Thysananthus patrickmuelleri K. Feldberg, Gradst., Schäf.-Verw., Mamontov, 2025

Thysananthus patrickmuelleri K. Feldberg, Gradst., Schäf.-Verw., Mamontov sp. nov.

Holotype.

Geoscientific Collection of the University of Göttingen, Germany ( GZG), GZG.BST.22086 .

Additional specimens examined.

Patrick Müller Amber Collection (Zweibrücken, Germany) MEX 70, MEX 71.

Etymology.

The specific epithet honors the amber collector Patrick Müller who has generously supported our research by providing numerous amber fossils for study, including all known specimens of this species.

Age and stratigraphic level.

15‒23 Ma, Langhian – Aquitanian (early to middle Miocene), Simojovel, Chiapas, Mexico.

Diagnosis.

Incubously foliated liverwort with a ventral merophyte at least five cells wide; flagelliform branches present on lower parts of the shoots; lateral leaves conduplicate-bilobed; dorsal lobe convex, asymmetrically oblong-ovate with a rounded to obtuse apex, median lobe cells mostly elongate with distinctly cordate trigones; ventral lobule Lejeunea - type, ovate-rectangular with one hooked apical tooth and a truncate apex; underleaves suborbicular to rounded quadrate to obovate with a rounded to emarginate, recurved apex and entire, mostly revolute margins.

Description.

Gametophyte fragments yellowish to dark reddish brown, 2.7–5.04 mm long, main shoots 0.52–1.16 mm wide with leaves (Figs 4 A – C View Figure 4 , 5 A – C View Figure 5 , 6 A – E View Figure 6 ). Branching irregular, Lejeunea - type, primary branches either similar to main shoot, or flagelliform and microphyllous (Figs 4 B, D View Figure 4 , 6 C, D View Figure 6 ); main shoot-like branches 1.26–4.8 mm long, 0.41–1.4 mm wide with leaves; flagelliform branches 0.2–0.24 mm wide; one secondary microphyllous branch becoming main shoot-like on upper part. Stem dark reddish brown to blackish, 40–70 µm in diameter; cortical cells possibly thick-walled and elongated [mostly obscured by underleaves], ventral merophyte ca. 5 cells wide; stem on large branches 40–60 µm in diameter, on flagelliform branches ca. 30 µm in diameter. Lateral leaves incubous, densely imbricate, alternate, widely spreading to erect-spreading or erect-appressed (especially on upper shoot parts), conduplicate-bilobed with large dorsal lobe and a Lejeunea - type ventral lobule folded against the lobe (Figs 4 C, E View Figure 4 , 5 C View Figure 5 , 6 E View Figure 6 ), insertion line J-shaped. Dorsal lobe convex, asymmetrically oblong-ovate, on upper shoot parts falcate, 480–770 µm long × 220–300 µm wide in the middle, length: width ratio 1.4–3.5: 1, margin entire, postical margin nearly plane or slightly incurved for up to 0.7 × the lobe length, exterior to keel first weakly then abruptly arched towards apex, apex rounded to obtuse, antical margin regularly arched, lobes overlapping stem 0.5–2 × the stem width beyond the farther edge of the stem, dorsal base not visible; lobes on larger branches very similar to main shoot, short ovate to oblong-ovate, slightly falcate to straight, 230–840 µm long × 160–430 µm wide in the middle, length: width ratio 1.4–2.1: 1; lobes on flagelliform branches erect-spreading to erect, 150–160 µm long × 70–110 µm wide in the middle, length: width ratio 1.5–2.1: 1. Lobe cells rectangular to hexagonal (Figs 4 F View Figure 4 , 5 D, E View Figure 5 , 6 E View Figure 6 ), elongated at the lobe base and in the middle, slightly shorter to (sub) isodiametric near apex, up to 3 × as long as wide [often collapsed and deformed], basal cells 20–35 µm long × 7.5–20 µm wide, median cells 15–35 µm long × 7.5–17.5 µm wide, apical cells 7.5–20 µm long × 7.5–15 µm wide; cell walls thin, trigones cordate, not coalesced, few intermediate thickenings present. Ventral lobule ovate-rectangular, lower part inflated, free antical margin not inflexed (Figs 4 E View Figure 4 , 5 C, D View Figure 5 , 6 E View Figure 6 ), 0.3–0.4 × the length of the lobe, 200–310 µm long × 80–200 µm wide in the middle, length: width ratio 1.6–2.9: 1, free antical margin nearly straight to slightly arched, free lateral margin truncate and mostly forming an angle of ca. 90 ° with the postical lobe margin, apex with a single, often hooked tooth (Figs 4 E View Figure 4 , 5 D View Figure 5 ), 10–15 µm (1–2 cells) long × 10–15 µm (1–2 cells) wide at base, hyaline papilla not seen, keel curved, leaves slightly emarginate at end of keel; no appendages at the base of the lobule or the keel seen; lobules on larger branches 0.3–0.5 × as long as lobes, 110–310 µm long × 70–160 µm wide in the middle, length: width ratio 1.3–2.4: 1; lobules on flagelliform branches 0.5–0.7 × as long as lobe, 90–110 µm long × 60–80 µm wide in the middle, length: width ratio 1.3–1.8: 1. Underleaves imbricate to contiguous, symmetrical, suborbicular to rounded quadrate to obovate, generally wider than long to as long as wide (Figs 4 C, E View Figure 4 , 5 C View Figure 5 , 6 E View Figure 6 ), 3–6 × wider than the stem, 150–310 µm long × 160–390 µm wide, length: width ratio 0.6–1: 1; slightly squarrose, concave with recurved margins to occasionally nearly plane, apex recurved, rounded to truncate to emarginate, margins entire, insertion line arched, base auriculate, not adnate to lateral leaf; underleaves on larger branches more often nearly plane, on lower parts slightly longer than wide, 160–280 µm long × 130–360 µm wide, length: width ratio 0.7–1.2: 1, apex rounded, often not recurved; underleaves on flagelliform branches not clearly visible, possibly ovate. Rhizoids in bundles near underleaf insertion, bundles up to 230 µm long, rhizoids ca. 10 µm in diameter with truncate and ampliate tips. Sterile.

Discussion.

The conduplicate-bilobed leaves with Lejeunea - type lobules and the presence of underleaves clearly identify this specimen as a member of Lejeuneaceae (Figs 4 C, E View Figure 4 , 5 A – C View Figure 5 , 6 E View Figure 6 ). The robust size, the undivided underleaves, and at least five cells wide ventral merophyte allow an assignment to subfamily Ptychanthoideae (Figs 4 A, C View Figure 4 , 5 A – C View Figure 5 , 6 A, B, E View Figure 6 ). Although all specimens are sterile, the ovate, apically rounded dorsal lobes, the elongate median lobe cells with distinct cordate trigones, as well as the entire-margined underleaves, the probably thick-walled stem cells, and the presence of flagelliform branches on lower shoot parts are indicative of the genus Thysananthus (Figs 4 View Figure 4 , 5 View Figure 5 , 6 A – E View Figure 6 ).

Thysananthus is a large genus with 30 extant species and several fossil taxa ( Gradstein 1993; Sukkharak and Gradstein 2014, 2017; Sukkharak 2015; Wang et al. 2016; Feldberg et al. 2021 a). It includes the former genus Mastigolejeunea which was ranked at subgenus level based on molecular phylogenetic analyses and morphology ( Sukkharak and Gradstein 2017). The oldest fossil of the genus is T. contortus (Göpp. & Berendt) Sukkharak & Gradst. from Paleogene Baltic and Bitterfeld amber. This species has predominantly Frullania - type branches, which become as vigorous as the main axis, and rarely Lejeunea - type branches, as well as four lobule teeth ( Grolle and Meister 2004, plates 14, 15, as Mastigolejeunea contorta (Göpp. & Berendt) Gradst. & Grolle ; Sukkharak and Gradstein 2017). More fossil species of Thysananthus have been found in tropical Miocene ambers. The genus is known from Dominican amber with the extant species Thysananthus auriculatus (Wilson & Hook.) Sukkharak & Gradst. ( Gradstein 1993, as Mastigolejeunea auriculata (Wilson & Hook.) Steph. ; Sukkharak and Gradstein 2017) as well as the extinct species T. bidentulus (Gradst.) Sukkharak & Gradst. ( Gradstein 1993, as Mastigolejeunea bidentula Gradst. ; Sukkharak and Gradstein 2017) and T. weiweianus N. - N. Yu & Gradst. ( Yu et al. 2020). Thysananthus auriculatus can be differentiated from T. patrickmuelleri by the less elongate leaf lobes, the more strongly incurved postical leaf margin, the oblique apical free margin of the lobule which continues into the postical margin of the lobe, and the either small or absent apical lobule tooth ( Gradstein 1993, fig. 7; Sukkharak and Gradstein 2014, figs 4, 5). Thysananthus bidentulus is more similar to T. patrickmuelleri in its general habit, but differs in having oblong lobules with oblique apices, which continue into the postical margin of the lobe, and the presence of two one-celled lobule teeth ( Gradstein 1993, fig. 8). Thysananthus weiweianus differs from the new fossil species in having larger, often confluent trigones, a very small or absent lobule tooth, and more elongate underleaves with plane lateral margins ( Yu et al. 2020, figs 1, 2). Another amber deposit that includes Thysananthus is Miocene Ethiopian Shewa amber ( Bouju et al. 2021, fig. 3). Thysananthus aethiopicus Bouju et al. differs from the new Mexican fossil in having lobes with more strongly incurved postical margins and occasionally apiculate apices, as well as lobules with a very coarse apical tooth and sometimes a second tooth.

A specimen similar to T. patrickmuelleri in its general habit has already been found in Mexican amber but was only identified as belonging to Ptychanthoideae and possibly to Mastigolejeunea (Fig. 6 F View Figure 6 ; Scheben et al. 2014). Many important diagnostic characters are not visible in this specimen, especially the walls of the lobe cells, which are crucial for correctly identifying the genus, and the structure of the lobules.

Compared to the extant species of Thysananthus , the new fossil is very similar to the neotropical T. plicatiflorus (Spruce) Sukkharak & Gradst. , a species widely distributed in the rainforests of northern South America ( Gradstein 1994, 2021; Sukkharak and Gradstein 2014, fig. 21, as Mastigolejeunea plicatiflora (Spruce) Steph. ). Both species share elongate, ovate-oblong leaves with rounded apices and a plane postical margin, lobules with a single short tooth and a truncate apical free margin that terminates at the end of the keel, as well as auriculate underleaves. The two species differ markedly, however, in the orientation of the underleaves, which are very flat and have a plane apex in T. plicatiflorus while being somewhat squarrose with a distinctly recurved apex in T. patrickmuelleri (apex only occasionally plane on branch leaves). Another quite similar species is the Australasian T. ligulatus (Lehm. & Lindenb.) Sukkharak & Gradst. ( Sukkharak and Gradstein 2014, figs 18, 19, as Mastigolejeunea ligulata (Lehm. & Lindenb.) Schiffn. ), which also has rather elongated lobes with rounded apices, plane margins, and auriculate underleaves. However, it can be differentiated by the lobules being smaller in relation to the lobes (0.2–0.3 times the lobe length vs. 0.3–0.4) and underleaves that are often longer than wide, spathulate, and not overlapping the stem as much as those of T. patrickmuelleri (3–4 times wider than the stem vs. 3–6). Since the new fossil shows significant morphological differences to all known extinct and extant species of Thysananthus , it is described as a new species.