Selaginella isophylla 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 : 208-210

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

Holotype: GZG. BST.22005 [ Fig. 19], Geoscientific Collection of the University of Göttingen. – Fig. 2B; 19 (only specimen available).

Diagnosis — Axes compressed. Trophophylls monomorphic, close to imbricate, arranged in 4 rows, symmetric, ovate, non-carinate, base peltate, rounded or truncate, apex short-acuminate, basal acroscopic margin short-ciliate, medial margins sparsely dentate, distal margins close to apex nearly entire. Strobilus terminal, compact, tetrastichous, quadrangular. Sporophylls monomorphic, imbricate, conduplicate, lanceolate, carinate, carina smooth, apex long-attenuate, medial and proximal margins denticulate, distal margins close to apex nearly entire, sporophyll-pteryx absent.

Description — Rhizophores not preserved. Axes compressed. Trophophylls monomorphic, close to imbricate, arranged in four rows, symmetric, ascending, 0.8–1.2 × 0.4–0.5 mm, ovate, non-carinate, base peltate, rounded or truncate, apex short-acuminate, basal acroscopic margin short-ciliate, medial margins sparsely dentate, distal margins close to apex nearly entire, teeth or cilia 25–50 µm long ( Fig. 19A, E–G). Epidermis cells polygonal, sometimes elongate, with long axes orientated uniformly parallel to long axis of trophophyll. Strobilus terminal, compact, tetrastichous, quadrangular, 2.6 × 1.2 mm ( Fig. 19A, B). Sporophylls monomorphic, imbricate, conduplicate, 20 in only available strobilus, 1.0–1.2 × 0.4–0.6 mm, lanceolate, carinate, carina smooth, base not seen, apex long-attenuate, medial and proximal margins denticulate, distal margins close to apex nearly entire, teeth 10–20 µm long, sporophyll-pteryx absent ( Fig. 2B; 19C, D). Epidermis of sporophylls similar to trophophyll epidermis. Sporangia immature, obscure, not seen.

Remarks — The monomorphic trophophylls arranged in rows and monomorphic sporophylls arranged in tetrastichous quadrangular strobili are suggestive of affinities of this fossil to Selaginella subg. Ericetorum . The trophophylls are typically decussately arranged (at least in the proximal portions of the plant) in all members of this subgenus ( Jermy 1986a; Schulz & al. 2013). However, because the attachment sites of the leaves are not clearly visible in the amber fossil it is impossible to assess whether the vegetative leaves are truly decussately arranged in the fossil. Other diagnostic characters of S. subg. Ericetorum, such as megaspores with wing-like laesurae and that are highly porose at the proximal pole ( Schulz & al. 2013; Weststrand & Korall 2016b), could also not be positively identified in the fossil due to the immature stage of the preserved strobilus.

Among extant members of Selaginella subg. Ericetorum , S. isophylla can be easily distinguished from S. lyallii , S. moratii and S. pectinata by its monomorphic trophophylls, which are dimorphic in the three extant species ( Stefanović & al. 1997). Additionally, the presence of trophophylls that are short-ciliate on their basal acroscopic margin and sparsely dentate in both basiscopic and acroscopic margins of the middle portion ( Fig. 19F, G) discriminate the fossil from several extant species such as S. aboriginalis , S. gracillima , S. pygmaea , S. royenii and S. uliginosa , which all have nearly entire-margined vegetative leaves ( Jermy & Holmes 1998).

Etymology — The specific epithet is derived from the Greek words isos: equal and phyllon: leaf. It emphasizes that the species is isophyllous in its vegetative part, i.e. possessing monomorphic trophophylls.

Fossils with unresolved infrageneric affinities

Subgenus not determined