Gopadispermum papillatus, Bhowmik & Parveen, 2014

Bhowmik, Nupur & Parveen, Shabnam, 2014, Fossilized pollination droplet in a new seed genus from the Middle Triassic of Nidpur, India, Acta Palaeontologica Polonica 59 (2), pp. 491-503 : 493-496

Gopadispermum papillatus sp. nov.

Figs. 2B, 3A, 4F.

Etymology: In reference to the papillate feature in outer cuticle of seed integument.

Type material: Holotype: DDPMAU 53,313 has an external coat (Figs. 2, 4) and clumped pollen grains in pollen chamber (Fig. 3).

Type locality: Nidpur village , Marhwas area, Sidhi District, Madhya Pradesh, India .

Type horizon: Middle Triassic (Anisian–Ladinian).

Diagnosis.—Seeds platyspermic, rounded to orbicular when compressed along principle plane, averaging 2.5 × 2 mm, broadly oblong in secondary plane, averaging 3 × 2 mm. Both micropylar and chalazal ends broadly rounded. Apex of micropylar end shortly mucronate forming the short, straight micropylar tube. Chalazal end showing a narrow, median, shallowly concave chalazal scar, about 600 μm wide. Surface of seeds generally smooth to finely papillate, sometimes rough, showing irregular folds. Seed margin smooth to finely papillate. Seeds often adhering to an external folded to wrinkled carbonaceous tissue enclosing the seed. Seed micropyle generally extending beyond margin of adhering tissue. Seeds yielding four membranes on maceration. Outer and inner cuticles of integument, nucellar cuticle and megaspore membrane. Outer cuticle of integument delicate, thin, about 2 μm thick, cells polygonal, often medianly papillate. Inner cuticle of integument thinner, about 1 μm thick, granular, extended apically into a short, narrow micropylar tube. Cells narrow rectangularly elongated. Nucellar cuticle thick, tough, about 7 μm thick, cells rectangularly elongated to polygonal, averaging 35 × 15 μm. Cell walls about 5 μm thick, deeply sinuous. Nucellar tissue distally modified into a pollen chamber having a dark coloured collar-like base. Cell walls at distal end of pollen chamber vertically extended, forming acutely pointed processes protruding into cavity of micropylar canal. Micropylar canal and pollen chamber containing saccate pollen grains averaging 55–75 × 25–55 μm, circular, oval or of other shapes, corpus and sacci faintly distinct, corpus circular to subcircular ranging between 25–35 μm in diameter, thick and dark brown coloured, ornamentation indistinct. Saccus generally folded, dark coloured, exine coarsely intrareticulately ornamented. Pollen grains in pollen chamber generally clumped together, rarely occurring isolated. Megaspore membrane large, tough, occupying more than 2/3 rd of total seed area, measuring 1.5 × 1.8 mm. Cells large, polygonal. Seeds pollinated by droplet mechanism or its modification. Carbonaceous layer external to seed coat yielding two non-stomatiferous cuticles on maceration.

Description.—The description of Gopadispermum papillatus is based on the study of 30 dispersed seed specimens flattened in various planes (Fig. 2A–P). The seeds were isolated after bulk maceration of carbonaceous shales of the Nidpur beds in hydrofluoric acid. The seeds are rounded to oblong, depending on the plane of preservation and broadly flattened. In most specimens, a small beak-like micropyle is seen prominently projecting out of the micropylar end of the seed body (Fig. 2A–N). The surface generally appears smooth to finely papillate, lacking folds or wrinkles but occasionally, irregular folds are visible at the apical end of some (Fig. 2A, D, E, K). The margin in most specimens is smooth but in a few seeds it may appear faintly papillate to dentate (Fig. 2B, H, L, N). The chalazal end is also rounded showing a narrow, transversely elliptical, shallowly concave, chalazal hole at the centre (Fig. 2B–D, I, N). Frequently, the seed outline appears irregular due to the persistence of a closely adherent tissue almost encompassing the seed like a cupule (Fig. 2A– D, F–H, K–P). This external tissue measuring approximately 2.7 × 2.4 mm generally shows folds or feeble wrinkles on the surface suggesting a fleshy nature. Interestingly, most seeds on maceration yielded two additional cuticles besides the cuticles of integument, nucellus and the megaspore membrane. Both the cuticles are structurally distinct from the cuticles of the seed coat. The outer cuticle appeared to be thin and delicate, about 2 μm thick showing faint polygonal cell outlines while the inner cuticle appeared comparatively thick (5 μm), granular and displayed distinct outlines of elongated rectangular cells. Besides carbonized seeds, a number of naturally macerated seeds were also recovered in the macerated residue. In such specimens, cuticles of the adhering tissue and the delicate outer cuticle of seed integument is generally missing while the rest of the seed membranes are found to remain intact. Such naturally macerated seeds were probably dispersed from their cupules long before fossilization and their delicate outer cuticles were not resistant enough to be preserved. Maceration of carbonized seeds in Schulze’s fluid yielded four membranes including the integumentary layers, nucellus and megaspore membrane. The outer cuticle of integument is thin and delicate showing vague outlines of papillate to non-papillate polygonal cells arranged in longitudinal rows. The cells averaging 16 × 30 μm have thin, straight to undulated walls (Fig. 3A 4). The inner cuticle of integument is also thin and delicate, extending apically to form a short, narrow micropylar canal, about 250 μm long. The cells are narrow and rectangularly elongated, averaging 30 × 5 μm. The cell walls are thin and straight (Figs. 3A 2, A 5, C 1, D, 5A 2, B 2). The inner integument is free above but fused with the nucellar membrane below ( Figs. 4A 1 , A 2, B, 5A 2, B, C). The nucellar cuticle is the most resistant and perceptible layer of the seed. All the macerated seeds yielded a very tough and thick nucellar cuticle which is free above but fused to megaspore membrane below (Figs. 3A 1, 4A 1, B, E 1, 5 A 1, B 1). The cells are rectangularly elongated to polygonal with deeply sinuous periclinal walls and vague to obscure anticlinal walls (Figs. 3A 6, B, C 1, D, 5A 2, B 2, C). The amplitude of sinuosity ranged between 8 to 15 μm and the wave length varied from 20–38 μm. In many seeds, the periclinal wall of distally situated nucellar cells is extended forward forming pointed processes that seem to protrude into the cavity of the micropyle (Figs. 3B, D, 5B 2, C). The nucellus is distally modified forming a pollen chamber having a prominent dark coloured, collar-like base (Figs. 3A 1, A 2, C 1, D, 4A 1, A 2, D 1, E, 5A, B). The cavity of the pollen chamber contains saccate pollen grains clumped together into a dark colored pollen mass of irregular shape, measuring about 400 × 220 μm (Figs. 3A 1 –A 3, B–D, 4A, D 1, E; 5). Incidentally, the pollen grains of a pollen clump are so tightly adherent that it is not possible to separate them for examination of morphological features. Besides the clumped pollen grains, a few pollen

Fig. 2. Compressed dry seed specimens of the ginkgoopsid Gopadispermum papillatus gen. et sp. nov. from the Triassic of Nidpur, India. A. A seed showing a short, micropylar tube. DDPMAU 53,361. B. Holotype, showing the externally adherent layer near base of the short micropylar → tube and a shallow concave chalazal hole at the opposite end. DDPMAU 53,313. C, F–H, J–N. Specimens showing a short micropylar tube extending beyond the margin of cupule-like externally adherent layer. DDPMAU 53,362; 53,365; 53,366; 53,374; 53,369; 53,372; 53,368; 53,377; 53,380 respectively. D, E, I, O, P. Specimens compressed in various planes showing feebly wrinkled or folded, faintly papillate surface. DDPMAU 53,363; 53,364; 53,367; 53,381; 53,382 respectively. Scale bars 500 μm.

grains may occasionally also occur isolated (Figs. 3A 2, A 3, B, D, 5B).

In addition to the clumped pollen grains in the pollen chamber, a specimen of the new seed also showed a fossilized pollination droplet extending from its micropylar orifice. Initially, this unmacerated specimen exhibited a small sized carbonized fragment clinging to the micropylar end of the seed. Prior to subjecting the seed to further maceration in Schulze’s solution several attempts were made to disengage the foreign fragment from the seed body and when all attempts failed, the seed was macerated together with the attached fragment. Maceration and later mounting of the seed on a slide revealed a convex mass of non-cellular, inorganic, crystalline resinous substance adhering to the tip of the micropyle. Entrapped within the non-cellular residue, could be seen obscure outlines of a few scattered, saccate pollen grains. Surprisingly, the morphological nature of the foreign fragment sticking to the micropylar end of the seed was revealed to be a carbonized fossilized pollination droplet ( Figs. 4E, 5A). The pollen grains trapped within the non-cellular mass of the droplet are not clearly evident but appeared to be saccate and quite similar to the pollen grains found clumped as well as isolated inside the pollen chamber and micropyle of the same seed and pollen chambers of other seeds (Figs. 3A 2, A 3, D, 4A 2, 5A, B). They are saccate, appear to be superficially similar to bisaccate grains due to the lateral expansion of saccus. The central body appears circular to subcircular, about 25–35 μm in diameter. The saccus exine is dark coloured, averagely thick and coarsely intrareticulate consisting of moderately broad muri enclosing irregular meshes (Figs. 3A 3, C 2, D, 4 E 2, 5).

The innermost layer of the seed consists of the megaspore membrane which is dark brown, spherical to oblong in outline occupying a large portion of the seed area ( Figs. A 1 , 4A 1, E 1 , 5A 1, B 1 ). The cells are large, averaging 150 × 100 μm, showing polygonal outlines and are often filled with dark cell contents. The cell walls are uniformly thickened and straight (Fig. 3A 7). In some seeds, one or more rounded dark-coloured spots are visible near the middle region of the megaspore membrane. Such spots presumably represent embedded archegonia ( Fig. 4D).

Stratigraphic and geographic range.—Middle Triassic (Anisian–Ladinian) of Nidpur, Madhya Pradesh, India.

Figures

Fig. 1. A. Map showing the location of Nidpur locality in Madhya Pradesh, India. B. Geological map of north-west Singrauli Coalfield showing Marhwas area, where the Nidpur beds are situated. Circles with dots, yielding samples taken along the traverses; asterisk, position of samples NID-4, 5, 8, 10. F-1–3, faults. Modified after Raja Rao (1983).