Selaginella heterosporangiata 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 : 187-190

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

Holotype: GZG. BST.21999 [ Fig. 4A, C, E], Geoscientific Collection of the University of Göttingen. – Fig. 1B; 4–6 (two specimens available).

Diagnosis — Axes compressed. Monostele located in a central canal, connected to cortical tissue by trabeculae. Trophophylls dimorphic. Axillary trophophylls ovate, apex obtuse to acute, base obtuse, margins slightly dentate. Dorsal trophophylls distant, lanceolate, apex acuminate to aristate, base decurrent, margins serrulate. Ventral trophophylls distant, broadly ovate, oblong or obovate, carinate, base obtuse, apex acute to obtuse, sometimes rounded, margins serrulate. Strobili terminal, bilateral, dorsiventrally complanate, resupinate. Sporophylls dimorphic. Dorsal sporophylls distant, oblong-lanceolate, non-carinate, apex obtuse, rarely acute, margins slightly denticulate; sporophyll-pteryx incomplete, broadly enlarged at base, with serrulate margins. Ventral sporophylls distant, ovate-lanceolate, slightly carinate, apex acuminate, margins sharply serrate-dentate. Sporangia suborbicular, 2 rows of smaller dorsal sporangia and 2 rows of larger ventral sporangia. Microspores proximally rugulate.

Description — Rhizophores not preserved. Axes compressed. Monostele located in an amber-filled central canal, connected to cortical tissue by trabeculae ( Fig. 4J). Trophophylls dimorphic ( Fig. 4I; 5G). Axillary trophophylls somewhat asymmetric, 0.5–1.2 × 0.3–0.5 mm, ovate, apex obtuse to acute, base obtuse, margins slightly dentate, with a few minute teeth. Dorsal trophophylls distant, adpressed, ascending, 0.7–1.5 × 0.2–0.4 mm, lanceolate, slightly carinate, base decurrent, apex acuminate to aristate, margins serrulate ( Fig. 4I; 5G). Ventral trophophylls distant, spreading, slightly asymmetric, 0.6–2.1 × 0.4–0.8 mm, broadly ovate, oblong or obovate, carinate, base obtuse, apex acute to obtuse, sometimes rounded, margins serrulate ( Fig. 4I; 5G). Strobili terminal, bilateral, dorsiventrally complanate, resupinate, 5.2–5.8 × 2.0– 2.7 mm ( Fig. 4A–C; 5A, B). Sporophylls dimorphic. Dorsal sporophylls distant, spreading, asymmetric, 16–18 per strobilus, 1.4–1.9 × 0.4–0.7 mm, oblong-lanceolate, non-carinate, base short-truncate, apex obtuse, rarely acute, margins slightly denticulate, teeth 10–20 µm long; sporophyll-pteryx incomplete, broadly enlarged at base, with serrulate margins, teeth 10–20 µm long ( Fig. 1B; 4E; 5F). Ventral sporophylls distant, spreading, conduplicate, 10–14 per strobilus, 1.0–1.2 × 0.5–0.8 mm, ovate-lanceolate, slightly carinate, base obtuse to rounded, apex acuminate, margins sharply serrate-dentate, teeth 25–40 µm long ( Fig. 1B; 4F; 5D). Epidermis of ventral sporophylls composed of elongate cells, with long axes orientated uniformly parallel to carina ( Fig. 4F). Sporangia suborbicular, smaller dorsal sporangia 330–460 × 310–480 µm and larger ventral sporangia 750–900 × 780–970 µm, composed of uniform isodiametric cells with thick anticlinal walls ( Fig. 4D; 5C, D; 6). Microspores numerous, 25–40 µm in diam., proximally rugulate ( Fig. 4G, H; 5H, I).

Remarks — Specimen GZG. BST.21999, from which one plant fragment was selected as the holotype of Selaginella heterosporangiata , preserves one of the morphological synapomorphies of the Selaginellaceae , namely the monostele situated in an air-filled (amber-filled in the fossils) central canal and connected to the cortical tissues by trabeculae ( Fig. 4J). Preservation of the anatomical structure of the axis in transverse section is not cellular because the tissue is replaced by pyrite.

Selaginella heterosporangiata differs from the other Kachin fossil Selaginella species with bilateral strobili by the conspicuous differences in the size of the dorsal and ventral sporangia ( Fig. 4C; 5C; 6), a condition that has not been observed in any other amber inclusion with bilateral strobili. However, it was not possible to verify if smaller dorsal sporangia correspond to microsporangia and larger ventral sporangia to megasporangia, because all sporangia are empty.

There are several differences between the two examined specimens, specifically with regard to the shape of the dorsal and ventral leaf apices: JZC-Bu-1869 usually has obtuse to rounded ventral trophophylls and aristate dorsal trophophylls, whereas GZG. BST.21999 mostly shows acute ventral trophophylls and short-acuminate dorsal trophophylls. Despite these minor differences, both specimens are congruent and we therefore regard them here as belonging to the same species.

Selaginella kouytcheensis H. Lév. is an extant species endemic to China that resembles the fossil morphology in having bilateral strobili with oblong or lanceolate, denticulate dorsal sporophylls and ovate, acuminate ventral sporophylls, which are dentate at the margins. However, it differs from the fossil in its ovate or elliptic dorsal trophophylls with obtuse apices ( Zhang & al. 2013). Additionally, the proximal microspore surface is also different in these two taxa, namely nearly smooth in S. kouytcheensis ( Zhou & al. 2015a) and rugulate in S. heterosporangiata ( Fig. 4G, H; 5H, I).

Etymology — The specific epithet is derived from the Greek words heteros: different, spora: spore and angeion: case, capsule or vessel. It emphasizes the conspicuous size difference between the (smaller) dorsal and (larger) ventral sporangia.

Additional specimens studied — JZC-Bu-1869, American Museum of Natural History ( Fig. 5, 6).