Selaginella grimaldii 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 : 190-192

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

Holotype: SMNS Bu-355 [ Fig. 7A, C–G], State Museum of Natural History Stuttgart. – Fig. 1C; 7 (only specimen available).

Diagnosis — Axes compressed. Trophophylls dimorphic. Axillary trophophylls symmetric, broadly ovate, non-carinate, base rounded, apex acute to obtuse, margins finely dentate. Dorsal trophophylls close to imbricate, adpressed, somewhat asymmetric, oblong to slightly obovate, non-carinate, base inequilateral, basiscopic portion rounded, acroscopic portion straight, apex acuminate to apiculate, margins denticulate. Ventral trophophylls close to imbricate, ascending, symmetric, broadly ovate, non-carinate, base rounded to slightly truncate, apex acute to slightly acuminate, margins denticulate. Strobili terminal, bilateral, dorsiventrally complanate, resupinate. Sporophylls dimorphic. Dorsal sporophylls close to imbricate, spreading, slightly asymmetric, lanceolate, non-carinate, base inequilateral, apex acute to obtuse, margins serrulate; sporophyll-pteryx incomplete, with serrulate margins similar to those on sporophyll margins. Ventral sporophylls imbricate, slightly conduplicate, symmetric, broadly ovate, slightly carinate, base decurrent, apex acuminate, margins denticulate. Sporangia suborbicular to sometimes ellipsoid. Microspores distally and proximally rugulate.

Description — Rhizophores not preserved. Axes compressed. Trophophylls dimorphic ( Fig. 7A, B). Axillary trophophylls symmetric, 0.7–0.8 × 0.5–0.6 mm, broadly ovate, non-carinate, base rounded, apex acute to obtuse, margins finely dentate, teeth 10–20 µm long ( Fig. 7C). Dorsal trophophylls close to imbricate, adpressed, somewhat asymmetric, 0.8–0.9 × 0.4–0.6 mm, oblong to slightly obovate, non-carinate, base inequilateral, basiscopic portion rounded, acroscopic portion straight, apex acuminate to apiculate, margins denticulate, teeth 10–15 µm long. Ventral trophophylls close to imbricate, ascending, symmetric, 1.2–1.8 × 0.7–1.1 mm, broadly ovate, non-carinate, base rounded to slightly truncate, apex acute to slightly acuminate, margins denticulate, teeth 10–20 µm long. Epidermis of dorsal and ventral trophophylls not preserved. Strobili terminal, bilateral, dorsiventrally complanate, resupinate, 1.4–3.0 × 0.8–2.0 mm ( Fig. 7D, E). Sporophylls dimorphic. Dorsal sporophylls close to imbricate, spreading, slightly asymmetric, 7–10 per strobilus, 0.9–1.2 × 0.3–0.4 mm, lanceolate, non-carinate, base inequilateral, apex acute to obtuse, margins serrulate, teeth 10–20 µm long ( Fig. 1C; 7F, G); sporophyll-pteryx incomplete, with serrulate margins similar to those on sporophyll margins ( Fig. 1C; 7G). Ventral sporophylls imbricate, slightly conduplicate, symmetric, 7–9 per strobilus, 0.8–1.0 × 0.6–0.7 mm, broadly ovate, slightly carinate, base decurrent, apex acuminate, margins denticulate, teeth 10–20 µm long ( Fig. 1C; 7E, H). Epidermis of both dorsal and ventral sporophylls not preserved. Sporangia suborbicular to sometimes ellipsoid, 260–400 × 180–240 µm ( Fig. 7I). Microspores sometimes in tetrads when released, 25–30 µm in diam., distally and proximally rugulate ( Fig. 7J, K).

Remarks — Among the fossil-taxa with bilateral resupinate strobili, Selaginella grimaldii closely resembles S. ohlhoffiorum and S. cretacea ( Li & al. 2022) in its general morphology. They all share the presence of sporophylls with serrulate-denticulate margins ( Fig. 1C–E; 7D–H; 8A–C; 9A–C; 11B, C). They also share the presence of an incomplete sporophyll-pteryx, with serrulate margins in dorsal sporophylls ( Fig. 1C–E; 7G; 8C; 9D; 11C). However, some apparent differences were also detected. For example, S. ohlhoffiorum and S. cretacea have distant and predominantly oblong ventral trophophylls ( Fig. 8A; 11E). This contrasts with the imbricate, broadly ovate ventral trophophylls of S. grimaldii ( Fig. 7B). Strobili in S. grimaldii and S. cretacea may be up to 3.2 mm long and sporangia are generally ellipsoidal, whereas S. ohlhoffiorum strobili are longer than 3.5 mm long and contain orbicular sporangia. Furthermore, S. grimaldii and S. cretacea have strobili of similar sizes although both differ in the number of dorsal and ventral sporophylls: S. grimaldii has only 7–10 dorsal sporophylls per strobilus, while S. cretacea has c. 14 dorsal sporophylls ( Li & al. 2022). Similarly, the number of ventral sporophylls per strobilus differs between both species, i.e. 7–9 ventral sporophylls in S. grimaldii but 12–13 in S. cretacea ( Li & al. 2022).

Several present-day Asian Selaginella species produce strobili that are gross-morphologically similar to the bilateral resupinate strobili of S. grimaldii in that they have dentate dorsal and ventral sporophylls, the dorsal ones bearing a sporophyll-pteryx. These species include S. decipiens Warb. from India, China and Vietnam ( Zhang & al. 2013), S. monospora Spring , native to Bhutan, China, India, Myanmar, Nepal, Thailand and Vietnam ( Zhang & al. 2013), S. trichophylla K. H. Shing from Bhutan, China, India and Vietnam ( Zhang & al. 2013) and S. miniatospora (Dalzell) Baker and S. tenera (Hook. & Grev.) Spring , both endemic to India ( Dixit 1992; Fraser-Jenkins & al. 2017). However, the following characters discriminate the extant taxa from S. grimaldii : S. decipiens , S. miniatospora and S. tenera all have asymmetric ventral trophophylls, with the acroscopic base strongly enlarged, broader, sometimes overlapping the branches in S. decipiens ( Zhang & al. 2013), and the acroscopic base is dilated in the other two taxa ( Dixit 1992). In contrast, S. grimaldii produced symmetric ventral trophophylls with a rounded to truncate base ( Fig. 7B). All the other above-mentioned species are distinct from the fossil in that they produce verrucate microspores ( Dixit 1992; Zhou & al. 2015a), which differ from the rugulate ornamentation seen in the fossil ( Fig. 7K). Additionally, S. monospora and S. trichophylla are characterized by a complete sporophyll-pteryx in dorsal sporophylls ( Zhang & al. 2013), in contrast to the incomplete sporophyll-pteryx in S. grimaldii ( Fig. 1C; 7G).

Etymology — The specific epithet honours Professor David A. Grimaldi (American Museum of Natural History, New York, U.S.A.), in recognition of his outstanding contributions to our knowledge of amber, amber deposits and amber inclusions.