ИСТИНА

Dispersed monotype genus Maiaspora Mamontov, McLean, Orlova, Gavrilova, 2021 from Late Visean of European Russia comprises rare trilete miospores with triangular-concave or triangular outline and delicate metareticuloid sculpture. This complex ornament incorporates both the polygonal muri and the circular scrobiculae at the base of the lumina. Such sculptural surface has been detected only on the few in situ spores from the Moscovian of the Czech Republic and North France. On the one hand, suggestions of miospores affinities based on morphological comparison with in situ spores. On the other hand, morphological similarities between miospores and microspores often reflect some sort of convergence rather than a natural link to parent plants. However, reliable evidence of relations between micro- and miospores is inferred only from the comparison at the ultrastructural level. Unfortunately, the ultrastructure of the metareticuloid dispersed spores remains uncertain. Therefore, the main purpose of this contribution is to study the ultrastructural details for the sporoderm organization of Maiaspora to elucidate its natural relationships. Studied miospore specimens of Maiaspora panopta were collected from the sample MS-7 of upper Visean (middle Mississippian) from Kaluga Region (Moscow syneclise). All material was prepared and analyzed using LM (Light Microscopy), SEM (Scanning Electron Microscopy) in the Department of Palaeontology (Lomonosov Moscow State University (MSU)), and Borissiak Paleontological Institute of Russian Academy of Science. Individually picked miospore specimens were cut on a diamond knife. Ultrathin sections were stained with uranyl acetate followed by lead citrate solution, then examined using TEM (Transmission Electron Microscopy) in the Electron Microscopy Laboratory of the Biological Faculty (EMLBF) of MSU. TEM analysis of ultrathin sections through the Maiaspora revealed a ‘specialized’ level of exospore organization. It is characterized by a well-developed proximal flange (= kyrtome-like bands) with a distinct foliated substructure of the middle exospore layer. The exospore is three-layered (inner, middle, and outer layers) between the aperture and flange in contrast with the two-layered (inner and outer layers) structure that continues outside of the flange up to the distal pole. This remarkable spore ultrastructure is shared exclusively by modern primitive families of leptosporangiate ferns: Gleicheniaceae, Matoniaceae, and Dipteridaceae. However, none of them produce a metareticuloid exospore surface. In the in situ record, only microspores of fructifications Raditheca dobranyana from the Moscovian (Brousmiche-Delcambre et al. 1997) display a close correlation with Maiaspora in the ultrastructure and sculpture features. Thus, the discovery of ‘specialized’ three-layered exospore in Maiaspora allows us to introduce the earliest record (Late Visean) of Gleicheniaceae based on the ultrastructure evidence. The study was supported by the Russian Foundation for Basic Research № 19-04-00498. The author thanks Svetlana V. Polevova and Julia V. Kupriyanova from EMLBF for their invaluable assistance with the preparations miospore material for TEM.