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

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

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

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.

Two gametophyte fragments. Intact shoot ca. 2.64 mm long, 0.33–0.35 mm wide with leaves, dark reddish brown (Figs 1A, B, 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 1A–C, 2A). Dorsal lobe in dorsal aspect convex to nearly flat, partially with reflexed margin, obliquely ovate to elliptical (Figs 1A–D, 2A), 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. 1D, F), 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 1C–E, 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), 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 1D, E, 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 1C, E, 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. 2B). Asexual reproduction not observed. Sterile.

Figure 1. 

Frullania chiapasensis sp. nov. from Mexican amber, holotype (GZG.BST.22085). A. Shoot fragments in dorsal view; B. Shoot fragments in ventral view; C. Detail of intact shoot fragment in ventral view; D. Dorsal lobe (right) with mammillose cells and ventral lobules (left) in dorsal view; E. A toothed underleaf and a lobule with a triangular stylus (indicated by arrow) in ventral view; F. Lobe cells with large central mammilla.

Figure 2. 

Frullania chiapasensis sp. nov. from Mexican amber, holotype (GZG.BST.22085). A. Upper part of intact shoot fragment in ventral view; B. Lower part of intact shoot fragment in ventral view, the uppermost underleaf shows a rhizoid bundle.

The entire conduplicate-trilobed leaves with an entire-margined dorsal lobe and Frullania-type lobules (Figs 1A–C, 2A) as well as the bilobed underleaves (Figs 1C, E, 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. 2021a, 2021b), and 16 species have been described from Cenozoic European and African ambers (Bouju et al. 2021; Feldberg et al. 2021a). 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. 3A, 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, 2; Grolle and Meister 2004, plate 5; Mamontov et al. 2019, figs 2, 3b, 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. 2021a), 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 19th and early 20th 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. 21A). 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).

Carsten Gröhn Amber Collection (Glinde, Germany) 10410; syninclusion Spruceanthus extinctus (Heinrichs et al.) Gradst. & Sukkharak, with antheridia (Feldberg et al. 2021a).

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

Gametophyte fragment ca. 0.73 mm long, 0.19–0.34 mm wide with leaves [the damaged leaves make the width difficult to assess], light yellowish to nearly transparent (Fig. 3A, B). Branching Lejeunea-type, leaves and underleaves somewhat smaller than on main shoot, form similar; intact branch 0.43 mm long, ca. 0.2 mm wide with leaves (Fig. 3C). Stem light yellowish brown, ca. 30 µm wide, surface cells elongated, possibly thick-walled, ventral merophyte ca. 2 cells wide (Fig. 3A); stem on branch somewhat thinner than main stem [~20 µm wide; appears to be shrunken]. Lateral leaves incubous, conduplicate-bilobed, imbricate to (probably) distant, alternate, obliquely spreading on lower part of main axis, on shoot apex and on branch erect (Fig. 3A–D), insertion line possibly J-shaped [not clearly visible]. Dorsal lobe flat, elliptical to ovate, slightly falcate, on main stem ca. 220 µm long × 150 µm wide in the middle [apex probably damaged], length : width ratio ca. 1.5 : 1, margins entire, antical margin regularly arched towards apex, postical margin slightly arched, forming a wide angle with the keel (Fig. 3A–C); lobes of branch leaves 160–180 µm long × 100–130 µm wide in the middle, length : width ratio 1.4–1.6 : 1, apex rounded. Lobe cells isodiametric to oblong, mostly hexagonal (Fig. 3C–E), up to 1.5× as long as wide, 15–30 µm long × 10–20 µm wide; cell surfaces smooth; cell walls thin, trigones small, triangular (Fig. 3E). Ocelli not seen. Ventral lobules Lejeunea-type, vertically inserted, inflated, ovate (Fig. 3A–C, E), ca. 0.5–0.6× the length of the dorsal lobe [difficult to assess, because the lobe seems to be damaged at the apex], ca. 150 µm long × 100 µm wide in the broadest part (lower half), length : width ratio ca. 1.5 : 1; free antical margin arched, slightly involute; apical margin truncate, with 1 short, blunt tooth 10–20 µm long (ca. 1 cell) (Fig. 3E, indicated by black arrow); keel arched, possibly crenate (Fig. 3C, E) [this might be an artifact due to damaged and collapsed cells with the cell walls remaining as ridges]; lobules of branch leaves similar to stem lobules. Underleaves distant, ovate (Fig. 3C, D), ca. 110 µm long × 95 µm wide in the middle, length : width ratio ca. 1.2 : 1, ca. 2.5–3× wider than stem, base cuneate, insertion line straight, margin entire; bilobed to 0.5 of their length, lobes slightly divergent, triangular, ca. (30–)55 µm (3–4 cells, incl. short triangular tip cell) long × 30–35 µm (3 cells) wide basally, apices acute, sinus broadly V-shaped, cell surface smooth; underleaves on branch 70–80 µm long × ca. 65 µm wide in the middle, length : width ratio 1.1–1.2 : 1, triangular lobes acute to obtuse, ca. 30 µm (3–4 cells, incl. short triangular tip cell) long × 20 µm (3 cells) wide basally. Rhizoids in tight bundle at the base of one underleaf, bundle ca. 120 µm long × 20 µm wide, rhizoids ca. 5 µm wide. Sterile.

Figure 3. 

Lejeunea sp. from Mexican amber (Carsten Gröhn Amber Collection, 10410). A, B. Gametophyte fragment attached to a specimen of Spruceanthus extinctus, ventral view; C. Lejeunea-type branch in ventral view; D. Shoot apex with bilobed underleaf; E. Apical part of a Lejeunea-type lobule (tooth indicated by black arrow).

The conduplicate-bilobed leaves with Lejeunea-type lobules (Fig. 3A–C) and the presence of underleaves (Fig. 3C, D) clearly identify this specimen as a Lejeuneaceae, and the narrow, only two cells wide ventral merophyte (Fig. 3A) allows an assignment to subfamily Lejeuneoideae. The bilobed underleaves (Fig. 3C, D) indicate that a member of the tribe Lejeuneeae is at hand.

The assignment to a genus is more difficult. The fragment is very small, partially obscured, and not many leaves are intact. However, the obliquely spreading leaf lobes, the large, often flask-shaped lobules which measure 0.5–0.6 times the leaf length and bear a short, straight tooth, and the thin-walled leaf cells align the fossil with Lejeunea. This genus comprises ca. 200 extant species and occurs in tropical and warm-temperate regions, where it grows on bark, living leaves, and other substrates (Gradstein 2020; Lee 2013). It is characterized by the absence of ocelli, lobules with one straight tooth, entire or bifurcated underleaves, and Lejeunea-type branches.

The fossil differs from extant representatives of this genus by the small overall size and the size of the lobule, which is rather large in relation to the lobe, though this might be due to the damaged lobe apex. Lejeunea species are usually larger than the fossil, although some of the smallest extant species approximate it in size and are generally morphologically very similar, e.g., Lejeunea elliottii Spruce (Reiner-Drehwald and Goda 2000, fig. 9) which is 450–700 µm wide or L. erostrata E.Reiner & Goda (Reiner-Drehwald and Goda 2000, fig. 10) with a width of only 400–450 µm. Typically, the lobules of Lejeunea are maximally 0.5 times the lobe length.

Other genera with a similar size to the fossil are Metalejeunea and Microlejeunea. However, both differ in having lobes oriented parallel to the stem and lobules which are not flask-shaped. Furthermore, the genus Microlejeunea is characterized by lobules which are more than 0.5 times the lobe length and bear a very distinct curved tooth which is usually larger and more prominent than those of the fossil (Dong et al. 2013; Bischler et al. 1963, tab. 63–70). Metalejeunea has larger lobules than the fossil (ca. 0.7 times the length of the dorsal lobe) which are attached to the stem over a wider area (by more than four stem cells) and bear a long, incurved tooth (Miller et al. 1963, plates 129, 130, as Microlejeunea; Grolle 1995; Zhu and So 2001, fig. 65). Due to these differences and despite the small size of the fossil, we decided to classify it as a Lejeunea. The size difference to extant Lejeunea could be due to the fragmentary nature of the fossil; it could represent a branch, which is often smaller than the main shoot, or it could be just an unusually small, extinct Lejeunea species.

Four fossil species of Lejeunea have been found in Dominican amber, namely L. hamatiloba Lee et al., L. resinata Lee et al., L. urbanioides Lee et al., and L. miocenica Heinrichs et al. (Reiner-Drehwald et al. 2012; Kaasalainen et al. 2017a; Lee et al. 2017). Among these species, L. urbanioides shows the strongest similarities to the Mexican fossil, e.g., in having obliquely to widely spreading leaves, suborbicular to ovate leaf lobes with broadly rounded apices, and distant underleaves which are bilobed to ca. 0.5 times their length (Lee et al. 2017, plates I3, II5, 6, fig. 3; Reiner-Drehwald et al. 2012, figs 1–4). However, L. urbanioides differs from the Mexican fossil in several aspects. Apart from being larger than the new fossil, it possesses rounded lobules which are ca. 0.25–0.3 times as long as the lobes and bear an elongated apical tooth. Lejeunea hamatiloba has more strongly falcate lobes with often triangular apices and tiny underleaves barely wider than the stem, while L. resinata has lobes with a straight ventral margin and underleaves with prominent lateral teeth. Both also have a lower lobule-lobe size relation: in L. hamatiloba the lobule is 0.3–0.5 and in L. resinata 0.27–0.35 times the lobe length. Lejeunea miocenica might have relatively large lobules (ca. 0.4(–0.5) times the lobe length), but the obtuse to subacute to apiculate lobe apices clearly differentiate it from the new fossil. While the new fossil certainly represents a different species from the Dominican amber fossils, a detailed comparison with the extant Lejeunea diversity is not possible due to the fragmentary state, and also because it is sterile. Therefore, we cannot rule out that it represents an extant species. The generic placement remains somewhat tentative, but due to the highest number of similarities, we describe it as Lejeunea sp.

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

Patrick Müller Amber Collection (Zweibrücken, Germany) MEX70, MEX71.

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.

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

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.

Gametophyte fragments yellowish to dark reddish brown, 2.7–5.04 mm long, main shoots 0.52–1.16 mm wide with leaves (Figs 4A–C, 5A–C, 6A–E). Branching irregular, Lejeunea-type, primary branches either similar to main shoot, or flagelliform and microphyllous (Figs 4B, D, 6C, D); 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 4C, E, 5C, 6E), 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 4F, 5D, E, 6E), 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 4E, 5C, D, 6E), 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 4E, 5D), 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 4C, E, 5C, 6E), 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.

Figure 4. 

Thysananthus patrickmuelleri sp. nov. from Mexican amber, holotype (GZG.BST.22086). A. Upper part of the gametophyte in dorsal view; B. Lower part of the gametophyte with a short Lejeunea-type flagelliform branch (indicated by white arrow) in dorsal view; C. Upper part of the gametophyte in ventral view; D. Flagelliform branch in ventral view; E. Underleaves and leaf lobules with single apical teeth; F. Lobe cells with cordate trigones.

Figure 5. 

Thysananthus patrickmuelleri sp. nov. from Mexican amber (Patrick Müller amber collection, MEX70). A. Gametophyte with erect-spreading leaves in dorsal view; B. Same specimen in ventral view; C. Upper part of the shoot in ventral view; D. Apical part of a ventral leaf lobule with a hooked tooth (indicated by white arrow); E. Lobe cells, mostly collapsed.

Figure 6. 

A–E. Thysananthus patrickmuelleri sp. nov. (A–E, Patrick Müller amber collection, MEX71) and a representative of Ptychanthoideae (F, Stuttgart State Museum of Natural History, Mx357) from Mexican amber. A. Branched gametophyte of Thysananthus patrickmuelleri in dorso-lateral view; B. Left shoot with erect-spreading leaves similar to MEX70 and right shoot with spreading leaves similar to holotype GZG.BST.22086; C. Lower part with several flagelliform branches, the one on the left becoming main shoot-like; D. Flagelliform branch in ventral view; E. Detail of the main shoot in ventral view; F. Ptychanthoideae sp., gametophyte in ventral view.

The conduplicate-bilobed leaves with Lejeunea-type lobules and the presence of underleaves clearly identify this specimen as a member of Lejeuneaceae (Figs 4C, E, 5A–C, 6E). The robust size, the undivided underleaves, and at least five cells wide ventral merophyte allow an assignment to subfamily Ptychanthoideae (Figs 4A, C, 5A–C, 6A, B, E). 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, 5, 6A–E).

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. 2021a). 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. 6F; 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.

Stuttgart State Museum of Natural History, Germany (SMNS), SMNS-DO-4928-M; syninclusion unidentified leafy liverwort of the order Porellales.

15‒20 Ma, Langhian–Burdigalian (early to middle Miocene), La Toca Formation, Dominican Republic.

Lichen fragment approximately 5.4 × 2.6 mm in diameter. Thallus foliose, lobate. Lobes robust, flat, and linear, 0.4–0.9 mm wide, lobe apices truncate (Fig. 7B). Upper surface slightly uneven, brown, with prominent dark margins (Fig. 7C). Medulla not visible. Lower cortex dark. Marginal cilia long (up to at least 0.8 mm), dark, thick and tapered (Fig. 7B, C). Rhizines dark, shorter than cilia. Isidia abundant, laminal, finger-shaped, up to 150 µm long (Fig. 7C). Apothecia, soredia, or pycnidia not present.

Figure 7. 

Parmotrema fossils from Dominican amber. A−C. Parmotrema specimen 1 (SMNS-DO-4928-M); A. Parmotrema inclusion with a bryophyte (leafy liverwort, Porellales); B. Lobes seen from above showing the truncate lobe apices, dark lobe margins, and long marginal cilia. The lighter spots visible on the upper surface of the thallus are caused by slight depressions and their brighter color derives from air-filled spaces between the thallus surface and amber. These depressions seem to correspond to the attachment points of the rhizines on the lower surface; C. Close-up of the slightly uneven upper surface and finger-like isidia. D−E. Parmotrema specimen 2 (SMNS-DO-4929-M); D. Close-up of the upper surface with abundant pycnidia; E. General view of the inclusion, showing the very long, branching marginal cilia.

The general habit and long, thick, and tapered marginal cilia identify the specimen as Parmotrema. Parmotrema is an extant genus with approximately 300 species mainly distributed in tropical and subtropical regions, especially in the Pacific Islands and South America (Thell et al. 2012). It is part of the parme­lioid crown group of the Parmeliaceae (Pizarro et al. 2018) where apothecial and conidial characters, growth form, cortical and medullar chemistry, and presence, absence and/or type of cell-wall polysaccharides, marginal cilia, rhizines, and surface features, like epicortex and pseudocyphellae, have been used to separate genera (Crespo et al. 2011). Like in many parmelioid genera, molecular phylogenetics have repeatedly shaped the generic boundaries of Parmotrema (e.g., Blanco et al. 2005; Divakar et al. 2005, 2017; Crespo et al. 2011). As the species level taxonomy of Parmotrema mainly relies on chemical and other characters not observable in the fossil inclusions, comparisons of fossils to extant species are often ineffectual. However, the genus currently includes several infrageneric groups that, at some point, have been acknowledged as separate genera based on morphology, anatomy, and chemistry but which were found to be nested within Parmotrema in molecular phylogenetic analyses (Elix 1993; Blanco et al. 2005; Divakar et al. 2005, 2017; Crespo et al. 2011). Parmotrema specimen 1 with the smooth upper surface, dark lower surface, and very long, branched marginal cilia resembles the Parmotrema s. str. group within the genus Parmotrema. The group is, for example, characterized by broad lobes, broad naked marginal zone on the lower surface, marginal cilia, and the lack of maculae on the upper surface (Elix 1993). The extant Parmotrema s. str. group contains more than 250 species distributed especially in the tropical regions of the world (Elix 1993).

Stuttgart State Museum of Natural History, Germany (SMNS), SMNS-DO-4929-M.

15‒20 Ma, Langhian–Burdigalian (early to middle Miocene), La Toca Formation, Dominican Republic.

Three lichen fragments situated very closely together, most probably originating from the same thallus (Fig. 7E); whole inclusion approximately 5.0 × 2.4 mm in diameter. Thallus foliose, lobate. Lobes flat, broad to linear, 0.4–1.0 mm wide, lobe apices truncate to rounded (Fig. 7E). Upper surface smooth and brown, with dark margins (Fig. 7D). Marginal cilia dark, branched, and long (up to 0.8 mm) (Fig. 7E). Medulla not visible. Lower cortex dark. Rhizines dark. Pycnidia abundant on the upper surface (Fig. 7D). Apothecia, isidia, or soredia not present.

The general habit, very long, branched marginal cilia, and dark lower surface identify the specimen as Parmotrema. Of the infrageneric groups, the dark lower surface without rhizines reaching the margins could represent, for example, the Parmotrema s. str. group.

American Museum of Natural History, New York, AMNH DR-14-294.

15‒20 Ma, Langhian–Burdigalian (early to middle Miocene), La Toca Formation, Dominican Republic.

Lichen fragment approximately 1.7 × 1.3 mm in diameter. Thallus foliose, lobate. Lobes flat, linear, approximately 0.5 mm wide and 40 µm thick, lobe apices truncate to rounded. Upper surface slightly uneven with effigurate maculae, with prominent dark margins (Fig. 8A). Marginal cilia long (up to 0.9 mm), branching, tapering (Fig. 8A). Medulla pale (Fig. 8C). Lower cortex dark (Fig. 8B, C). Rhizines dark, mainly simple (Fig. 8B). Apothecia, isidia, soredia, or pycnidia not present.

Figure 8. 

Parmotrema fossils from Dominican and Mexican ambers. A−C. Parmotrema specimen 3 (AMNH DR-14-294) from Dominican amber; A. Maculate upper surface and long, branching marginal cilia; B. Lower surface with rhizines; C. Dark lower cortex and a cross-section of the lobe revealing the medulla; D, E. Parmotrema specimen 4 (GZG.BST.22085) from Mexican amber; D. Smooth upper surface and long, dark marginal cilia. The dark, robust rhizines are also visible from the upper side through the semitransparent thallus; E. Branching rhizines on the lower surface.

The general habit and long, branching and tapering marginal cilia identify the specimen as Parmotrema. The effigurate maculae on the upper surface and tapered marginal cilia of Parmotrema specimen 3 resemble the former genus Canomaculina (Elix 1993) which has been synonymized with Parmotrema (Blanco et al. 2005). The Canomaculina group is characterized by tapered cilia, effigurate-maculate upper surface of the lobes, and simple and furcate to branched rhizines (Elix 1993). The Canomaculina group contains more than ten extant species and has a center of distribution in South America (Elix 1993; Blanco et al. 2005).

Geoscientific Collection of the University of Göttingen, Germany (GZG), GZG.BST.22085; syninclusion Frullania chiapasensis.

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

Lichen fragment approximately 2.3 × 4.5 mm in diameter. Thallus foliose, lobate. Lobes flat and linear, 0.5−1.0 mm wide, lobe apices truncate to slightly rounded (Fig. 8D). Upper surface smooth, brown (Fig. 8D). Marginal cilia dark, mainly simple, up to 0.5 mm long (Fig. 8D). Medulla not visible. Lower cortex concolorous with the upper surface (Fig. 8E). Rhizines dark, abundantly branched (Fig. 8E). Apothecia, isidia, soredia, or pycnidia not present.

The general habit and long, tapering marginal cilia identify the specimen as Parmotrema. The branching rhizines and long, tapered marginal cilia resemble the Canomaculina group. However, unlike the Canomaculina group, the fossil inclusion seems to possess a smooth upper surface of the lobes that visibly lack maculae.