Lejeunea aristovii Sch, 2024

Lejeunea aristovii Sch ä f.-Verw., Mamontov, K.Feldberg & Perkovsky sp. nov. ( Fig. 1 View Figure 1 , 2 View Figure 2 )

Type material: Holotype. SIZK-Be-18c, Rovno amber, late Eocene (Schmalhausen Institute of Zoology in Kiev). Syninclusions: Leptoscyphus davidii , Nipponolejeunea solodovnikovii , Odontoschisma dimorpha (see below), Plagiochila sp. , and Radula oblongifolia .

Diagnosis: The Lejeunea gametophyte is characterized by elliptic-ovate, apiculate to obtuse or narrowly rounded leaf lobes, reduced to absent leaf-lobules, and bilobed underleaves, and differs from the Paleogene Cheilolejeunea latiloba by the less falcate and longly inserted leaf lobes, reduced or absent leaf lobules, and underleaves that are longer than wide and widest in their upper third.

Description: Shoot yellow-brownish, 1.5 mm long and 0.36–0.48 mm wide, creeping, unbranched. Rhizoids not observed. Stem straight, colourless, up to 64–68 μm in diameter (where observable). Epidermis of ventral merophytes at least 3 cells wide, cells with equally somewhat thickened walls, rounded-rectangular, (17.3–)18–32(–49) µm long × (9.8–)10.2–13.5 μm wide, from 1.64–1.92 to 3.3–3.8× as long as wide. Cells of dorsal stem cortex with equally somewhat thickened walls, ± rectangular, 13–21 µm long × 8–11 μm wide, ca 1.6–2.5× as long as wide. Leaves with a very long, J-shaped insertion, insertion line ca. (0.61–)0.66–0.85 of leaf width, incubous, usually planodistichous, remote to loosely imbricate, dorsally almost not interlocking, not reaching beyond the farther edge of stem. Lobe diverging at an angle of 40–80° to stem, usually almost flat, sometimes slightly elevated, slightly convex dorsally, shape widely to narrowly ovate or almost oblong, 181–255 μm long × 100–214 μm wide, ca 1.07–1.81× as long as wide, free margin entire. Dorsal margin semilunate to strongly arcuate in basal half, not ampliate at base. Apex sometimes decurved, ± rounded or obtuse or apiculate with a single acute protruding cell. Free ventral margin in proximal half straight to slightly incurved, near the end of the keel-like part gradually curved, not forming a part of the ventral lobule opening, in distal half slightly to strongly incurved. Cells in upper middle ± isodiametric, hexagonal, 13–26 μm long × 12–21 μm wide, ca 1.04–1.25× as long as wide, in lower middle 14–25 μm long × 12–19 μm wide, with brownish slightly thickened walls and distinct trigones. Ocelli absent. Keellike part (where present) 0.24–0.45(–0.55) length of lobe, smooth, convex to almost straight, with gradual transition into free margin of lobe. Lobule (if present) not available for observation. Underleaves with shallowly arcuate insertion, remote, the only available evident underleaf ( Figs. 2E, 2G View Figure 2 ) flat, 167 μm long × 145 μm wide, ca. 2.5× wider than the stem, obovate, widest in its upper third, ± cuneate at base, bilobed to 0.39–0.43× the length, sinus acutangular with U-shaped base, lobes slightly converging, triangular, ca. 4 cells long, ca. 4–5 cells wide at base, moderately acute, formed by a single rounded cell, subterminally with 2 cells side by side, lateral margin in the upper half unidentate because of a protuberant cell, in the lower half somewhat sinuous. Asexual reproduction, gynoecia, and androecia not observed.

Etymology: The species is named in honor of Dr. Daniil Sergeevich Aristov, an eminent paleoentomologist.

Comparison: Lejeunea aristovii differs from Cheilolejeunea latiloba by its yellow-brownish colour (red-brown in C. latiloba ), less falcate leaf lobes, much less prominent and frequently reduced leaf lobules, and an apiculate leaf apex with a single acute cell (if apiculate in C. latiloba , then with a single protruding rounded cell). It is therefore obvious that L. aristovii cannot be identical with C. latiloba . Lejeunea aristovii is a very small species, but possibly only a less developed branch is at hand. The ovate to almost oblong and often apiculate leaf lobes with partly (or completely?) reduced lobules in combination with the remote and distinctly bilobed (to ca. 0.4), obovate (to ovate) underleaves seem to be very characteristic. It is possible that the majority of leaf lobes were apiculate, but the apiculus might be hidden because it is incurved and therefore not (well) visible, especially when the plant is enclosed in amber and cannot be examined like an extant species. Other fossil species of Cheilolejeunea can be easily excluded either by undivided underleaves ( C. antiqua W.Ye & R.L.Zhu and C. suzannensis (Grolle) Grolle & R.L.Zhu , both from Dominican amber) or by rather different shapes of leaf lobes and underleaves ( C. lamyi (Casp.) Grolle , also from Dominican amber). Fossil Lejeunea species are only known from the Miocene Dominican and Mexican amber. Dominican amber includes the species L. hamatiloba G.E.Lee, Schäf. -Verw., M.A.M.Renner & Heinrichs, L. miocenica Heinrichs, Schäf. -Verw., M.A.M.Renner & G.E.Lee, L. resinata G.E. Lee, Schäf. -Verw., M.A.M.Renner & Heinrichs and L. urbanoides G.E.Lee, Schäf. -Verw., M.A.M.Renner & Heinrichs ( Lee et al. 2017). Recently, a small specimen of Lejeunea sp. was discovered in Mexican amber (Feldberg et al. in press). None of these species are similar to L. aristovii .

There are some small extant species of Lejeunea and Cheilolejeunea with which L. aristovii may be compared. Lejeunea apiculata Sande Lac. is easily distinguished by a much more prominent apiculus, while L. cocoes Mitt. and L. malaysiana G.E.Lee & Pócs differ by their narrowly rounded to sometimes obtuse leaf lobe apex. Moreover, these Lejeunea species all have shorter leaf lobe insertion lines, ca. 0.38–0.67 of leaf width (according to our measures of the leaves imaged in Lee 2013). One of the smallest Lejeunea species in Asia, L. exilis (Reinw., Blume & Nees) Grolle , has distant, ovatelanceolate leaf lobes with a leaf lobe insertion line ca. 0.44–0.74 of leaf width, with mostly well-developed leaf lobules, and small, deeply divided or undivided lanceolate underleaves, which are not or only slightly wider than the stem. Another small species, L. tuberculosa Steph. , has ovate-orbicular, broadly rounded leaf lobes without apiculus, a leaf lobe insertion line ca. 0.44–0.74 of leaf width, rarely reduced leaf lobules and underleaves that are mainly as long as wide. In fact, all other extant Lejeunea species known to us are clearly different from L. aristovii .

There are no small-sized extant Cheilolejeunea species with ovate-oblong leaf lobes and an apiculate leaf apex combined with underleaves as in Lejeunea aristovii (not even among the Neotropical Cheilolejeunea species). Cheilolejeunea subopaca (Mitt.) Mizut. differs from L. aristovii by its ovate-triangular, acute leaf lobes and small underleaves, which are entire-margined and nearly orbicular, as long as wide or wider than long. Other small or tiny species have either almost orbicular, broadly rounded leaf lobes like C. intertexta (Lindenb.) Steph. or ovate to ovate-triangular leaf lobes with obtuse or acute apices like C. osumiensis (S.Hatt.) Mizut. or C. obtusifolia (Steph.) S.Hatt.