Hepatics from Rovno amber (Ukraine). 11. Radula oblongifolia and R. tikhomirovae sp. nov.

© 2024 The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Hepatics from Rovno amber (Ukraine). 11. Radula oblongifolia and R. tikhomirovae sp. nov. YURIY S. MAMONTOV1,*, MICHAEL S. IGNATOV2, DMITRY V. VASILENKO3,4, ANDREI A. LEGALOV5,6,7 & EVGENY E. PERKOVSKY8 1 Polar-Alpine Botanical Garden-Institute, Kola Science Centre, Russian Academy of Sciences, Kirovsk 184256, Russia; https://orcid.org/0000-0003-3851-0738 2Lomonosov Moscow State University, Biological Faculty, Moscow 119234, Russia; https://orcid.org/0000-0001-6096-6315 3 Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya 123, Moscow 117997, Russia; https://orcid.org/0000-0002-4827-7290 4 Cherepovets State University, Lunacharsky Prospect 5, Cherepovets 162600, Russia. 5 Institute of Systematics and Ecology of Animals, Siberian Branch of the Russian Academy of Sciences, 630091 Novosibirsk, Russia; https://orcid.org/0000-0001-7347-8169 6 Department of Ecology, Biochemistry and Biotechnology, Altai State University, 656049 Barnaul, Russia. 7 Department of Forestry and Landscape Construction, Tomsk State University, 634050 Tomsk, Russia. 8 Natural History Museum of Denmark, Universitetsparken 15, Copenhagen 2100, Denmark; https://orcid.org/0000-0002-7959-4379 * Corresponding author. E-mail: yur-mamon[email protected] Received 2 February 2024 │ Accepted by V. Pešić: 29 March 2024 │ Published online 17 April 2024. Abstract A study of Rovno amber has revealed two fossil leafy liverwort species of the genus Radula (Radulaceae, Marchantiophyta) that are new to the late Eocene flora of Rovno amber. One of these species, R. oblongifolia, was described previously from Eocene Baltic and Bitterfeld amber, whereas the other species, R. tikhomirovae, is described here as new. Radula tikhomirovae differs from R. oblongifolia and two other Eocene species of that genus, R. sphaerocarpoides and R. baltica, by the presence of microphyllous branches and by the shape of its leaf lobules. Key words Fossil liverworts, taxonomy, Europe, Eocene, Jungermanniales. Introduction The extant genus Radula Dumort. [hereinafter in the sense of Devos et al. (2011), that is, including the newly separated genera Cladoradula (Spruce) M.A.M.Renner, Gradst., Ilk.-Borg. & F.R.Oliveira-daSilva and Dactyloradula (Devos, M.A.M.Renner, Gradst., A.J.Shaw & Vanderp.) M.A.M.Renner & Gradst.] of the family Radulaceae is one of the richest genera of leafy liverworts Marchantiophyta, Jungermanniopsida, Jungermanniidae) with approximately 220 currently accepted species (Renner Ecologica Montenegrina 72: 189-199 (2024) This journal is available online at: www.biotaxa.org/em https://dx.doi.org/10.37828/em.2024.72.18 HEPATICS FROM ROVNO AMBER 190 2015). The genus is subcosmopolitan (with representatives on all continents, except Antarctica), although largely tropical and subtropical in range (Yamada 1979; Schuster 1980; Renner 2015). The modern representatives of Radula are normally corticolous or epiphyllous in humid, tropical or warmtemperate regions (Heinrichs et al. 2016), but some species penetrate into cool climates where they live rocks, the ground, and the bark of trees (Schuster 1980). The genus has an ancient age: Its extinct members are known from the Cretaceous (Bechteler et al. 2017; Wang et al. 2022), and three extinct species (R. baltica Heinrichs, Schäf.-Verw. & M.A.M.Renner, R. oblongifolia Casp., and R. sphaerocarpoides Grolle) have been described from Eocene Baltic and Bitterfeld amber (Grolle & Meister 2004; Heinrichs et al. 2016). Until now the members of this genus were not known from Rovno amber; however recently two specimens from the Rovno and Zhitomir regions of Ukraine have been studied and found to include representatives of Radula. The discovered plants lack underleaves and are characterized by having incubously inserted leaves that are unequally bilobed with a larger dorsal lobe and a ventral lobe (lobule) folded under the dorsal lobe and attached to the lobe and the stem. The first studied plant (from the Zhitomir Region) is rather small-sized and probably represents an underdeveloped (etiolated) form. However, this plant is similar in the shape and proportions of its leaf lobes and lobules to R. oblongifolia described from Baltic and Bitterfeld amber (Grolle & Meister 2004; Heinrichs et al. 2016), and is, therefore, considered to belong to the latter species. The second studied plant (from the Rovno Region) is comparatively large and well developed, with rather dense foliation where the leaf lobes are imbricate and overlap the nearby lobes (both opposed and overlying), and in plane its leaf lobes are patent from the stem at a right or obtuse angle (between the stem axis and the long axis of the lobe). This plant is furcately branched, but also bears characteristic microphyllous branches, which likely helped facilitate vegetative reproduction. Such branches are unknown in the species already described from European amber, so this plant is considered to belong to a new fossil species R. tikhomirovae sp. nov. The descriptions and illustrations of the discovered taxa are provided below. Materials and methods The studied amber specimens are a part of the Rovno amber collection of the Schmalhausen Institute of Zoology in Kiev (SIZK-Be-18 and SIZK-DO-233F). The age of Rovno amber is late Eocene, about 35– 37 Ma (see discussion in Mitov et al. 2021; Perkovsky et al. 2007, 2010; Radchenko et al. 2021). One of the studied specimens was collected in the Dubrovitsa District of Rovno Region, where several insects in amber were already reported (Khaustov et al. 2021; Inshyna et al. 2022; Chemyreva et al. 2024). The other studied specimen was collected in the Zhitomir Region; data on the amber biota from this region were recently summarized in Mamontov et al. (2024). The specimen SIZK-DO-233F was collected in the western part of the Dubrovitsa District (Osova, Grani, Volnoje), Rovno Region. The partially clear amber piece has a weight of 13.6 g, length of 40 mm, width of 30 mm and a height of 28 mm. It contains syninclusions of the liverwort Frullania sp. and several microphyllous branches more likely detached from the plant(s) of the studied here Radula species. The studied specimen SIZK-Be-18 was collected in a quarry 5 km south of Belokorovychi Village, Korosten District, Zhitomir Region. After primary preparation the amber piece has a weight of 11.22 gram and a spheroidal shape with a diameter of ca. 24 mm. The specimen contains syninclusions of other liverworts, including Leptoscyphus davidii Mamontov, Ignatov, Vasilenko & Perkovsky (Mamontov et al. 2024) and a species of the genus Nipponolejeunea S.Hatt. (description of this species is in press). The liverwort inclusion in the specimen SIZK-Be-18 (Fig. 1A, C) was studied and photographed using a stereomicroscope, an Olympus SZX16, equipped with a digital camera, a Canon 1100D. The inclusions in SIZK-DO-233F (Figs. 2A–E and 3C) were studied and photographed using a stereomicroscope, an Olympus MVX10, equipped with a digital camera, an Infinity Lumenera 3-6. The photomicrograph Fig. 3A was obtained using a stereomicroscope, a Nikon SMZ25, equipped with a digital camera, a Nikon DS-Fi3. To optimize visualization of the three-dimensional inclusions, photomicrographs in Figs. 1–3 were combined from several optical sections using the focus stacking software Helicon Focus 8 (Kozub et al. 2008) or the focus stacking package EDF built into the NISElements imaging software that controls the Nikon SMZ25. Two of the final images (Fig. 3A and C) MAMONTOV ET AL. Ecologica Montenegrina, 72, 2024, 189-199 191 were reconstructed in line drawings (Fig. 3B and D, respectively). The leaf lobes and lobules of the studied plants are described following to the methodology proposed in Renner (2005). The concept of the amentulose and microphyllous branches of Radula is provided here according to Yamada (1979: 219). Systematic paleobotany Order Porellales Schljakov, 1972 Suborder Radulineae R.M.Schust., 1963 Family Radulaceae Müll.Frib., 1909 Genus Radula Dumort., 1822 Radula oblongifolia Casp., Schriften der Königlichen Physikalisch-Ökonomischen Gesellschaft zu Königsberg 27: 4. 1887. Fig. 1. Type material: Neotype: Geoscientific collections of the Georg August University Göttingen, Germany, GZG.BST.K7.323 (Heinrichs et al. 2016: Plate II, Fig. 5.v). Description: Pale yellowish gametophyte, the available part of shoot unbranched, ca. 2.3 mm long, up to 0.6 mm wide. Stem ca. 75–80 µm in diameter, cortical cells elongate-rectangular, 14–19 µm long, 13–16 µm wide. Leaves continuous, rather remote, the parts of the stem free from leaves are comparable with the leaf lobes in their length. Leaf lobes spreading, obliquely patent in plane, with the angle between stem axis and the long axis of a lobe (from the middle of lobe apex to the middle of lobe insertion) is about 44–75°, elliptic, the length 232–367 μm, the overall width 208–314 μm, postical margin straight to curved, always short, exterior margin rounded, antical margin weakly curved, interior margin weakly ampliate, usually extending onto the dorsal stem surface, but leaving the stem visible from above. Leaf lobules extend to ca. 0.4–0.6 times the area of the leaf lobes; elliptic to rhomboidal, the lobule breadth 152–265 μm, the lobule depth 157–202 μm, insertion longitudinal; keel straight, arising from stem at 44–75° angle; exterior and antical margins straight to slightly curved, apex obtuse; interior margin likely not ampliate, not extending onto ventral surface of the stem. Strip-shaped dorsal leaf-free zone present, one cortical cell row wide; presence or absence of the ventral leaf-free strip not ascertained. Cells on leaf lobe margin quadrate to rectangular, 15–23 μm long, 13–20 μm wide, long axis either perpendicular to, or parallel with, the lobe margin; medial cells rectangular to hexagonal, 20– 24 μm long, 14–20 μm wide, basal cells similar to mid-leaf cells, sometimes larger; cell wall thickenings not resolved. Asexual reproduction not observed. Gynoecia, androecia, and sporophytes not observed. Specimen examined: Rovno amber: SIZK-Be-18c (Schmalhausen Institute of Zoology in Kiev). Comparison: As stressed above, the studied plant can be considered to be poorly developed (perhaps etiolated) because it is rather small-sized (with the shoot width up to 0.62 mm) and rather remotely foliated, with obliquely oriented leaves. By contrast, the neotype plants of R. oblongifolia are up to 1.43 mm wide and are more densely foliated, with overlapping leaf lobes. In the studied shoot, the proportions between the overall width of the leaf lobe and the breadth of the leaf lobule are similar to those of R. oblongifolia. However, the leaf keel in the Rovno plant is relatively shorter, giving the leaf lobules a somewhat different in shape from the majority of leaf lobules of the latter species. Indeed, in the Rovno plant the leaf lobules are elliptic to rhomboidal, whereas in the Baltic plants of R. oblongifolia they are quadratic to rectangular. However, the less-developed leaves in the Baltic plant of R. oblongifolia (Heinrichs et al. 2016: Plate II, Fig. 1) have lobules that are similar to those of the studied here plant. Because no other differences from the Baltic plants of R. oblongifolia have been found, the plant studied here is considered more likely belong to this species. HEPATICS FROM ROVNO AMBER 192 Figure 1. Radula oblongifolia (SIZK-Be-18c, photo and drawing): A – habit, dorsal aspect, B – same, C – leaf lobe cells. Radula tikhomirovae Mamontov & Perkovsky sp. nov. (Figs. 2 – 3) Type material: Holotype. SIZK-DO-233F, Rovno amber, late Eocene. Syninclusion: Frullania sp. (Schmalhausen Institute of Zoology in Kiev). Diagnosis. The Radula gametophyte characterized by elliptic-rectangular leaf lobules, rotund leaf-lobes that spread in plane with the stem, and the presence of microphyllous branches, differs from MAMONTOV ET AL. Ecologica Montenegrina, 72, 2024, 189-199 193 Cretaceous R. heinrichsii K.Feldberg, Schäf.-Verw., M.A.M.Renner, von Konrat & A.R.Schmidt by the morphology of its microphyllous branches and the less elongate leaf lobes. It also differs from the habitually most similar extant R. brunnea Steph., R. auriculata Steph. and R. amentulosa Mitt.in the absence of amentulose branches and appendages in its leaf lobe and lobule bases (vs. the presence of amentulose branches and appendages in R. brunnea), in its rather narrow microphyllous branches (vs. rather wide branches in R. auriculata), and in the subtransverse insertion of its leaf lobules (vs. a longitudinal insertion in R. amentulosa). Figure 2. Radula tikhomirovae sp. nov. (holotype, Rovno amber, SIZK-DO-233F, photo), A – shoot, dorsal aspect. B, C – parts of shoot, dorsal aspect (close-ups of areas selected in Fig. 2A). D, E – microphyllous branches. HEPATICS FROM ROVNO AMBER 194 Description: Shoot up to 7 mm long, 1080–1670 μm wide, with lateral, Radula-type branches. Stem 180–210 μm in diameter. Leaves imbricate, leaf lobes broadly elliptic, spreading, obliquely patent, in dorsal view mostly convex, but concave in the area of insertion, at places with narrowly reflexed margins; the lobe length 424–642 μm (along a line parallel with the stem), the overall width of the lobe 690–1020 μm (along a line perpendicular with the stem), thus the lobes ca. 1.25–1.83× as wide as long; antical margin more or less straight or convex at midpoint, interior margin narrowly reflexed, curved, ampliate, extending onto the dorsal stem surface and covering it, without leaving the stem visible from above; keel arising from the stem at 49–79° angle, arched at the base and towards the lobe-lobule junction. Leaf lobules (barely visible) variable in outline, oblong-obovate to rounded-quadratic, the lobule depth 257–356 μm, the lobule breadth 345–579 μm, thus the lobules ca. 1.1–1.79× as wide as long; insertion subtransverse, exterior and antical margins convex, entire, apex rounded; interior margin ampliate and extending onto the ventral stem surface; dorsal and ventral leaf-free strip seem to be present. Leaf lobe medial cells almost isodiametric, rounded-hexagonal, irregularly arranged, 19–26 μm long, 18–24 μm wide, with bulging trigones and medial wall thickenings. Asexual reproduction via microphyllous branches of Radula-type (Fig. 2B–E) arising from the bases of the majority (if not all) lateral leaves, 0.49–2.43 mm long, 0.17–0.26 mm wide, with 9–14 pairs of reduced, elliptic or ovate or widely fusiform leaves. Gynoecia, androecia and sporophytes not observed. Etymology. The species is named in honor of Dr. Anna L'vovna Tikhomirova, an eminent entomologist and paleontologist. Comparison. The studied fossil demonstrates a combination of the following morphological characteristics that distinguish this species from all extinct and extant Radula, namely: oblong-obovate to rounded-quadratic leaf lobule with subtransverse insertion, and the presence of comparatively long microphyllous branches consisting of up to 14 pairs of reduced leaves. The presence of several detached microphyllous branches located near the main shoot of the R. tikhomirovae suggests the branches have served as structures of vegetative reproduction. With the presence of microphyllous branches R. tikhomirovae greatly differs from other Radula species already described from European Eocene amber, because these species are not known to have such branches. The comparatively long microphyllous branches are found in R. heinrichsii described from the Cretaceous (Wang et al. 2022); moreover, the latter species is also somewhat similar to R. tikhomirovae in the shape of leaf lobes and lobules. However, the leaf lobules in the former species were inserted longitudinally, whereas in R. tikhomirovae the leaf lobule insertion was subtransversely. Furthermore, in R. heinrichsii the leaf lobes and lobules of the microphyllous branches were similar in their shape and size (Wang et al. 2002); according to Feldberg et al. (2022) the microphyllous branches of R. heinrichsii have no exact equivalent among extant species. In R. tikhomirovae, the microphyllous branches differ from those of R. heinrichsii in having leaf lobules that were smaller than lobes and were different from the lobes in their shape (Fig. 3C, D). Almost all extant Radula species (except species of the subgenus Cladoradula Spruce) bearing microphyllous branches differ from R. tikhomirovae in the longitudinal insertion of leaf lobules. The species of the subgenera Cladoradula and Dactyloradula Devos, M.A.M.Renner, Gradst., A.J.Shaw & Vanderp. have leaf lobules that are inserted subtransversely, as in R. tikhomirovae; however the sole species of the subgenus Dactyloradula (R. brunnea) bears amentulose branches and teeth or laciniate appendages at the bases of leaf lobes and lobules. By contrast, in R. tikhomirovae the branches are microphyllous, and no appendages at the bases of leaf lobes and lobules have been observed. Among the species of the subgenus Cladoradula, the only R. auriculata Steph. is known to have microphyllous branches (Bakalin & Klimova 2020; Renner et al. 2022). However, the microphyllous branches in this species are illustrated to have their width comparable with the length [in the sense of Renner (2005)] of their associated stem leaves (Bakalin & Klimova 2020: 135, Fig. 1: 1), whereas in R. tikhomirovae all microphyllous branches are nearly twice (or more) narrower than the length of their associated stem leaves. Moreover, in R. auriculata the interior leaf lobe margin is flat, while the line of the leaf lobe insertion is ca. ½ of the lobe length, according to the illustration in Yamada (1979: 307, Fig. 59d). By contrast, in R. tikhomirovae the interior leaf lobe margin was narrowly reflexed (Fig. 2A–C, 3A, B) and the line of the leaf lobe insertion was more likely shorter (according to that is suggested by the shape and arrangement of the leaf lobes in Fig. 3A) and similar to that of R. brunnea (Yamada 1979: 272, Fig. 38). Therefore, the combination of these morphological characteristics distinguishes R. tikhomirovae from all extant and fossil members of the genus and confirms its separation as a distinct species. MAMONTOV ET AL. Ecologica Montenegrina, 72, 2024, 189-199 195 Figure 3. Radula tikhomirovae sp. nov. (holotype, Rovno amber, SIZK-DO-233F, photo and line drawings), A – part of shoot, dorsal aspect. B – same, the graphic reconstruction. C – leaf, ventral aspect. D – same, the graphic reconstruction. Discussion The discovery of R. oblongifolia in Rovno amber, in addition to its records from Baltic and Bitterfeld amber, makes this species the most widely distributed among European Eocene liverworts. Such comparatively wide distribution of an epiphytic species is, on the one hand, not unique for the extant HEPATICS FROM ROVNO AMBER 196 members of the genus Radula, where some species have a wide distribution in the Holarctic (e.g., R. complanata (L.) Dumort.) or on different islands in the Paleotropics (Yamada 1979). On the other hand, SIZK-Be-18c belonging to R. oblongifolia rather than to a new species may be questioned in the future because of the way this specimen differs from the Baltic plants of R. oblongifolia in the shape of its leaf lobules. Resolving this taxonomic issue would require the study of further specimen(s) attributable to the same species as SIZK-Be-18c to obtain more information about variability of the lobule shape, as well as finding and studying the gynoecia of the species. Considering the general distribution of Radula species in European amber, it should be noted that three species of this genus are known from Baltic amber and three species in Bitterfeld + Rovno ambers as well. Among Baltic amber there are 17 specimens of Radula, and the same number of Radula specimens have been discovered in Bitterfeld and Rovno amber taken together. This fact may be significant if one takes into account that the extraction of Baltic amber greatly exceeds the extraction of Bitterfeld and Rovno amber taken together by age and volume. Due to this fact, the representation of specimens and species of this genus in Bitterfeld and Rovno amber may indicate that the warmer climates in these two amber forests were more favorable for the members of Radula than the climate of the more northern Baltic amber forest. The similar pattern of the distribution is characteristic of many cryophobic amber taxa of arthropods (e.g. Colombo et al. 2021; Telnov et al. 2021, 2023; Lyubarsky et al. 2023; Jenkins Shaw et al. 2023; Sokolov et al. 2024; Melnitsky et al. 2024) and the extant members of Radula. In fact, the vast majority of Radula species (as well as the species of other epiphytic lineages of Porellales including Frullaniaceae, Lejeuneaceae and Porellaceae) are distributed in areas with rather warm climates; that is, equatorial, sub-equatorial, tropical, and subtropical areas in the sense of Alisov (1936). Whereas the number of Radula species occurring in areas with temperate, subarctic/subantarctic, and arctic/antarctic climates (lying below 40 degrees south, and above 40 degrees north) is about 55– 60, according to the lists in Yamada (1979), Paton (1999), So (2005), Renner (2005), Hässel de Menéndez & Rubies (2009), Stotler & Crandall-Stotler (2017), and Choi et al. (2021). This number represents ca. ¼ of the current diversity of the genus. The phylogenetic relationships of Eocene species of Radula are of particular interest, although they are difficult to determine despite the number of discovered taxa and the excellent preservation of the Baltic and Bitterfeld specimens of Radula illustrated in Heinrichs et al. (2016). This genus is characterized with extraordinary uniformity in basic leaf form (Schuster 1980); according to Renner (2015), the genus has a lot of morphological homoplasy in lobule shape evolution that “poses a challenge to relating fossils of known age to extant lineages, particularly when fossils are sterile”. In the case of the newly discovered taxa, it is still impossible to attribute R. oblongifolia to a subgenus within Radula because this species has a morphology that is characteristic of different subgenera, namely Radula, Odontoradula K.Yamada, and Volutoradula Devos, M.A.M.Renner, Gradst., A.J.Shaw & Vanderp. By contrast, the morphological traits of R. tikhomirovae suggest that this species is phylogenetically closely allied with the basal subgenera of Radula [in the sense of Devos et al. (2011)], namely Cladoradula and Dactyloradula. As mentioned above, R. tikhomirovae is similar to Cretaceous R. heinrichsii in possessing microphyllous branches. However, Feldberg et al. (2022) noted that the relationship of the latter species is obscure because of the morphology of these branches. Devos et al. (2011) synonymized the microphyllous and amentulose branches and noted that these branches occur only in the subgenus Amentuloradula Devos, M.A.M.Renner, Gradst., A.J.Shaw & Vanderp. of the extant Radula. However, Renner et al. (2022) noted that amentulose branches occur also in the genus Dactyloradula (≡ subgenus Dactyloradula of the genus Radula) represented by the only extant species D. brunnea (Steph.) M.A.M.Renner & Gradst. (≡ Radula brunnea). The latter coincides with that which was shown for R. brunnea by Yamada (1979: 272) and Bakalin & Klimova (2020: Fig. 1: 3). Regardless of this, a close relationship between R. tikhomirovae and the subgenus Amentuloradula may be doubted because the lobule insertion in all species of the latter subgenus is longitudinal (parallel to stem, directed to stem apex), according to Devos et al. (2011), whereas in R. tikhomirovae the lobule insertion was most likely subtransverse or at least oblique and directed to stem ventral midline (Fig. 3C, D). Based on the latter feature, R. tikhomirovae is similar to the genera Cladoradula and Dactyloradula separated by Renner et al. (2022), as the leaf lobule insertion in the species of both these genera is sinuate-oblique to transverse, and is directed to stem ventral midline, too (Devos et al. 2011; Renner et al. 2022). However, the distinctions of these genera from the genus Radula sensu Renner et MAMONTOV ET AL. Ecologica Montenegrina, 72, 2024, 189-199 197 al. (2022) includes the presence (vs. absence in Radula) of a stem subepidermis, the character state that cannot yet be examined in R. tikhomirovae. The two mentioned genera, the oligospecific Cladoradula and the monospecific Dactyloradula, differ from each other in the number of layers of the subepidermis and the presence/absence of subfloral innovations (Renner et al. 2022). Therefore, an assignment of R. tikhomirovae to any of these genera is challenging. Based on the presence of the microphyllous branches R. tikhomirovae is similar to Cladoradula auriculata (Steph.) M.A.M.Renner, Gradst., Ilk.-Borg. & F.R.Oliveira-da-Silva (≡ Radula auriculata) and differs from Dactyloradula brunnea, which has amentulose (not microphyllous) branches. However, based on the shape of its leaf lobes and lobules and probably by the shape of the insertion of its leaf lobes, R. tikhomirovae is more similar to D. brunnea, although it differs from this species in the absence of teeth and/or appendages at the bases of its leaf lobes and lobules. Moreover, the studied plant of R. tikhomirovae has probably been attached to substrate and was not a part of the large, regularly bior even tri-pinnate shoot systems with rigidly expressed branching architecture as in Cladoradula species, the characteristic that the latter genus shares with Porella L. (Renner et al. 2022). Due to this, R. tikhomirovae may be considered more likely to belong to the genus Dactyloradula. However, such an assumption requires examination of additional specimen(s) of this species to ascertain the presence or absence of subfloral innovations and teeth and/or appendages at the bases of its leaf lobes and lobules. Both species morphologically most similar to R. tikhomirovae are distributed mainly in East Asia, but also on the Pacific coast of North America; of these species, C. auriculata occurs in Himalaya, China, Korea, Japan, Primorye Territory (Russia), southern Alaska, and British Columbia, while D. brunnea is known from Japan, Shikotan, Moneron, and Oregon (Yamada 1979; Bakalin et al. 2009; Bakalin & Klimova 2020). If this is indeed the case that the morphological similarity between R. tikhomirovae and the mentioned C. auriculata and D. brunnea reflects their close phylogenetic relationships, R. tikhomirovae is yet one more European amber species close to the modern liverwort flora of the North Pacific, in addition to European Eocene species of the genera Metacalypogeia (S.Hatt.) Inoue and Nipponolejeunea. Acknowledgements The authors are very grateful to Dr. Nadezhda A. Konstantinova (Polar-Alpine Botanical GardenInstitute, Kirovsk, Russia) for valuable comments. Special thanks are due to Madeline V. Pankowski (Rockville, USA) for linguistic corrections. The study by E.E. Perkovsky was supported by the Scholars at Risk Ukraine (SARU) program jointly funded by the Villum Foundation, Carlsberg Foundation and the Novo Nordisk Foundation. References Alisov, B.P. (1936) Geographical types of climates. Meteorologiya i Gidrologiya, 6, 16–25. [In Russian] Bakalin, V.A., Cherdantseva, V.Ya., Ignatov, M.S., Ignatova, E.A. & Nyushko, T.I. (2009) Bryophyte flora of the South Kuril Islands (East Asia). Arctoa, 18, 69–114. https://doi.org/10.15298/arctoa.18.03 Bakalin, V.A. & Klimova, K.G. (2020) A review of Radulaceae (Marchantiophyta) in the Russian Far East. Botanica Pacifica, 9(2), 133–153. https://doi.org/10.17581/bp.2020.09204 Bechteler, J., Schmidt, A.R., Renner, M.A.M., Wang, B., Pérez-Escobar, O.A., Schäfer-Verwimp, A., Feldberg, K. & Heinrichs, J. 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