Lejeunea sp.

Specimen.

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

Age and stratigraphic level.

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

Description.

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. 3 A, 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. 3 C). Stem light yellowish brown, ca. 30 µm wide, surface cells elongated, possibly thick-walled, ventral merophyte ca. 2 cells wide (Fig. 3 A); 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. 3 A – 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. 3 A – 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. 3 C – 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. 3 E). Ocelli not seen. Ventral lobules Lejeunea - type, vertically inserted, inflated, ovate (Fig. 3 A – 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. 3 E, indicated by black arrow); keel arched, possibly crenate (Fig. 3 C, 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. 3 C, 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.

Discussion.

The conduplicate-bilobed leaves with Lejeunea - type lobules (Fig. 3 A – C) and the presence of underleaves (Fig. 3 C, D) clearly identify this specimen as a Lejeuneaceae, and the narrow, only two cells wide ventral merophyte (Fig. 3 A) allows an assignment to subfamily Lejeuneoideae. The bilobed underleaves (Fig. 3 C, 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. 2017 a; 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 I 3, II 5, 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.