, d = 7 µm (IRSNB b7100);I-M, Discoaster sublodoensis Bramlette & Sullivan,1961,Mont-des-Récollets,116.45 m (IRSNB b7101,b7102); J, d = 15.2 µm; I, d = 16 µm; K, Mont-des-Récollets, 118.35 m, d = 11.6 µm (IRSNB b7103); L, Vossem, otolith sample Nolf 1974, d = 7 µm (IRSNB b7104); M, Gisors, Bassin de Paris,sample 3,d = 14 µm (IRSNB b7105);N, Martiniaster fragilis (Martini,1961), Mont-des-Récollets,122.85 m, wm = 12 µm (IRSNB b7106);O-R, Discoaster wemmelensis Achuthan & Stradner,1969;O, Mont-des-Récollets, 122.85 m, d = 5.6 µm (IRSNB b7107); P-R, Vossem,otolith sample Nolf 1974 (IRSNB b7108, b7109, b7110);P, d = 5.3 µm; Q, d = 4.4 µm; R, d = 4.3 µm;S-V, Blackites minusculus n. sp.; S, Mont-des-Récollets,117.46 m, paratype, l = 8.8 µm (IRSNB b7111); T-V, Vossem, otolith sample Nolf 1974; T, U, paratypes (IRSNB b7112, b7113); T, I = 8.2 µm;U, I = 7.2 µm;V, holotype,I = 8.56 µm (IRSNB b7114); W-Y, Trochoaster nodosus n. sp.; W, Chaumont-en-Vexin,sample 16, 0.80 m above base of the Lutetian,holotype, d = 16 µm (IRSNB b7115);X, Vilvoorde BH, 5.80 m depth,paratype,d = 15 µm (IRSNB b7116); Y, Oedelem BH, 11.70 m depth, paratype, d = 15 µm (IRSNB b7117); Z-AA, Sphenolithus recolletensis n. sp.; Z, Mont-des-Récollets, 116.45 m, paratype, htot = 20 µm (IRSNB b7118);AA, Mont-des-Récollets, 117.46 m, holotype (a, c, parallel to the polarization directions;b, at 45°), htot = 11.4 µm (IRSNB b7119).