Selaginella obscura 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 : 225-226

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

Holotype: PB25334 [ Fig. 31A, B], Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. – Fig. 2K; 31, 32 (three specimens available).

Diagnosis — Axes compressed. Trophophylls dimorphic. Axillary trophophylls symmetric, narrowly elliptic, non-carinate, base obtuse, apex acute, margins somewhat to conspicuously dentate. Dorsal trophophylls close, adpressed, lanceolate or elliptic, non-carinate, apex aristate, margins nearly entire to sparsely serrate. Ventral trophophylls distant, mostly symmetric, spreading, ascending, elliptic or ovate, non-carinate, base rounded, apex acute to acuminate, margins serrate to denticulate. Strobili terminal, compact, tetrastichous, quadrangular. Sporophylls monomorphic, imbricate, conduplicate, lanceolate, carinate, carina sparsely dentate as in sporophyll margins, base rounded, apex long-attenuate to aristate, margins sparsely dentate, sporophyll-pteryx absent. Sporangia suborbicular. Megaspores distally reticulate-rugulate. Microspores in tetrads, distally rugulate.

Description — Rhizophores not preserved. Axes compressed. Trophophylls dimorphic ( Fig. 31A, B; 32A, E). Axillary trophophylls symmetric, 0.8–1.3 × 0.4–0.6 mm, narrowly elliptic, non-carinate, base obtuse, apex acute, margins somewhat to conspicuously dentate, teeth 40–50 µm long (15–20 µm long in GZG. BST.22006). Dorsal trophophylls close, adpressed, 0.7–1.3 × 0.3–0.4 mm, lanceolate or elliptic, non-carinate, base not seen, apex aristate, margins nearly entire to sparsely serrate, teeth 40–50 µm long. Ventral trophophylls distant, mostly symmetric, spreading, ascending, 1.1–1.7 × 0.5–0.8 mm, elliptic or ovate, non-carinate, base rounded, apex acute to acuminate, margins serrate to denticulate, teeth 30–45 µm long (10–25 µm long in GZG. BST.22006) ( Fig. 31D; 32F). Epidermis of dorsal and ventral trophophylls poorly preserved. Strobili terminal, compact, tetrastichous, quadrangular, 4.0–5.4 × 1.3–1.6 mm ( Fig. 32B, C). Sporophylls monomorphic, imbricate, conduplicate, (12–)28–56 per strobilus, 0.8–1.4 × 0.3–0.4 mm, lanceolate, carinate, carina sparsely dentate as in sporophyll margins, base rounded, apex long-attenuate to aristate, margins sparsely dentate, teeth 30–40 µm long, sporophyll-pteryx absent ( Fig. 2K; 31C; 32D). Sporophyll epidermis poorly preserved. Sporangia suborbicular, 300–340 × 220–260 µm, composed of uniform isodiametric cells with thick anticlinal walls. Megaspores 160–240 µm in diam., distally reticulate-rugulate ( Fig. 31F, G). Microspores in tetrads, distally rugulate ( Fig. 31E; 32G, H).

Remarks — Selaginella obscura shares the gross-morphology with S. wangboi , S. tomescui and S. pellucida . However, it differs from those species in having 4.0– 5.4 mm long strobili and sporophylls with a long-attenuate to aristate apex ( Fig. 2K; 31C; 32D). Conversely, the strobili of S wangboi , S. tomescui and S. pellucida are less than 3.4 mm long and the sporophyll apex is attenuate. Although there are minor differences in the size of the axillary and ventral trophophyll indentations between the two examined specimens ( PB 25334 and GZG. BST.22006), they are both regarded as belonging to the same species because they correspond in all other characters.

Like Selaginella wangboi , S. tomescui and S. pellucida , S. obscura resembles several extant species from Asia and the Pacific, including Selaginella brachyblepharis , S. involvens , S. moellendorffii and S. radicata . However, these extant species all have baculate or verrucate microspores ( Van Alderwerelt van Rosenburgh 1915; Dixit 1992; Zhou & al. 2015a), while they are rugulate in S. obscura ( Fig. 31E; 32G, H).

Etymology — The specific epithet is derived from the Latin word obscurus: dark, indistinct or obscure, and refers to the dark and poorly preserved tissue.

Additional specimens investigated — GZG.BST.22006, Geoscientific Collection of the University of Göttingen, PB25335, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences.

Additional sterile morphologies

A few additional specimens have been discovered that represent vegetative branches with trophophylls, but without any connection to strobili. These fossils further substantiate the local abundance and diversity of Selaginella in the mid-Cretaceous Kachin amber forest, but cannot presently be assigned to any of the above-described species with confidence. They are here briefly described informally as Selaginella species 1 , 2 and 3.