Selaginella cretacea Ya Li, H. Schneid. & Y. D. Wang

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 : 195-197

Selaginella cretacea Ya Li, H. Schneid. & Y. D. Wang in Cretaceous Res. 133: 105143, page 3. 2022, emended here. – Holotype: PB22713, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. – Fig. 1E; 11.

Emended diagnosis — As provided by Li & al. (2022), with following addition: Axillary trophophylls symmetric, oblong, non-carinate, apex apiculate, margins serrulate. Dorsal sporophylls close, ascending, asymmetric, ovate to ovate-falcate; sporophyll-pteryx incomplete, with serrulate margins similar to those on sporophyll margins. Ventral sporophylls imbricate, slightly conduplicate, symmetric, ovate-lanceolate, slightly carinate, apex long-acuminate, margins denticulate to serrulate.

Additional description — Rhizophores not preserved. Axes compressed. Trophophylls dimorphic ( Fig. 11E, F). Axillary trophophylls symmetric, c. 1.2 × 0.4 mm, oblong, non-carinate, base not seen, apex apiculate, margins serrulate, teeth <20 µm long ( Fig. 11G). Dorsal trophophylls distant, adpressed, 0.6–1.0 × 0.2–0.3 mm, lanceolate, non-carinate, base decurrent, apex long-acuminate to aristate, margins denticulate to serrulate, teeth 7–9 µm long ( Fig. 11E). Ventral trophophylls distant, ascending, slightly asymmetric, 0.9–1.6 × 0.3–0.7 mm, oblong to lanceolate, non-carinate, base rounded, non-auriculate, apex acute to apiculate, margins serrulate to serrate, teeth 5–25 µm long ( Fig. 11E, F). Epidermis of dorsal and ventral trophophylls not preserved. Strobili terminal, bilateral, dorsiventrally complanate, resupinate, 2.9–3.2 × 1.4–1.6 mm ( Fig. 11B). Sporophylls dimorphic. Dorsal sporophylls close, ascending, asymmetric, c. 14 per strobilus, 0.8–1.2 × 0.3–0.5 mm, ovate to ovate-falcate, non-carinate, base not seen, apex acute, margins serrulate, teeth 5–16 µm long ( Fig. 1E; 11C); sporophyll-pteryx incomplete, with serrulate margins similar to those on sporophyll margins ( Fig. 1E; 11C). Ventral sporophylls imbricate, slightly conduplicate, symmetric, 12 or 13 per strobilus, 0.5–0.9 × 0.3–0.5 mm, ovate-lanceolate, slightly carinate, base not seen, apex long-acuminate, margins denticulate to serrulate, teeth 7–21 µm long ( Fig. 1E). Epidermis of both dorsal and ventral sporophylls not preserved. Sporangia ellipsoid, 380–400 × 190–230 µm ( Fig. 11D). Microspores 28–30 µm in diam., surface not seen ( Fig. 11D).

Remarks — Our morphological analysis of images provided by Li & al. (2022), together with new photomicrographs of the type specimen, revealed that there are several discrepancies between the specimen and the original description of the taxon. We have, therefore, emended the diagnosis of Selaginella cretacea and provided a new and improved description, which includes also the morphology of the axillary trophophylls and the sporophyll-pteryx of the dorsal sporophylls.

This fossil-taxon is very similar to Selaginella grimaldii and S. ohlhoffiorum in sharing the presence of serrulate-denticulate trophophyll and sporophyll margins. However, it differs from S. grimaldii in its distant and predominantly oblong ventral trophophylls, ( Fig. 11E) and from S. ohlhoffiorum in its short strobili (up to 3.2 mm long) and ellipsoid sporangia (see remarks under S. grimaldii for further discussion).

The fossil displays morphological similarities to some extant species with bilateral resupinate strobili, including Selaginella decipiens , S. miniatospora , S. monospora , S. tenera and S. trichophylla . They all share the general shape, type of margins and arrangement of sporophylls and the presence of a sporophyll-pteryx in the dorsal sporophylls ( Dixit 1992; Zhang & al. 2013). They can be differentiated by features such as presence of a complete sporophyll-pteryx and details in the shapes of the ventral trophophylls. For example, S. monospora and S. trichophylla have carinate trophophylls and dorsal sporophylls with a complete sporophyll-pteryx ( Zhang & al. 2013). In contrast, S. cretacea has non-carinate trophophylls ( Fig. 11E, F) and dorsal sporophylls with an incomplete sporophyll-pteryx ( Fig. 1E; 11C). Selaginella decipiens differs from S. cretacea in having asymmetric ventral trophophylls, with the acroscopic base strongly enlarged, broader and sometimes overlapping the branches ( Zhang & al. 2013). Selaginella miniatospora and S. tenera have ventral trophophylls with an asymmetric base and a dilat- ed basal acroscopic side ( Dixit 1992), whereas S. cretacea has symmetric ventral trophophylls. Additionally, S. tenera has contiguous ventral trophophylls at the branches, which are sometimes imbricate ( Dixit 1992) and thus different from the pattern of distant ventral trophophylls seen in the fossil.