Phaeoceros aequatus Suwanmala & Chantanaorr. 2025, sp. nov.

Phaeoceros aequatus Suwanmala & Chantanaorr. sp. nov. Figs 2, 3, 10 A – D Type. Thailand • Chiang Mai: Chiang Dao, Pang Wao, 19°24'33.05"N, 098°51'35.46"E, 1,178 m elev., 16 Oct 2020, S. Chantanaorrapint & O. Suwanmala 4070 (holotype: PSU!; isotype: NICH!, QFA!). Diagnosis. Phaeoceros aequatus is similar to P. himalayensis and P. kashyapii but differs in the thallus lacking tubers, nearly smooth spores under light microscope (vermiculate under SEM), and the distal face of spores without verrucae. Description. Thallus green to dark green in fresh material, becoming yellowish green to brown when dry, growing prostrate, moderately adhering to substrate, forming irregular patches or fan-shaped colonies, dichotomous or irregularly branched into several lobes, thallus lobe lingulate to obovate, sometimes fan-shaped, 10–16 mm long, 3–7 mm wide; margins nearly entire to shallowly crenulate, usually flat; apex truncate to round, or shallowly lobulate, rarely attenuate, flat; tubers absent. Thallus in cross section plano-convex to concave-convex, 5–11 cells thick in the middle region. Dorsal epidermal cells irregular rectangular to hexagonal, 27–100 × 21–45 µm. Chloroplasts 1 per cell expanded occupying nearly entire cells, frequently contracted into round shape, pyrenoid stellate. Nostoc colonies scattered ventrally, appearing as dark dots in old thallus and pale brown near apex. Rhizoids sparse to densely scattered on ventral surface, smooth in early stage, become pegged at maturity, hyaline to pale brown. Sexuality monoicous, weakly protandrous. Androecia sparse, slightly raised over the dorsal surface of thallus, usually 2–7 antheridia per chamber; antheridia subglobose to globose, 2–3 - tiered stalk with quadriseriate cells, 160–180 × 100–126 µm. Archegonia embedded in thallus, connected to the upper surface, sparse, randomly scattered nearly thallus apex. Involucres erect, conical-cylindrical, (1.2 –) 1.5–3 mm long, 3–5 cells thick, mouth smooth, rarely shallowly sinuate. Sporophytes frequent, capsules erect, sometimes inclined, cylindrical, up to 13 mm long; epidermal cells of capsule elongate-rectangular, 145–243 × 15–25 µm; stomata 77–88 × 45–50 µm, surrounded by 5–6 epidermal cells; assimilative layer 3–5 cells thick in cross section; the innermost cells of capsule elongate-rectangular to hexagonal, 40–103 × 17–33 µm, pale brown to dark brown; columella consisting of 4–6 cells in cross section, reddish brown to dark brown. Spores yellowish brown to dark brown, rounded-triangular in polar view, 30–38 µm in equatorial diameter, nearly smooth under light microscope (LM); distal face convex with finely vermiculate ornamentation, without hump-like projection; proximal face with thin triradiate mark, bordered by vermiculate strip on each side of trilete mark, each facet covered with fine vermiculate pattern. Pseudoelaters pale to dark brown at maturity, thick-walled, occasionally branched, 1–3 cells long; pseudoelaters cells rectangular, without helicoidal band. Etymology. The epithet “ aequatus ” refers to smooth distal surface of spore, as observed under light microscope. Distribution, habitat, and ecology. Phaeoceros aequatus is currently known only from the type locality at Chiang Dao Wildlife Sanctuary, Chiang Mai province, and may represent an endemic species of northern Thailand. It occurs on soil on the edge of the mixed bamboo-pine-oak deciduous forests at elevations of 1100–1200 m. It grows associated with other bryophytes such as Entodon macropodus (Hedw.) Müll. Hal., Fissidens spp., Notothylas javanica (Sande Lac.) Gottsche, and Phaeoceros carolinianus (Michx.) Prosk. Conservation status. The type locality of Phaeoceros aequatus is within the Chiang Dao Wildlife Sanctuary, a protected area. Only two small populations of the species have been found along the walking trail, occupying less than a quarter of a square metre. These populations have persisted over six years (2016–2021) of observations. The site remains susceptible to human activities and might be destroyed by fire. However, as suitable habitats appear to occur in the surrounding landscapes and additional survey effort is needed, we propose to treat P. aequatus as Data Deficient (DD) until further information becomes available. Specimens examined. Thailand • Chiang Mai: Chiang Dao, beside the road from Chiang Dao to Muang Kong, 19°24'47.41"N, 098°49'58.88"E, 1,118 m elev., 28 Oct 2018, S. Chantanaorrapint & O. Suwanmala 3447 (PSU); • Pang Wao, 19°24'33.05"N, 098°51'35.46"E, 1,178 m elev., 13 Nov 2016, S. Chantanaorrapint & O. Suwanmala 716 (PSU), 16 Oct 2020, S. Chantanaorrapint & O. Suwanmala 4070 (PSU), 3 Oct 2021, S. Chantanaorrapint & O. Suwanmala 4092 (PSU). Taxonomic notes. The distinctive features of P. aequatus are its monoicous sexuality, absence of thallus tubers, nearly smooth spores under a light microscope, vermiculate spores under SEM, and absence of hump-like projections on the distal surface of the spore. Phaeoceros aequatus could be confused with P. himalayensis and P. kashyapii, as they share similar characters of gametophytes and sporophytes, such as ligulate to obovate thallus lobes, monoicous sexual condition, cylindrical capsules with yellowish brown to dark brown color at maturity, adherent tips of capsule valves, and yellowish brown to dark brown spores and pseudoelaters. However, P. himalayensis and P. kashyapii differ by having irregular verrucose ornamentation on the distal face of the spores that is visible with a light microscope and by producing thallus tubers. In spore ornamentation, P. aequatus is similar to P. perpusillus Chantanaorr. var. scabrellus Suwanmala & Chantanaorr., which also has nearly smooth spores under a light microscope. However, P. aequatus differs in having yellowish brown to dark brown spores and longer pseudoelater cells that are more than 1.5 times the spore diameter. In contrast, P. perpusillus var. scabrellus has yellow spores and shorter pseudoelater cells that are as long as the spore diameter. Similarly, the spores of Phaeoceros dendroceroides (Steph.) Hässel resemble those of P. aequatus, as both appear nearly smooth under a light microscope. However, P. dendroceroides can be distinguished by its broad thallus (6–10 mm wide), which is more consistent with members of the genus Phaeomegaceros (Villarreal pers. comm.).

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