Selaginella tomescui A. R. Schmidt & L. Regalado, 2022

Schmidt, Alexander R., Korall, Petra, Krings, Michael, Weststrand, Stina, Bergschneider, Lena, Sadowski, Eva-Maria, Bechteler, Julia, Rikkinen, Jouko & Regalado, Ledis, 2022, Selaginella in Cretaceous amber from Myanmar, Willdenowia 52 (2), pp. 179-245 : 223

Selaginella tomescui A. R. Schmidt & L. Regalado , sp. nov.

Holotype: JZC-Bu-1692 [ Fig. 29A, B, F, G], American Museum of Natural History. – Fig. 2I; 29 (only specimen available).

Diagnosis — Axes compressed. Trophophylls dimorphic. Axillary trophophylls symmetric, elliptic, non-carinate, base obtuse to rounded, apex acute to obtuse, margins short-ciliate or denticulate. Dorsal trophophylls close, adpressed, symmetric, ovate to elliptic, non-carinate, base decurrent, apex aristate, margins sparsely denticulate. Ventral trophophylls close, spreading, ascending, somewhat asymmetric, ovate to oblong-ovate, non-carinate, base rounded to short-truncate, apex obtuse to acute, margins sparsely denticulate. Strobili terminal, compact, tetrastichous, quadrangular. Sporophylls monomorphic, imbricate, conduplicate, lanceolate carinate, carina sparsely dentate-denticulate similar to sporophyll margins, base rounded, apex attenuate, margins sparsely denticulate, sporophyll-pteryx absent. Sporangia suborbicular. Megaspores reticulate-rugulate. Microspores in tetrads, distally rugulate.

Description — Rhizophores not preserved. Axes compressed. Trophophylls dimorphic ( Fig. 29A, F). Axillary trophophylls symmetric, 0.7–1.0 × 0.5–0.6 mm, elliptic, non-carinate, base obtuse to rounded, apex acute to obtuse, margins sparsely short-ciliate or denticulate, cilia or teeth 30–60 µm long ( Fig. 29F). Dorsal trophophylls close, adpressed, symmetric, 0.6–0.8 × 0.2–0.3 mm, ovate to elliptic, non-carinate, base decurrent, apex aristate, margins sparsely denticulate, teeth 20–30 µm long ( Fig. 29F). Ventral trophophylls close, spreading, ascending, somewhat asymmetric, 0.8–1.2 × 0.4–0.6 mm, ovate to oblong-ovate, non-carinate, base rounded to short-truncate, apex obtuse to acute, margins sparsely denticulate, teeth 20–40 µm long ( Fig. 29G). Epidermis of dorsal and ventral trophophylls mostly composed of uniform isodiametric cells ( Fig. 29G). Strobili terminal, compact, tetrastichous, quadrangular, 2.0–3.4 × 1.6–2.2 mm ( Fig. 29B–D). Sporophylls monomorphic, imbricate, conduplicate, 28–36 per strobilus, 0.9–1.1 × 0.3–0.4 mm, lanceolate, carinate, carina sparsely dentate-denticulate similar to sporophyll margins, base rounded, apex attenuate, margins sparsely denticulate, teeth 10–40 µm long, sporophyll-pteryx absent ( Fig. 2I; 29E). Epidermis of sporophylls composed of elongate cells, with long axes orientated uniformly parallel to carina. Sporangia suborbicular, 280–300 × 300–340 µm, with uniform isodiametric cells bearing thick anticlinal walls ( Fig. 29H. Megaspores 130–150 µm in diam., reticulate-rugulate ( Fig. 29I). Microspores in tetrads, distally rugulate ( Fig. 29J).

Remarks — Of all the Kachin Selaginella inclusions, S. tomescui has the highest level of morphological correspondence to S. wangboi , S. pellucida and S. obscura . They all have in common the general outline and apex shape of the dorsal trophophylls and sporophylls, the type of sporophyll margins, the shape of the sporangia and the type of microspore ornamentation, among other features. However, S. tomescui differs from the other three taxa by its axillary trophophylls, which are short-ciliate or denticulate at the margins, with teeth or cilia 30–60 µm long ( Fig. 29F). In comparison, the margins of the axillary trophophylls of S. wangboi , S. pellucida and S. obscura are mostly serrate or dentate, with teeth 15–50 µm long.

There are four extant species ( Selaginella brachyblepharis , S. involvens , S. moellendorffii and S. radicata ) that are somewhat similar morphologically to S. tomescui . However, the fossil differs from them in having distally rugulate microspores ( Fig. 29J) (see remarks under S. wangboi for further details). The strobili being less than 5 mm long is also a feature that separates S. tomescui from S. brachyblepharis , S. involvens and S. moellendorffii , all of which have strobili longer than 5 mm, usually between 10 and 20 mm long.

Etymology — The specific epithet honours Professor Alexandru Mihail Florian Tomescu (Humboldt State University, Arcata, California, U.S.A.), for his contributions to the knowledge of fossil cryptogams.