Radula tikhomirovae Mamontov & Perkovsky, 2024

Mamontov, Yuriy S., Ignatov, Michael S., Vasilenko, Dmitry V., Legalov, Andrei A. & Perkovsky, Evgeny E., 2024, Hepatics from Rovno amber (Ukraine). 11. Radula oblongifolia and R. tikhomirovae sp. nov., Ecologica Montenegrina 72, pp. 189-199 : 192-199

Radula tikhomirovae Mamontov & Perkovsky sp. nov. ( Figs. 2 – 3 )

Type material: Holotype. SIZK-DO-233 F, 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 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 ).

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.

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 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 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 bi- or 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 Garden-Institute, 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.

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Figures

Figure 1. Radula oblongifolia (SIZK-Be-18c, photo and drawing): A – habit, dorsal aspect, B – same, C – leaf lobe cells.

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.

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.