Late Oligocene petrified wood remains from alluvium sediments of Radomir Depression, Bulgaria

© Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, 2025 DOI 10.3897/phytolbalcan.31.e177410 This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). PHYTOLOGIA BALCANICA vol. 31: 109-118 • Sofia • 2025 Late Oligocene petrified wood remains from alluvium sediments of Radomir Depression, Bulgaria Stănilă Iamandei1, Eugenia Iamandei1, Vladimir S. Bozukov2, Valeri V. Sachanski3,4 & Milorad D. Vatsev3 1 Geological Institute of Romania, 1 Caransebeș Str., sect. 1, Bucharest, Romania, e-mail: [email protected], [email protected] 2 Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, Akad. G. Bonchev Str, Bl. 23 1113 Sofia, Bulgaria, e-mail: [email protected] 3 University of Mining and Geology, Studentski Grad, Prof. Boyan Kamenov Str, 1700 Sofia, Bulgaria; e-mail: [email protected]; [email protected] 4 Geological Institute, Bulgarian Academy of Sciences, Acad. G, Bonchev Str., bl. 24, 1113 Sofia, Bulgaria; e-mail: [email protected] Received: June 09, 2025|Accepted: July 30, 2025|Published: November 07, 2025 Abstract The paper presents a xylotomic study of some samples of petrified wood collected from alluviums in the Vladimir village (Radomir Depression, Bulgaria). Most likely they were transported from Late Oligocene coal-bearing sediments (Chattian) of Pernik or Bobov Dol areas. The samples have been identified as wood of Taxodioxylon taxodii, a fossil equivalent of the recent species of Taxodium distichum. The latter is a tree which grows presently in restricted areas of South USA, Mexico and Guatemala, but used to be widespread in the fossil records of the European Cenozoic flora as leaf imprints, wood remains and even as an important member in the Tertiary coal forests. Keywords: Chattian, Paleogene, Radomir Depression, Taxodioxylon, Taxodium, xylotomic study Citation: Iamandei, S., Iamandei, E., Bozukov, V.S., Sachanski, V.S. & Vatsev, M.D. 2025. Late Oligocene petrified wood remains from alluvium sediments of Radomir Depression, Bulgaria. – Phytologia Balcanica, 31: 109-118 -- ISSN 1310-7771 (print), 1314-0027 (online). Introduction and geological settings The purpose of this study is to identify the original tree from which the fossil remains have come. The specimens studied have been found dispersed in alluviums around the Vladimir village (Radomir Depression). Most probably they came from coal-bearing formations in the nearby located Pernik or Bobov Dol coal basins, brought by the active hydrological processes from there. Palaeobotanical studies in that area (see Palamarev & al. 1998) have revealed the presence of numerous trees, including Taxodium, Glyptostrobus and Sequoia, as species usually found in many Cenozoic European coal deposits. For example, Hadjiev (1956) had identified a Middle Miocene wood of Taxodium distichum (L.) Rich. miocenicum Heer in the Chukurovo Coal Mine (W Bulgaria) and Taxodioxylon taxodii Gothan in the Late Miocene sediments of Sofia Basin (Hadjiev 1962). Also, Bozukov & Ivanov (2020) have described a rich Late Oligocene paleoflora (microflora + macroflora) from the Pernik Coal Basin, which includes Glyptostrobus, Taxodium, Sequoia and other conifers. Iamandei, S. & al. | Late Oligocene wood remains of Radomir Depression, Bulgaria110 Fig. 1. Alpine tectonic subdivision of Bulgaria by Dabovski & Zagorchev (2009) with location of the fossiliferous site of Vladimir on a map (star): A. 1-Moesian Platform, South Carpathian orogenic system; 2-Kraynenska Zone; 3-Kula Zone, Balkan orogenic system; 4-Balkan Zone; 5-Srednogorie Zone; 6-Moravo-Rhodope Zone; 7-intraorogenic basins (post-tectonic: a-Neogene-Quaternary, b-Paleogene); B. Simplified tectonic map of the studied area [redrawn from Antonov & al. (2011a, b) with corrections] with Bobov Dol and Pernik areas, and the location of samples as fossiliferous site of Vladimir (star); C. The Paleogene sedimentary sequences (stratigraphic columns) in the Pernik and Bobov Dol Grabens. 111 PHYTOLOGIA BALCANICA, vol. 31 • Sofia • 2025 Mantzouka & al. (2021 and references therein) have communicated preliminary observations of some Oligocene-Miocene fossil woods from Bulgaria, (Ustren, Bobov Dol and, respectively, Staroseltsi), and have identified some conifers as Taxodioxylon sp. and some Taxaceae, Taxodiaceae and Pinaceae woods, partially petrified and partially coalified. Mantzouka & al. (2020) have reported a new species Castanopsis bulgarica Mantzouka, Ivanov & Bozukov from the Late Miocene plant fossiliferous locality eastwards of Boboshevo town (SW Bulgaria). Representatives of the genera Cupressinoxylon, Glyptostroboxylon, Magnolioxylon, Populoxylon, Quercoxylon, Rhysocaryoxylon, and Sequoioxylon have been also identified on the territory of Bulgaria (Hadjiev 1956, 1962; Hadjiev & Mädel 1962; Iamandei & al. 2014, 2016, 2023). In a study on the presence of Taxodioxylon type charcoal in some Late Miocene coal deposits from West Bulgaria (Stanyantsi Coal Basin), the authors have suggested occurrence of wildfires in the geological past (Uhl & al. 2014). The samples investigated in the present study (see Fig. 4) also appeared partially as charcoal, i.e. charred wood remains, suggesting that fires occurred in the peat bogs during various phases of peat accumulation. The authors have studied some petrified wood samples collected from the alluviums in the Vladimir village (42°26’22.36”N; 23°05’0.27”E), in the southeastern part of the Radomir Depression, revealing a complex of floodplain and floodplain terraces. There, subsidence had led to the formation of accumulative Quaternary surface, consisting of alluvial-proluvial deposits (Fig. 1A-B). Not far away lie the Pernik and Bobov Dol Grabens, filled with Paleogene sediments, where the Pernik and Bobov Dol Coal Basins are situated, both of Late Oligocene age, (specifically, Middle Chattian, see Fig. 1C). These deposits have been assigned to four lithostratigraphic units: a conglomerate-sandstone formation (Lyulin Formation in the Pernik Graben), a variegated molassoid formation, a coal-bearing formation, and an argillite-marly formation (Antonov & al. 2011a, b). Their almost identical profiles show that deposition of sediments took place in a common lacustrine basin (or lacustrine complex), with postsedimentary separation and differentiation. The conglomerate-sandstone formation and the Lyulin Formation consist of predominantly pebbly to cobbly polymict conglomerates. The variegated molassoid formation consists of irregularly alternating conglomerates, sandstones, siltstones, and argillites. The sediments have grey, greygreen, green, rusty yellow, and locally red-purple colors. Thin coal lenses have been also encountered in some places. The coal-bearing sequence is a clayey-bituminous formation and consists of irregularly alternating sandy and greasy clays, thin-bedded argillites, yellow-brown sandstones with clayey and calcareous cement, and coal seams. The argillite-marly formation consists of homogeneous grey-bluish, thin-bedded argillites and clayey to sandy marls. The chronostratigraphic range of the conglomerate-sandstone formation and Lyulin Formation has been recently assumed as Early Oligocene, on the basis of its stratigraphic position. The chronostratigraphic ranges of the variegated molassoid formation, coal-bearing formation, and argillite-marly formation are based on sporepollen and paleoflora determinations [Palamarev & al. (1998) and the references therein; Antonov & al. (2011a, b) and references therein; see also Fig. 1C]. The coal-bearing formation is a clayey-bituminous formation, with fine layered slightly bituminous clays and marls (Palamarev & al. 1998; and see Fig. 1C). Since some petrified wood has been also found in the Pernik Coal Mine, a specimen is now exposed in front of the Pernik Museum of Mining (see Fig. 2), so it is very likely that the specimens found in the neighborhood alluviums are also of that age - Chattian. Imprints of leafy branches of Taxodium dubium (Sternberg) Heer (Fig. 3) have been found in both coal basins. The recent species T. distichum (L.) Rich. has been accepted as the nearest living relative of the fossil T. dubium (Konjaroff 1932; Palamarev & al. 1998; Stefanova & al. 2013; Bozukov & Ivanov 2020). Material and methods Thin petrographic sections have been made from the studied samples, oriented transversely, radially and tangentially, and studied under a microscope. Identification is based on the microscopic structure (xylotomy) and its comparison with similar studies in the specialized literature, which may include current or fossil plants. The authors have studied some samples of petrified wood collected from the Quaternary alluviums around the Vladimir village, in the neighborhood of Pernik and Bobov Dol Late Oligocene (Chattian) Coal Basins, apparently their most probable source, as specified above. The samples are deposited now in the Palaeobotanical Collection of the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, Sofia, Bulgaria, under indexes Vld-1, Vld-2, Vld-3, Vld-4, Vld-5, Vld-6, Vld-7 (Fig. 4A-G). For the xylotomical study, standard oriented thin sections of petrographic type have been prepared (transversal, tangential, radial) and studied by a transmitted light microscope. All observed anatomical details are described following the IAWA Lists of Microscopic Features for Softwood and Hardwood Identification (Wheeler & al. 1989). Photographs of the xylotomical details have been made with an EverFocus Video-Camera adapted to the microscope, with AVerMedia software, and the images have been processed with specialized computer programs. Identification of the unknown original tree was subsequently performed by comparison with previously described similar aspects of fossil or recent wood structures from published scientific papers, all included in the References below. The systematics follow classification of the gymnosperms according Gadek & al. (2000), Farjón (2005) and Christenhusz & al. (2011). Iamandei, S. & al. | Late Oligocene wood remains of Radomir Depression, Bulgaria112 Fig. 2. Pernik Underground Mining Museum – a petrified trunk exposed at the entrance (https://museumpernik.com/galeria/). Fig. 3. Leafy branches of fossil Taxodium dubium and recent T. distchum. The fossil material is from the Pernik Coal Basin (No Пк-92 in the Paleocollection of IBER-BAS). 113 PHYTOLOGIA BALCANICA, vol. 31 • Sofia • 2025 Systematics Gymnosperms Order Cupressales Link Family Cupressaceae Gray Subfamily Taxodioideae Endl. Genus Taxodioxylon Hartig emend. Gothan Taxodioxylon taxodii Gothan, 1906 (Figs 5, 6). Material code: Vld-1, Vld-2, Vld-3, Vld-4, Vld-5, Vld-6, Vld-7. Locality: the samples were collected from Quaternary alluviums around the Vladimir village, Radomir Depression (see Fig. 1A-B) (42°26’22.36”N; 23°05’0.27”E). Formation: the samples came, most probably, from the Pernik or Bobov Dol Oligocene Coal Basins. From a coal-bearing formation, known as clayey-bituminous formation of the Middle Chattian Age (see Fig. 1C). Repository: The samples are deposited in the Paleobotanical Collection of the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, Sofia, Bulgaria, under the inventory indexes: Vld-1 to 7. Fig. 4. Samples of petrified wood before making the thin sections. 1. Vld-1; 2. Vld-2; 3. Vld-3; 4. Vld-4; 5. Vld-5; 6. Vld-6; 7. Vld-7 (measuring bar 1 cm). Iamandei, S. & al. | Late Oligocene wood remains of Radomir Depression, Bulgaria114 Fig. 5. Photos of Taxodioxylon taxodii (Specimen Vld-2): A, B, C – cross section: growth rings with gradual transition from early to late wood, with distinct ring boundaries, scanty parenchyma, diffuse, resin ducts absent; D, E, F – tangential section: uniseriate rays, parenchyma with nodular end-walls (D-F) and with dark resin content; G, H, I – radial section: tracheidal and cross-field pitting, parenchyma with dark resin content as plugs, granules, homocellular rays, and poorly preserved cross fields with 1-3 taxodioid pits. 115 PHYTOLOGIA BALCANICA, vol. 31 • Sofia • 2025 Fig. 6. Photos of Taxodioxylon taxodii (Specimen Vld-7): A, B, C – cross section: growth rings with gradual transition from early to late wood, with distinct ring boundaries, scanty parenchyma, diffuse; D, E, F – tangential section: uniseriate rays, parenchyma with nodular end-wall (E-F), and granular resin content; G, H, I – radial section: tracheidal and cross-field pitting, parenchyma with dark resin content as plugs, granules, homocellular rays, and poorly preserved cross fields with 1-3 taxodioid pits. Iamandei, S. & al. | Late Oligocene wood remains of Radomir Depression, Bulgaria116 Microscopic description Growth rings distinct, variably sized, of 10-30(-50) tracheids with gradual transition from early to late wood. Ring boundaries are marked by 3-7(-10) tangential rows of smaller, flattened, thick-walled cells of late wood. Normal or traumatic axial resin ducts are absent. The tracheids in cross section appear usually larger in the early wood, occasionally slightly deformed by compression. Between two successive rays, there are 2-10 regular radial rows of tracheids, sometimes more. The earlywood tracheids have a polygonal section, with rounded corners, occasionally determining intercellular spaces, and radial / tangential diameters of 16-50(63)/(16)20-48 μm, smaller in the late wood: 8-12/8-16 μm r/tg. The wall thickness is 3-6(-8) μm (double wall), thicker in the late wood: 8-10(12) μm double wall. Density of the structure amounts to 950-1350 tracheids per mm2. On the tangential walls, pitting is missing or, if appears occasionally, it is either uniseriate, spaced, or in irregularly arranged pairs, with round to oval pits of 8-10 μm in diameter, and round to elliptic apertures of 1.5-3.0 μm. Radial pitting is uniseriate or biseriate, of abietinean type, opposite, spaced or contiguously arranged, with round borders, 1219 μm in diameter, and with round apertures of 3-5 μm. Crassulae are usually absent, but occasionally oblique striations appear. In cross section, axial parenchyma is relatively frequent, diffuse or in discontinuous tangential lines, and constitutes of rounded cells, smaller than the tracheids, or similarly sized, and usually with dark globular granular remains or plugs. There are vertically present thin walls of 1.0-1.5 μm (simple walls) and nodular horizontal (end) walls, with up to eight bead-like knots. Inside, parenchyma cells have dark resin content, or granular remains or rounded plugs, as big and small globules. Occasionally, the cross fields with the ray cells display up to four simple pits in two superposed rows. Medullary rays appear in the cross section quite linear and in the tangential sections have a uniseriate aspect, often with some biseriate storeys, 1-16 cells high, or more. In the tangential view, ray-cells appear polygonal-rounded to round, or vertical oval, 12-18-24 μm in diameter, with lateral intercellular triangular spaces not always very obvious. In terms of frequency, there are 6-12 rays per tangential millimeter. Radially, the rays show a homocellular structure, all cells procumbent, 16-20(24) μm long, the marginals longer: 20-24(32) μm. The horizontal walls are smooth, and have 3-5 μm thick (double wall). The tangential walls are thin and smooth, or slightly rugose, straight and inclined. Indistinct or even, indentures are absent. The typical taxodioid cross fields have 1-2(3) taxodioid pits, oval to round, of 7-9 μm, with large elliptic apertures, inclined to horizontal, of 2-4 μm. They are arranged in 1-2 horizontal, diagonal or triangular rows, sometimes badly preserved or covered by traces of resin globules with a false aspect of unilaterally composed pits. In the taller marginal fields, pitting appears in vertical pairs, up to six on two superposed rows. Occasionally, cross fields with parenchyma can be seen, with up to four simple pits in two superposed pairs. Discussion A xylotomic description of all studied specimens allowed to outline a combination of characters similar to those of the taxodiaceous Cupressaceae, namely: shape and distribution of the tracheids and of the parenchyma in cross section, tracheidal pitting especially on the radial walls, taxodioid type of the cross fields with typical taxodioid pits. However, taking into account the obvious nodular aspect of the end (horizontal) walls of the parenchyma cells, with up to eight knots, or even denticles, it could be assumed that the studied specimens have the greatest affinity to Taxodium–type wood. And to the fossil genus Taxodioxylon (Hartig) Gothan, respectively. From the already described species of Taxodioxylon, the studied specimens mostly resembled Taxodioxylon taxodii, as defined by many authors (Greguss 1967; Gottwald 1992; Selmeier 2001; Iamandei & al. 2005, 2008, 2012), a fossil species equivalent to the current species Taxodium distichum, a tree growing presently in restricted areas in South USA, Mexico and Guatemala, in riparian and wet habitats (Earle 2017). Thus, remarkable features in the studied specimens are the radial pitting of the tracheids, nodular end wall of the parenchyma, and a typical aspect of the cross rings, very similar to the current species T. distichum. Following the discussion of affinities of the studied material, the authors decided to assign all the seven studied specimens to the Taxodioxylon taxodii species. Quite possible, the process of petrification of the wooden parts had taken place in a common lacustrine basin in the two above mentioned coal basins, or in a basin that was part of the lacustrine complex in the studied area. The palaeofloras from the Pernik Basin (Bozukov & Ivanov 2020) and most of the Bobov Dol Basin (Palamarev & al. 1998) reflect the period of transition and transformation of the flora of the Balkan Peninsula from palaeotropical to arctotertiary. Reduction of the palaeotropic elements and preservation of some of them in refugia was a main tendency, along with the simultaneously enhanced role of arctotertiary species in the structure of vegetation, namely, representatives of the genera Acer, Alnus, Carpinus, Fagus, Populus, Quercus, Salix, Ulmus, and Zelkova. According Palamarev & al. (1998) and Bozukov & Ivanov (2020), the climate of the Late Oligocene could be described as humid and warm temperate. Existence of such a climate in the study area seems suitable for the distribution of forests of fossil species, whose nearest living relative is Taxodium distichum. 117 PHYTOLOGIA BALCANICA, vol. 31 • Sofia • 2025 Conclusions The paper presents a xylotomical study of some samples collected near the Vladimir village, close to the Pernik and Bobov Dol Coal Basins. The samples have been identified as Taxodioxylon taxodii, a fossil equivalent of the current Taxodium distichum, which presently grows in riparian and wet habitats in restricted areas of South USA, Mexico and Guatemala. However, it seems to have been widespread in the European Cenozoic flora, since it is present in the fossil records as leaf imprints, wood remains and even as important species identified and described in many coal deposits in Europe. The presence of fossils of the genus Taxodium in the coal basins of the Late Oligocene Age in immediate vicinity of the studied deposits also suggests a probable transport of the fossil material in the form of sediments by the paleo-rivers in the area. It is also likely that there was a system of water basins between those of Pernik and Bobov Dol, where a spread of marsh vegetation, including the fossil species of Taxodioxylon from which the studied material originated, would have been possible. Further studies in the area will be necessary to answer definitively this question. Acknowledgements. This work has been carried out within the framework of the National Science Program „Environmental Protection and Reduction of Risks of Adverse Events and Natural Disasters“, approved by a Resolution of the Council of Ministers No 577/17.08.2018 and supported by the Ministry of Education and Science (MES) of Bulgaria (Agreement No D01-27/06.02.2024). 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