Frullania chiapasensis Mamontov, K. Feldberg, Schäf.-Verw., Gradst., 2025

Frullania chiapasensis Mamontov, K. Feldberg, Schäf.-Verw., Gradst. sp. nov.

Holotype.

Geoscientific Collection of the University of Göttingen, Germany ( GZG), GZG.BST.22085 ; syninclusion Parmotrema specimen 4.

Etymology.

The species is named after the location of the amber deposit in Chiapas, Mexico.

Age and stratigraphic level.

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

Diagnosis.

Incubously foliated liverwort with conduplicate-trilobed, entire-margined leaves; dorsal lobe obliquely ovate to elliptical, apex rounded to obtuse, lacking ocelli; ventral lobule Frullania - type, saccate, adnate to lobe at a distance of ca. 0.8 of the stem width, parallel or somewhat converging to stem and at places leaning against it, clavate to obovate, not constricted above the postical opening, surface smooth, opening extending along the abaxial side of the lobe for ca. 0.32–0.50 of the lobule length, sickle-shaped; underleaves triangular to obovate, longer than wide, widest in the upper half, bilobed 0.2–0.3 × their length, upper half of underleaves armed with 2 short teeth on both sides.

Description.

Two gametophyte fragments. Intact shoot ca. 2.64 mm long, 0.33–0.35 mm wide with leaves, dark reddish brown (Figs 1 A, B View Figure 1 , 2 View Figure 2 ); with three broken, lateral branches [not clearly visible]. Stem only slightly darker in color than leaves, 35–45 µm in diameter, surface cells not visible. Lateral leaves incubous, alternate, contiguous to imbricate near apex, widely spreading, conduplicate-trilobed (Figs 1 A – C View Figure 1 , 2 A View Figure 2 ). Dorsal lobe in dorsal aspect convex to nearly flat, partially with reflexed margin, obliquely ovate to elliptical (Figs 1 A – D View Figure 1 , 2 A View Figure 2 ), 150–200 µm long × 120–150 µm wide, slightly longer than wide, length: width ratio ca. 1.2–1.3: 1; margin entire, apex rounded to obtuse, inner margin barely extending beyond the farther edge of the stem; lobe base not clearly visible. Lobe cells almost isodiametric or rectangular to 5–6 - angled (Fig. 1 D, F View Figure 1 ), 15–20 × 10–20 µm in the middle of the lobe, marginal cells somewhat smaller; cell walls slightly thickened, with indistinct trigones, no intermediate thickenings seen; with one central mammilla, ca. 5 µm in diameter. Ocelli not observed. Ventral lobules Frullania - type, inflated, saccate, explanate lobules not observed; mostly + / - parallel with the stem, isolated lobules obliquely spreading at an angle of ca. 35 ° to the stem, or leaning against it; remotely inserted, distance between anterior base of lobule and edge of stem ca. 0.8 × the stem width; lobules clavate or obovate (Figs 1 C – E View Figure 1 , 2 View Figure 2 ), 110–140 µm long × 75–90 µm wide at the widest part, longer than wide, length: width ratio 1.5–1.6: 1; apex broadly rounded or angulate; gradually applanate towards the postical opening, not constricted above it, the portion adjoining the opening dorsiventrally compressed in comparison to the gibbous upper half, pinched together; opening very wide, extending along the abaxial side of the lobe for ca. 0.32–0.50 × the lobule length, sickle-shaped (Fig. 2 View Figure 2 ), the apex of the opening with a discolored, gibbous, protuberant cell; the lobules almost symmetric, or somewhat asymmetric due to the sickle-shaped opening, widest in upper third, rarely near the middle; the valves (free margins of the opening) entire, parallel, equal in size; the lobule surface smooth, the margins of the opening crenulate-sinuate, especially near the apex (i. e., the upper end of the sinus), due to protuberant angles between the pairs of cells and strongly concave external walls of these cells. Styli indistinctly visible, triangular (Figs 1 D, E View Figure 1 , 2 View Figure 2 ), with expanded base from several cells and at least 6 cells high, with a short apex of 2 cells. Underleaves 1 per leaf pair, distant, transversely inserted, ca. 2.1–2.5 × the stem width, mostly longer than wide (Figs 1 C, E View Figure 1 , 2 View Figure 2 ), 110–140 µm long × 100–120 µm wide, triangular to obovate, flat, gradually broadening from a cuneate base; lower two thirds entire-margined, upper third armed on both sides with 1–2 short teeth of 1–3 cells; bilobed 0.2–0.3 × their length, sinus acute-angled, the sinus base broadly rounded; lobes widely triangular, obliquely directed to the stem axis, wider than long, with acute to obtuse apex, inner margins of lobes entire, slightly arcuate. Rhizoids possibly in fascicles arising from the upper part of the underleaves (Fig. 2 B View Figure 2 ). Asexual reproduction not observed. Sterile.

Discussion.

The entire conduplicate-trilobed leaves with an entire-margined dorsal lobe and Frullania - type lobules (Figs 1 A – C View Figure 1 , 2 A View Figure 2 ) as well as the bilobed underleaves (Figs 1 C, E View Figure 1 , 2 View Figure 2 ) allow for a reliable generic assignment. Frullania is a large genus of porellalean leafy liverworts with a center of diversity in tropical and subtropical regions, where it often grows epiphytic in the canopy, but it is also a common element of temperate and arctic zones and can grow under much drier conditions than Lejeuneaceae (e. g., Gradstein et al. 2001).

Frullania is the most diverse genus found in amber. Four species occur in Cretaceous Kachin amber ( Feldberg et al. 2021 a, 2021 b), and 16 species have been described from Cenozoic European and African ambers ( Bouju et al. 2021; Feldberg et al. 2021 a). To date, only three Frullania fossils are known from New World deposits: a small fragment described at subgenus level from Dominican amber ( Heinrichs and Schmidt 2010) and two fossils from Mexican amber, including Frullania sp. ( Juárez-Martínez et al. 2023) and the recently described F. delgadilloi Juárez-Mart. & Estrada-Ruiz , the latter representing the most completely preserved specimen ( Juárez-Martínez et al. 2024). The Dominican fossil differs from F. chiapasensis in possessing obliquely spreading lobules, small, acuminate styli, and more deeply bilobed, wider than long underleaves ( Heinrichs and Schmidt 2010, figs 1–5). Frullania sp. from Mexican amber is differentiated by its lobules which are not much longer than wide, constricted above the only slightly arched opening, and much smaller in relation to the lobes ( Juárez-Martínez et al. 2023, fig. 3). Moreover, the leaf lobules in this specimen have a crook-shaped mouth similar to those in the subgenus Meteoriopsis Spruce. By contrast, in F. chiapasensis the lobule mouth is sickle-shaped and resembles those of numerous species in the subgenera Diastaloba s. l. and Microfrullania (R. M. Schust.) R. M. Schust. (see also below). The underleaves of Frullania sp. are not well visible and cannot be compared. Frullania delgadilloi differs from F. chiapasensis in lobule and underleaf shape. The lobules of F. delgadilloi are significantly smaller in relation to the lobes and strongly oblique with the apices oriented outwards, resembling the Dominican Frullania sp. in this aspect. Furthermore, the lobules of F. delgadilloi are inserted further from the stem (ca. 1–2 times the stem width), while those of F. chiapasensis are larger in relation to the lobes, less remote (ca. 0.8 times the stem width), and oriented mostly parallel to the stem. The lobes of F. delgadilloi possess scattered larger cells, interpreted by the authors as ocelli, which seem to be absent from F. chiapasensis . The underleaves in F. delgadilloi are ovate, four or more times wider than the stem (according to our measurements of the shoots imaged in the description of F. delgadilloi ), bilobed to ca. 0.3 of their length, with the lobes narrowly acute and angular along the outer margins ( Juárez-Martínez et al. 2024, fig. 3 A, F). In contrast, the underleaves of F. chiapasensis are triangular to obovate, less than three times wider than the stem, and bilobed to 0.2–0.3 of their length, with 1–2 short but distinct teeth at the base of the obtuse to acute lobes. However, the species also resemble each other in several ways: F. chiapasensis is characterized by a sickle-shaped lobule mouth that extends to ca. 0.32–0.50 of the lobule length and has crenulate margins as well as a protuberant cell at the apex. Juárez-Martínez et al. (2024) describe the opening as “ asymmetrical slightly crenulate with incision reaching about ca. 0.25 of lobule length ”; a protuberant cell is not mentioned. Furthermore, both species have large, foliose styli. We consider especially the differences in lobule shape and orientation to be significant enough to describe a second Frullania species for Mexican Chiapas amber, though both species might be related at subgenus or section level.

Other liverwort inclusions similar to F. chiapasensis are F. baltica Grolle from Baltic and Bitterfeld amber and F. schmalhausenii Mamontov et al. from Rovno amber. However, these species are not only older than F. chiapasensis but can easily be differentiated based on their lobules, which are obliquely spreading, with the apices oriented away from the stem like in F. delgadilloi , instead of lobules oriented mostly parallel to the stem. They can also be distinguished by their low abaxial lobule opening, which ends at less than 0.1 of the lobule length, while the lobe opening of F. chiapasensis is strongly extended along the lateral lobe margin and hence sickle-shaped (Figs 1 View Figure 1 , 2 View Figure 2 ; Grolle and Meister 2004, plate 5; Mamontov et al. 2019, figs 2, 3 b, c, g, j, k).

Because Mexican amber is a rather young deposit and several extant species have been found in Dominican amber ( Gradstein 1993; Kubilius et al. 2017; Feldberg et al. 2021 a), it is very important to compare the new fossil to the extant diversity. As one of the largest and taxonomically most complex genera of leafy liverworts, Frullania contains more than 2,000 published names ( von Konrat et al. 2010; Hentschel et al. 2015) and 675 currently accepted taxa ( Söderström et al. 2016; Schäfer-Verwimp and Winter 2023). Many extant Frullania species and subspecies are insufficiently known, and information about their morphology is only available in protologues published in the 19 th and early 20 th centuries. Recent studies have greatly expanded the knowledge of the complex taxonomy of the genus, and it seems that its diversity is even greater than anticipated, as new species are found regularly (e. g., Hentschel et al. 2009, 2015; Heinrichs et al. 2010; Ramaiya et al. 2010; von Konrat et al. 2010, 2012, 2013; Carter et al. 2017; Silva et al. 2017; Mamontov et al. 2020; Schäfer-Verwimp and Winter 2020, 2022, 2023). Consequently, the assessment of morphological differences between F. chiapasensis and the extant species of the genus is difficult. Moreover, the morphological homoplasies within Frullania challenge the subgeneric assignment of numerous extant non-sequenced species, and even more so that of fossils. Based on the combination of the characteristics of leaf lobes, lobules, and underleaves, F. chiapasensis is similar to species of several subgenera: namely Caulisequa , Diastaloba I, Diastaloba II, Diastaloba III, Frullania, Mammillosae , Meteoriopsis , and Microfrullania . To compare them with F. chiapasensis , we have analyzed the information (protologues, descriptions, and figures) for all species of the mentioned subgenera, which are accepted in Söderström et al. (2016). Some members of these subgenera are well distinguished from F. chiapasensis by the presence of moniliate ocelli in leaf lobes or by the shape of the underleaves (if the underleaves are entire at apex or appendiculate at base, for example). However, many taxa can be distinguished from the newly described species only with difficulty. Therefore, we compare extant species similar to F. chiapasensis in a morphology matrix based on 5 + 11 critical characters (see material & methods and Appendix 1). Most species of subgenera Frullania and Meteoriopsis were excluded from consideration based on characters 1–5, while F. chiapasensis + 144 species meeting these criteria have been chosen to map their approximate differences and similarities based on 11 additional characters.

Based on the character comparison, the extant species morphologically most similar to F. chiapasensis is F. simmondsii Steph. , as described in Hattori (1979, fig. 42). According to Appendix 1, the species coincide with each other in all 11 characters. However, based on the description and illustration in Hattori (1979), the stylus of F. simmondsii is similar to that in numerous species of subg. Diastaloba s. l. by leaving a narrow foliose strip extending along the keel, and thus being wider than long. In contrast, the stylus of F. chiapasensis resembles the foliaceous styli found in numerous species of subg. Trachycolea by being longer than wide. Summarizing all differences between both species, F. chiapasensis can be distinguished from F. simmondsii as follows: (1) leaf lobes longer than wide in F. chiapasensis vs. wider than long in F. simmondsii ; (2) stylus foliaceous, longer than wide vs. filiform, when flat narrowly foliaceous, wider than long; (3) stem underleaves mostly longer than wide, narrowly obcuneate at base, with the margins concave in the lower two-thirds vs. stem underleaves mostly wider than long, widely obcuneate at base, with the margins largely convex in lower two-thirds.

Besides F. simmondsii , we have identified eight more species, each differing from F. chiapasensis in one of the listed 11 characters (Appendix 1). The species are F. colliculosa von Konrat et al. , F. eplicata Steph. , F. gabonensis Vanden Berghen , F. hodgsoniae von Konrat et al. , F. multilaceroides S. Hatt. , F. scalaris S. Hatt. , F. subtilissima (= F. taxodiocola and F. exilis Taylor ) and F. vaga Mitt. Among them, five species ( F. colliculosa , F. gabonensis , F. hodgsoniae , F. multilaceroides , F. subtilissima ) differ from F. chiapasensis in the lobule arrangement. The lobules in the mentioned five species are obliquely patent, spreading from the stem at angles of 25–55 ° or more, whereas in F. chiapasensis the lobules are slightly patent or largely parallel to the stem. Moreover, in F. colliculosa , F. gabonensis , F. hodgsoniae , and F. multilaceroides the underleaves of leading stems are usually slightly broader than long and broadest in the middle, whereas in F. chiapasensis the underleaves are mostly longer than wide and broadest at the upper third. In the studied extant specimens of F. subtilissima from Costa-Rica (Schäfer-Verwimp & Holz SV / H- 0486 / A, Schäfer-Verwimp & Holz SV / H- 0429 / Z, MHA), the width and length of leaf lobes exceed the length of leaf lobules ca. 2 and 2.8 times, respectively, thus the leaf lobes are ca. 8–10 times larger than lobules, while the lobe apex is often apiculate and the margins of underleaves entire or barely angulate. In contrast, in F. chiapasensis the length and width of leaf lobes exceeds the length of leaf lobules ca. 1.4 and 1.6 times, respectively, thus the leaf lobes are only ca. 2–3 times larger than lobules, while the lobe apex is obtuse to rounded, and the margins of underleaves are armed with 1–2 teeth of 1–3 cells on both sides. Among the remaining three species, F. eplicata differs from F. chiapasensis by the low position of the lobule mouth apex that is similar to species of subgenera Frullania , Meteoriopsis , and Trachycolea. Moreover, well-developed underleaves in F. eplicata are up to 3 times broader than the stem and wider than long (Vanden Bergen 1976, fig. 21 A). The second species, F. scalaris , is very similar to F. chiapasensis in the shape and size of leaf lobes and lobules. However, the leaf lobules in the former species are apically overlapping the stem, while the underleaves are described and illustrated as only slightly broader than the stem, entire margined, broadest in the middle, almost bilobed to 0.5 of their length, with triangular-lanceolate lobes ( Hattori 1977, fig. 15). In contrast, in F. chiapasensis the leaf lobules are mostly parallel to the stem, while the underleaves are 2.1–2.5 times broader than stem, toothed in the upper third, bilobed to 0.2–0.3 of their length, with widely triangular lobes. The last species, F. vaga , is described to have only shallowly bilobed, entire margined underleaves ( Mitten 1865), while other distinctions are not evident from the protologue.

The remaining 135 species in Appendix 1, apart from the 9 species discussed above, can be distinguished from F. chiapasensis by two to seven characters. Therefore, the latter taxon can be considered as a separate species which essentially differs from all the described extant and fossil Frullania species. The subgeneric assignment of F. chiapasensis remains problematic because the species morphologically most similar to it are treated now as belonging to different subgenera, namely Diastaloba I and Microfrullania ( Söderström et al. 2016).