Populoxylon Mädel-Angeliewa, 1968

Iamandei, Stănilă, Iamandei, Eugenia & Ursachi, Laurențiu, 2023, Late-Miocene Moldavian Petrified Forest, Acta Palaeontologica Romaniae 19 (1), pp. 61-85 : 74-77

Genus Populoxylon Mädel-Angeliewa, 1968 Populoxylon tremuloides Iamandei & Iamandei, 2006

Fig. 6 , a-i.

Material code: Pb14, Pb18, Pb25 and Pb37.

Locality: Simila gravel quarry (Vaslui county), central part of Moldova, Northward of Bârlad.

Repository: in the Collection of the Natural Sciences Section of the Museum “Vasile Pârvan”, from Bârlad city, Romania.

Age: the Maeotian age.

Formation: Fluvio-deltaic sediments with gravel levels, exploited in Simila gravel quarry, where the petrified wood samples appear as reworked centimetric elements with obvious ring porous and fibrous structure with vessels – visible even by naked eye, typical for a dicot.

Microscopic description

The growth rings – distinct, wide, are often unequally wide, since having distinct ring-boundaries, marked by a few rows of slightly flattened cells of ground tissue as final wood and the sudden start of the earlywood with large pores.

The vessels, in the cross-section seen, have quite similar sizes and are evenly distributed throughout the growth ring, so defining a diffuse-porous wood structure. They appear in radial pattern between two successive rays, usually as radial multiples of 2-4(-6) pores, sometimes mixed with a few solitary pores, whose contour is polygonal rounded to oval, or star-like. The radial groups use to be radially linked by 1-5 fibro-tracheids (described below). In the groups larger and smaller vessels are present. The mean tangential diameter of the solitary vessels lumina is 50-120 μm. The density is 80-160 vessels per square millimeter, sometimes more. In the longitudinal view, the vessels show simple perforation plates, inclined, and usually poorly preserved. The intervessel pits are of bordered type, numerous, alternate, polygonal, mean-sized of 8-10 μm in diameter, with a horizontalelliptic aperture of 3-4 μm. Vessel–ray pits with distinct borders, similar to intervessel pits in size and shape throughout the ray cell, but poorly preserved, and difficult to observe. The vessel-element length is probably ≥ 800 μm, but difficult to meassure, due to poor preservation. Thin-walled tyloses and dark gum remains or fungi, rarely appear inside vessels.

The ground tissue include fibers and parenchyma, usually mixed.

The libriform fibres have also polygonal cross-section, of 12-16 μm in diameter, and relatively thick walls: 4-6 μm the double wall. They show a poorly preserved pitting on the longitudinall walls and are non-septate.

Fibro-tracheids are present, linking the radial groups of vessels, as 1-5 cells, and are similar to the libriform fibers, or larger, of 15-20 μm, and are moderately thick-walled, of 3-6 μm the double wall. More microscopic details on the tracheids which are linking the radial groups of vessels are difficult to observe, due to poor preservation.

The wood parenchyma appears few and scarce, diffuse or terminal, rather indiscernible, even if sometimes it appears in the vertical sections.

The medullary rays appear thin in cross-section and formed by quassi-rectangular radially elongated cells. On the horizontal walls simple small pits difficultly can be seen, since granular gum remains usually are present inside the ray cells. The ray trajectory is usually linear, or slightly sinuous. Tangentially the rays appear exclusively uniseriate and have 5-35(-55) cells in height or even more. The ray-cells have polygonal rounded to vertical elongate shapes and, the marginals are slightly higher. The ray-frequency is ≥ 12 / mm, we counted even 18-22(- 35) rays per tangential mm. Radially, the rays are homocellular, constituted from thin-walled cells, all procumbent, the marginals slightly taller. The cross-fields with vessels are difficult to observe due to poor preservation, but present 1-2 weakly bordered pits to apparently simple, per field, round to oval, of 4-7 μm in diameter, and in the marginal fields appear usually in 2-3 horizontal rows.

Special details - as sheath cells or tile cells are not present. Also, storied structures, secretory elements, intercellular canals, and cambial variants, including phloem are absent.

Mineral inclusions are present as usually rounded crystals, as we showed above, present in chambered axial parenchyma cells and in ray parenchyma cells.

Affinities and discussions

The studied specimens show, in cross-section, some xylotomical features typical for the current members of Salicaceae family, such as the uniseriate rays, diffuse-porous distribution of vessels, usually as radial multiples of 2-4(-

6) pores, sometimes mixed with few solitary pores, and homocellular rays with cross-fields specifically pitted, suggesting the current genus Populus L. Quite similar features appear in the current Salix L., in which the cross-section shows the diffuse-porous distribution of vessels which appear usually solitary, and the rays are heterocellular (Greguss, 1959; Schweingruber, 1990; Schoch et al., 2004 – onwards; Wheeler et al., 2011; Akkemik & Yaman, 2012; Sakala et al., 2018).

From the above-cited scientific papers we found there are only five species of poplars, naturally growing in Europe today and in the Anatolian area: Populus euphratica Oliver P. nigra L., P. alba L., P. tremula L. and also, P. xcanescens (Aiton) Sm. , which is considered a hybrid between the last two species (see P. × canescens - Wikipedia). All these species are very similar since they cannot be distinguished from each other on the basis of their wood anatomy (see Schweingruber, 1990; Akkemik & Yaman, 2012). There is only one small difference: the rays are higher in the fossil species, and maybe the marginal axial parenchyma is more common in the fossil species.

There are no many fossil forms described by the study of wood remains, but the correspondent fossil genus - Populoxylon was created by Mädel-Angeliewa, in 1968, based on the study of the type species P. priscum Mädel-Angeliewa 1968 . The diagnosis indicates: ”Diffuse porous wood with usually grouped vessels, that has simple perforations and numerous alternate intervascular pitting. The uniseriate rays have all procumbent cells, the marginals higher, the cross-fields simple pitted, in 2-3 horizontal rows of slightly oval pits, and few parenchyma”. And our studied specimens have almost the same xylotomical features.

For comparison we consulted the paper of Nastschokin (1968) who described a Populus sp. ( P. tremula L.?) from the Quaternary of Yenisei-river basin ( Russia). Close to us, Greguss (1969) has described a Populoxylon sp. (cf. Populus tremula L.), from the Sarmatian of Mikfalva ( Hungary), which has a semi-ring porous wood with radially grouped vessels with simple perforation plates and alternate polygonal intervessel pits, and only uniseriate rays. Both these structures send to the current species Populus tremula L., (see Greguss, 1959; Schweingruber, 1990), and are very similar to our studied specimens. However, as Sakala (2006) and Sakala et al. (2018) observed, because of the lack of a radial section, it is not possible to confirm whether the rays are homocellular, similar to Populus L., or heterocellular, similar to Salix L. Dutrelepont et al. (1997) described a Populoxylon sp. , which has the vessel distribution in cross-section similar to the extant Populus euphratica Olivier , which is slightly different of our specimens.

From far-East, Blokhina & Snezhkova (2003) described from the Upper Miocene of the Erkovetskii brown coalfield (Amur River Region), a Populoxylon priamurensis Blokhina et Snezhkova , and also, Terada and Suzuki described from Japan, a Miocene Populus soyaensis (in Choi et al., 2010), which differ of our specimens by lower vessels density and arrangement in cross-section.

Iamandei et al. (2005, 2011) described some Mid- Miocene petrified woods collected from Prăvăleni (Apuseni Mts.) and, respectively, from Bala area (Mehedinţi Mts.) which presented xylotomical similarity to the current Populus alba L., as Populoxylon sp. (cf. Populus alba L.), which is slightly different, by their porosity.

From the Late Badenian of South Apuseni ( Romania) Iamandei & Iamandei (2006) and later, from Moldova Rep. (Iamandei et al., 2008b), have described Populoxylon tremuloides Iamandei et Iamandei as xylotomically identical with the extant species Populus tremula L., (see Greguss, 1959; Schweingruber, 1991) and with our studied specimens from Simila gravel quarry.

Also, Akkemik (2021) described from Turkey (Hoçaş village, Seben city, Bolu), a new species, as Populoxylon sebenense Akkemik , which is slightly different of our specimens by the obvious presence of marginal axial parenchyma.

Thus, consulting also the identification key of Akkemik (2021) and revising once again the description of the here studied specimens, we observed an identity of their xylotomical features with those of the current species Populus tremula L., and also with the fossil form Populoxylon tremuloides described from Carpathian area, so we attribute them to the species Populoxylon tremuloides Iamandei et Iamandei, 2006 .

Figures

Fig. 6 - Populoxylon tremuloides Iamandei & Iamandei 2006. Specimen Pb14. a, b, c – cross-section: aspect of growth rings, diffuse porous distribution of vessels, fine rays, regular libriform fibers; d, e, f – tangential section: poorly preserved alternate intervascular pitting (e), uniseriate rays; g, h, i – radial section, simple perforated plates (g), poorly preserved cross-fields with vessels.