Selaginella wangboi 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 : 218-221

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

Holotype: PB23159 [ Fig. 26], Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. – Fig. 2H; 26–28 (four specimens available).

Diagnosis — Rhizophores arising from branch dichotomies, ventrally orientated, base covered by axillary trophophyll. Axes compressed. Trophophylls dimorphic. Axillary trophophylls somewhat asymmetric, broadly ovate, non-carinate, base obtuse, apex acute to slightly acuminate, margins nearly entire or with a few teeth on proximal acroscopic margin. Dorsal trophophylls close to distant, adpressed, ovate, non-carinate, base obtuse, apex aristate, slightly decurrent, margins serrate-denticulate. Ventral trophophylls mostly symmetric, occasionally asymmetric, close to distant, spreading, ascending, broadly ovate, non-carinate, base rounded to obtuse, apex acute to slightly acuminate, basiscopic margin serrulate, acroscopic margin short-ciliate-denticulate. Strobili terminal, tetrastichous, quadrangular. Sporophylls monomorphic, imbricate, ascending, conduplicate, ovate, carinate, carina slightly serrate, base rounded, apex attenuate, margins dentate-serrate, sporophyll-pteryx absent. Sporangia suborbicular. Megaspores distally and proximally reticulate. Microspores proximally rugulate.

Description — Rhizophores arising from dichotomy of branch, ventrally orientated, base covered by axillary trophophyll ( Fig. 27E, F). Axes compressed. Trophophylls dimorphic ( Fig. 26G; 27C–F; 28B). Axillary trophophylls somewhat asymmetric, 1.2–1.3 × 0.6–0.8 mm, broadly ovate, non-carinate, base obtuse, apex acute to slightly acuminate, margins nearly entire or with a few teeth on proximal acroscopic margin, teeth 10–25 µm long ( Fig. 27F). Dorsal trophophylls close to distant, adpressed, 0.6–1.2 × 0.2–0.5 mm, ovate, non-carinate, base obtuse, apex aristate, slightly decurrent, margins serrate-denticulate, teeth 10–20 µm long. Ventral trophophylls mostly symmetric, occasionally asymmetric, close to distant, spreading, ascending, 1.1–1.7 × 0.5–0.9 mm, broadly ovate, non-carinate, base rounded to obtuse, apex acute to slightly acuminate, basiscopic margin serrulate, teeth 10–20 µm long, acroscopic margin short-ciliate-denticulate, cilia 50–70 µm long, teeth 20–30 µm long ( Fig. 26H; 27C). Epidermis of dorsal and ventral trophophylls composed of uniform isodiametric cells. Strobili terminal, tetrastichous, quadrangular, 2.3–3.0 × 1.4–1.9 mm ( Fig. 26C, D; 27B; 28A). Sporophylls monomorphic, imbricate, ascending, conduplicate, 12–32 per strobilus, 0.8–1.2 × 0.3–0.5 mm, ovate, carinate, carina slightly serrate, teeth of carina ≤ 10 µm long, base rounded, apex attenuate, margins dentate-serrate, teeth 20–30 µm long, sporophyll-pteryx absent ( Fig. 2H; 26E, F). Epidermis of sporophylls composed of elongate cells, with long axes orientated uniformly parallel to carina ( Fig. 26E, F). Sporangia suborbicular, (200–)290–350 × 210–290 µm, composed of uniform isodiametric cells with thick anticlinal walls. Megaspores 170–200 µm in diam., distally and proximally reticulate ( Fig 26I–K). Microspores 20–30 µm in diam., proximally rugulate ( Fig. 26L).

Remarks — Among the Kachin Selaginella species with quadrangular strobili, S. wangboi ( Fig. 2H; 26–28), S. tomescui ( Fig. 2I; 29), S. pellucida ( Fig. 2J; 30) and S. obscura ( Fig. 2K; 31, 32) are very similar in regard to overall morphology. They all show the following coincident characters: trophophylls dimorphic with dorsal trophophylls aristate, sporophylls dentate, serrate or at least erose, carinate with the carina serrate or erose, without sporophyll-pteryx, sporangia mostly suborbicular, relatively uniform in size throughout the strobilus, and rugulate microspores. However, minor differences between them also exist. For example, S. wangboi differs from the other three taxa in that it has nearly entire axillary trophophylls, with only a few sparse teeth on the proximal acroscopic margin, these teeth 10–25 µm long ( Fig. 27F). Additionally, the fertile specimens of S. wangboi released microspores in monads ( Fig. 26L). In contrast, S. tomescui ( Fig. 2I; 29), S. pellucida ( Fig. 2J; 30) and S. obscura ( Fig. 2K; 31, 32) have axillary trophophylls, which are short-ciliate, serrate or conspicuously dentate at the margins, and released the microspores in tetrads.

Among the extant species of Selaginella , the common pattern is that microspores are disseminated individually (as monads), but they may also remain in tetrads after their release due to different connections or fusions of the spore wall ( Morbelli & al. 2001; Tryon & Lugardon 2001; Morbelli & Lugardon 2012; Zhou & al. 2015a; Singh & al. 2016). Preservation of the fossil inclusions renders it impossible to examine if there were any connections among the individual microspores forming a tetrad in S. tomescui ( Fig. 29H, J), S. pellucida ( Fig. 30E) and S. obscura ( Fig. 31E; 32G, H), but this condition was taken into account when establishing boundaries between taxa, especially in order to separate S. wangboi from all other taxa with similar overall morphology. However, a much larger sample set would be necessary to verify whether these differences are truly constant and discrete.

We believe that there are four specimens (i.e. PB23159, PB23161, P288–12 and the sterile specimen JZC-Bu-1868) of Selaginella wangboi currently available. They all are very similar in regard to vegetative morphology, so they are attributed to the same species. However, preservation of tissues in specimen PB23159 made it sometimes difficult to recognize the details of the trophophyll margins .

Among modern species, four species display morphological similarities with this fossil, including Selaginella involvens (Sw.) Spring , from Bhutan, China, India, Japan, Korea, Nepal and SE Asia ( Dixit 1992; Zhang & al. 2013; Fraser-Jenkins & al. 2017; Shalimov & al. 2019), S. moellendorffii Hieron. , native to China, Japan, the Philippines and Vietnam ( Zhang & al. 2013), S. radicata (Hook. & Grev.) Spring , endemic to India and S. brachyblepharis Alderw. , endemic to Borneo ( Van Alderwerelt van Rosenburgh 1915). They have in common with the fossil the presence of quadrangular strobili with ovate-lanceolate sporophylls that are acute to acuminate at the apex, denticulate at the margins and carinate ( Van Alderwerelt van Rosenburgh 1915; Dixit 1992; Zhang & al. 2013; Shalimov & al. 2019). However, all extant species differ from S. wangboi by having strobili longer than 5 mm (up to 15 mm) and baculate or verrucate microspores ( Van Alderwerelt van Rosenburgh 1915; Dixit 1992; Zhou & al. 2015a), whereas the fossil has strobili 2–3 mm long and rugulate microspores ( Fig. 26L). Selaginella radicata differs from the fossil by having verrucate microspores ( Dixit 1992). Furthermore, S. involvens and S. moellendorffii appear to release microspores in tetrads ( Zhou & al. 2015a), whereas they are discharged singly in S. wangboi ( Fig. 26L).

Etymology — The specific epithet honours Professor Bo Wang (Nanjing), who has supported research collaborations on E Asian ambers for many years and has shared new discoveries with us.

Additional specimens studied — PB23161, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences ( Fig. 27), JZC-Bu-1868, American Museum of Natural History, Max Kobbert Amber Collection Münster, Germany, P 288–12 ( Fig. 28).