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 : 192-194

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 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.

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.