Selaginella minutissima 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 : 186-187

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

Holotype: GZG. BST.22002 [ Fig. 3], Geoscientific Collection of the University of Göttingen. – Fig. 1A; 3 (only specimen available).

Diagnosis — Strobilus terminal, bilateral, dorsiventrally complanate. Dorsal sporophylls imbricate, ovate, carinate, apex acute, margins serrulate; sporophyll-pteryx incomplete, nearly entire with only a few teeth. Ventral sporophylls imbricate, adpressed, broadly ovate, apex acuminate to aristate, margins dentate.

Description — Rhizophores, branches and trophophylls not preserved. Strobilus fragment terminal, compact, bilateral, dorsiventrally complanate, 1.1 × 0.9 mm ( Fig. 3A). Sporophylls dimorphic. Dorsal sporophylls imbricate, ascending, 9 in only available strobilus, 0.4–0.7 × 0.2–0.3 mm, conduplicate, ovate, carinate, base not seen, apex acute, margins serrulate, teeth 5–7 µm long ( Fig. 1A; 3B); sporophyll-pteryx incomplete, nearly entire with only a few teeth similar in size to those of margins ( Fig. 1A; 3B). Ventral sporophylls imbricate, adpressed, symmetric, 6 in only available strobilus, 0.3–0.6 × 0.3–0.4 mm, broadly ovate, non-carinate, base rounded, apex acuminate to aristate, margins dentate, teeth 20–40 µm long ( Fig. 1A; 3C). Epidermis of ventral sporophylls composed of elongate cells, with long axes orientated uniformly parallel to long axis of sporophyll ( Fig. 3C). Sporangia and spores not seen.

Remarks — Only a single fragment of a strobilus has been found. Bilateral strobili with a sporophyll-pteryx are known to be produced exclusively by extant species with resupinate strobili, and the seven species from Kachin amber with bilateral strobili and preserved vegetative shoots likewise show this pattern. Although vegetative leaves are not preserved in the Selaginella minutissima specimen, it is most probable that the strobili were resupinate and that the sporophylls with a sporophyll-pteryx were positioned dorsally.

Selaginella minutissima is distinguished from all other Selaginella inclusions with bilateral strobili by the small size of the strobilus with imbricate, adpressed ventral sporophylls, which are nearly orbicular and aristate at the apex ( Fig. 1A; 3A, C), together with the nearly entire-margined sporophyll-pteryx, which possesses only a few teeth in the margins ( Fig. 3B). The extant S. loriae Hieron. , endemic to New Guinea ( Van Alderwerelt van Rosenburgh 1915), resembles the fossil in regard to morphology, in particular concerning the small strobili (2–3 mm long), oblong-ovate, minutely dorsal sporophylls and broadly ovate ventral sporophylls ( Van Alderwerelt van Rosenburgh 1915). However, S. minutissima differs from S. loriae by the latter possessing dentate-pellucid-margined dorsal sporophylls with teeth 10–30 µm long and ventral sporophylls denticulate with teeth up to 20 µm long ( Hieronymus 1913), whereas the dorsal sporophylls of the fossil are serrulate with teeth up to 7 µm long, not pellucid-margined, and the ventral ones are dentate with teeth 20–40 µm long ( Fig. 3A, C).

Etymology — The specific epithet is the superlative of the Latin adjective minutus: very small, minute, which refers to the small size of the strobilus.