Trilamina nitescens (Greville) J. Witkowski, P.A. Sims, N.I. Strelnikova & D.M. Williams, 2015

Witkowski, Jakub, Sims, Patricia A., Strelnikova, Nina I. & Williams, David M., 2015, Entogoniopsis gen. nov. and Trilamina gen. nov. (Bacillariophyta): a survey of multipolar pseudocellate diatoms with internal costae, including comments on the genus Sheshukovia Gleser, Phytotaxa 209 (1), pp. 448-450 : 448-450

1. Trilamina nitescens (Greville) J. Witkowski, P.A. Sims, N.I. Strelnikova & D.M. Williams , comb. nov.

(SEM: Figs 217–222 ; LM: Figs 223–229 )

BASIONYM: Triceratium nitescens Greville (1865a , Transactions of the Microscopical Society of London, New Series 13: 8, pl. II, fig. 19).

TYPE:—‘Barbadoes deposit, Cambridge estate’ (BM3198, holotype! = Fig. 228 ; Williams 1988: 56).

Frustules rectangular to subrectangular in girdle view, valves tripolar ( Fig. 223–229 ) with deeply concave sides and wedge-shaped ( Figs 227, 229 ) to linear projections ( Figs 223, 228 ), rounded or tapering near the pole. An inconspicuous pseudocellus is located at each pole ( Fig. 219 ). The valve centre, with a triangular or hexagonal outline ( Figs 224 versus 223), is convex and mostly hyaline, with few irregular rings of sparse poroid areolae near the margins ( Fig. 218 ). The marginal parts of the projections are perforate, with poroid areolae arranged in transverse, short, parallel rows that continue down the mantle ( Figs 218–219 ). Size of areolae diminishes toward the mantle margin ( Figs 218–219 ). Valve face-mantle transition is gradual, mantle is steep and shallow ( Figs 217–218 ). The central part of each projection bears a prominent, depressed hyaline area tapering toward the pole ( Figs 217–218 ). A prominent network of robust costae ( Figs 220–221 ) is present on the valve interior. The most prominent of these costae are associated with the depressed hyaline areas within each projection ( Fig. 220 ). Tapering costae give rise to transverse, shorter costae that continue down the mantle and terminate at the inwardly expanded hyaline mantle margin ( Fig. 221 ). On the valve interior, each side of the central area is associated with a robust costa or a transverse fold ( Figs 220–221 ). A single, inconspicuous rimoportula is placed next to one of the folds ( Fig. 220 ). On the interior, it is a slit between two slightly raised lips ( Fig. 220 ). The rimoportula lacks a tube, and thus it is indistinguishable from other perforations on the exterior ( Fig. 218 ). Valvocopula is closed, comparable in depth to the mantle, attached to the valve by means of a fossa that embraces the inwardly expanded hyaline margin of the mantle, large clasping devices attached at mid-point of each side to internal costae or folds that line the central area ( Fig. 221 ), and small clasping devices located in regular intervals along the sides of each projection ( Fig. 222 ). The small clasping devices attach to each of the transverse costae that run down the mantle ( Fig. 222 ). Valvocopula is mostly hyaline, with a single row of poroids located next to the advalvar edge ( Fig. 221 ), interrupted at mid-point of each side, due to the presence of the large clasping devices. Measurements (n =13): average side length: 44.7–124.5 µm; 3–4 costae in 10 µm measured along the valve face margin; 3–5 areolae in 5 µm measured across a projection; 6–7 rows of areolae in 10 µm, measured along a projection.

Geographic and stratigraphic distribution ( Fig. 10, sites 14, 18):

(a) specimens:

Middle Eocene-early Miocene: Oceanic Formation outcrops at Cambridge Estate: BM3198 ( Fig. 228 , holotype); Joe’s River : BM63659 ( Fig. 223 ), BM65907, BM coll. Adams H 772.

ENTOGONIOPSIS GEN. NOV. AND TRILAMINA GEN. NOV. (BACILLARIOPHYTA)

Phytotaxa 209 (1) © 2015 Magnolia Press • 31

Late Eocene-earliest Oligocene: Oamaru , New Zealand: BM stub P.137 ( Figs 217 – 222 ), BM coll. Adams TS694 ( Fig. 224 ), BM63658 ( Fig. 225 ), BM61163, BM61164 ( Figs 226 and 229 ), BM coll. Adams GC 3331, GC: 3332 ( Fig. 227 ), H302, J3666.

(b) records:

Middle Eocene-early Miocene: Oceanic Formation outcrops at Cambridge Estate: Greville (1865a: 8, pl. II, fig. 19), Williams (1988: 56).

Late Eocene-earliest Oligocene: Oamaru , New Zealand: Schmidt (1888: taf. 127, fig. 19), Laporte & Lefébure (1929: pl. II, fig. 14, 1930: pl. XIX, fig. 131), Reed (1991: pl. 16, fig. 210); Oamaru Diatomite outcrops at Cormack’s Siding: Grove & Sturt (1886: 328); Allan’s Farm, Bain’s Upper, Jackson’s Paddock, Mavor’s: Desikachary & Sreelatha (1989 261, pl. 126, figs 4–8; not fig. 2); Otago, New Zealand; Totara, New Zealand: Desikachary & Sreelatha (1989: 261)

Observations:—Based on superficial resemblances, Greville (1865a: 8) suggested that T. nitescens was ‘allied’ to Triceratium lobatum Greville (1863: 233) . Prior to publishing the description of T. nitescens, Greville proposed a number of other similar taxa in addition to T. lobatum : Triceratium westianum ( Greville 1861a: 43, pl. IV, fig. 11), Triceratium rotundatum ( Greville 1861c: 75, pl. IX, fig. 6), Triceratium denticulatum ( Greville 1863: 233, pl. IX, fig. 14) and Triceratium obesum ( Greville 1864b: 90, pl. XIII , fig. 11). None of these, however, were indicated by Greville to be a possible close relative of T. nitescens . Of all these taxa, T. westianum has been available for SEM examination (see below). However, the presence of extensive hyaline areas and an overall similar pattern of internal costae in all of these species has prompted their transfer to Trilamina .

Figures

FIGURES 217–222. Scanning electron micrographs of Trilamina nitescens from Oamaru, New Zealand. Fig. 217: Oblique external view of a valve with attached valvocopula (VC). Fig. 218: Detail of the specimen in Fig. 217, showing the hyaline depressed sectors next to the valve centre. Arrow points to a poroid which might represent an external opening of a rimoportula. Fig. 219: Detail of the specimen in Fig. 217, showing the inconspicuous polar pseudocellus (arrowhead). Fig. 220: Flat internal view of a specimen with an attached valvocopula. Internal opening of the rimoportula is indicated by the white arrow. Arrowhead points to the prominent, tapering costa on the underside of the depressed area. Black arrow indicates the costa associated with the margin of the valve centre. Fig. 221: Oblique internal view of the specimen in Fig. 220, showing the differentiated clasping devices on the valvocopula. Large clasping mechanism is indicated by the arrowhead; small clasping mechanism is indicated by the white arrow. Note that the possible pseudocelli are bound by costae on the interior (black arrows). Fig. 222: Detail of the small clasping devices attached to the internal costae in the distal part of one of the projections.

FIGURES 223–229. Light micrographs showing the range of morphological variation in Trilamina nitescens. Fig. 223: BM63659, Joe’s River, Barbadoes. Fig. 224: BM coll. Adams TS694, frustule, Oamaru, New Zealand. Fig. 225: BM63658, Oamaru, New Zealand. Figs 226–227: BM coll. Adams GC3332, Oamaru, New Zealand. Fig. 228: BM3198, Cambridge, Barbadoes, holotype. Fig. 229: BM61164, Oamaru, New Zealand.