Quercoxylon bavaricum Selmeier, 1971

Iamandei, Stănilă, Iamandei, Eugenia & Ursachi, Laurențiu, 2023, Late-Miocene Moldavian Petrified Forest, Acta Palaeontologica Romaniae 19 (1), pp. 61-85 : 68-72

Quercoxylon bavaricum Selmeier, 1971

Fig. 4 , a-i.

Material code: Pb19, Pb20, Pb26, Pb27, Pb28, Pb38, Pb44, Pb45, Pb52, Pb58, Pb59, and Pb64Zo.

Locality: Simila gravel quarry (Vaslui county) and Zorleni, in the 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: 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 big vessels – visible by the naked eye, typical for a dicot.

Microscopic description

The growth rings are distinct in cross-section, the wood structure of the ring-porous type, with two-sized vessels and show distinct ring-boundaries marked by an abrupt change between the latewood with small vessels and earlywood suddenly starting with large vessels. Also, the wood structure is marked by the presence of two-sized rays.

The vessels of the earlywood appear large-sized in cross-section, are exclusively solitary, and are arranged in 1- 2(3) tangential rows. They appear as round to ovalshaped pores, radially elongated or slightly deformed by compression, and have the lumina size of 100-250(350) / 70-200(250) µm the radial/tangential diameters (r/tg d.), and the wall-thickness of 3–5(7) µm the simple wall. In the transitional to latewood, the vessels appear suddenly as small-sized pores, almost exclusively solitary, sometimes in small groups, the latewood slightly diminishing close to the ring boundary (as final wood). They appear in radial bundles between the numerous fine rays, floating in a mixed ground-mass, and they form in the transitional to latewood a quassi-triangular patern, having intermediary portions devoid of vessels. The solitary small vessels of the transitional and latewood have, usually, polygonal cross-section, rounded or deformed, even star-like, probably due to compression and fossilization processes, and have r/tg diameters of 20–40(50) / 15–30(50) µm, and thin walls, of 2–3 µm the simple wall. The vessels’ density is variable, with 2–3(5) large pores per tangential millimeter but more numerous in the transitional to latewood (often more than 100 pores per square millimeter). Longitudinally, the vessels show simple perforation plates, rather poorly preserved. Also, vascular bordered pits appear, numerous, alternate and in some vertical rows, often difficult to see due to poor preservation of all the wood structure. The vessel-ray parenchyma pits have reduced borders. Inside the vessels, thin-walled, large tyloses are common, but poorly preserved, so, difficult to observe. The vascular element size usually is difficult to measure.

The ground tissue is mixed, constituted by libriform fibers and fibrotracheids (described below) and parenchyma, often difficult to identify each other in cross sections, having quite a similar aspect.

The fibers in the cross-section appear slightly irregularly arranged mixed with the diffuse or banded parenchyma. They have polygonal rounded sections and are relatively thick-walled (4–5 µm double wall), and are unpitted and unsepted. The fibrotracheids appear mixed in the ground tissue and are relatively unclear in cross-section, because of bad preservation. Also, longitudinally they present small pitting, bordered, round or slightly elliptic, with small apertures, in 1(2) vertical rows arranged. As vasicentric tracheids appear with smaller rounded crosssections and are coiling the big vessels. Vertically they bear small, round, alternate, bordered pits with small round apertures, in 1-2 vertical rows arranged.

The axial parenchyma, in cross-section, appear diffuse, of apotracheal type, scattered among the libriforms, or diffuse-in-aggregate, or as short thin tangential bands, and scanty paratracheal. In longitudinal sections, the parenchyma cells difficultly can be seen amongst the elements of the groundmass. Sometimes vertical rows of large crystals in chambered parenchyma cells were observed in some of the studied specimens.

The rays are of two distinct sizes: fine rays, uniseriate and biseriate, low and numerous, linear or slightly curved molding the vessels in the earlywood, and broad rays, multiseriate, compact or compound, of 13–20 cells wide (up to 50–60 µm). In tangential section they appear often taller than 1 mm and sometimes dissected by libriform fibers, giving them an aspect of aggregate rays. They are constituted of rounded to polygonal cells, unequal in size (10–20 µm in diameter) and relatively thin-walled (2–3 µm the double wall). The ray frequency is variable, with 8–15 thin rays on tangential millimeters, the multiseriate being rare and at relatively uniform intervals. Radially the rays are homocellular and show cells all procumbent. Sometimes there is a tendency to heterocellularity, the rays showing 1–2 rows of square or upright marginal cells. In the cross-fields, 5–7 small bordered pits appear, rounded to elliptic (of 8-12 / 5-9 µm) usually hardly visible, since often gum remains and solitary crystals appear inside the ray cells are present, blurring the details. Sheath cells or tile cells are not present.

Special details – such as storied structures, secretory elements, intercellular canals, cambial variants, included 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

Evaluating the studied specimens, at least in cross-section, we observed their affinity to the fagaceous wood structures by their well-expressed ring-porosity. The two-sized rays (wide multiseriates and finer, mostly uniseriate) and the two-sized vessels with their distribution in a ring porous structure are typical for Quercineae (see Privé-Gill, 1975), especially for the extant genus Quercus L.

By consulting the books of Greguss (1959) and Schweingruber (1990), the site of Schoch et al. (2004) - onwards and Wheeler et al. (2011 - InsideWood – onwards) – we also have observed the xylotomical identity of our studied specimens to the quercineous taxa. For correct generic identification, some other keys of identification were consulted too, for example:

1. The key of Hadziev & Mädel (1962), which separate within the current Quercoideae subfamily, four types of wood structure:

• Type “Weisseichen” (white oaks), comprising most of the species of section Quercus (former Lepidobalanus) – having a ring-porous structure with small, polygonal, thin-walled vessels in the latewood, densely and flamelike arranged. This type of structure is quite similar to our here studied specimens.

• Type “Roteichen” (red oaks), comprising the species of the section Lobatae (former Eritrobalanus) and some species of Lepidobalanoideae, with ring-porous structure and relatively large, round, thick-walled vessels in the latewood.

• Type “Immergrüne eichen” (sempervirent oaks), comprising species of Quercus and of Lithocarpus with diffuse porous or half-ring-porous structure, the relatively small and spaced vessels, often radially disposed in the latewood.

• Type “Wurzelholz” (root-wood), present in all the Quercoideae and having diffuse porous structure and crowded large pores.

2. The key of Petrescu (1976), in which the author tried to systematize the fossil fagaceous wood structures, separating within the ring-porous structures those with uniseriate and compact pluriseriate rays, sometimes compactcompound or partially-aggregate, corresponding to the current Quercus genus and to the fossil genus Quercoxylon .

For fossil wood of this type was created, during the time, more genera, as it follows: Kloedenia, Quercinium, Quercites , Quercus and Quercoxylon - used by Goeppert, Felix, Unger, Conwentz, Mercklin, Schleiden, Edwards, Schüster, Platen, Knowlton, Pampaloni, Webber, Nee, Ogura, Watari, Shimakura (see Müller-Stoll & Mädel, 1957).

However, the accepted fossil morphogenus is Quercoxylon , which was created by Hofmann (1929), but was correctly defined later, by Kräusel (1939), with Q. retzianum Kräusel as type-species and with this diagnosis: "Secondary wood porous or ring porous with more or less obvious growth rings, usually solitary vessels, with simple perforations, with large alternate pits, bigger to parenchyma or to ray cells from irregular oval to polygonal, usually vertical; specific two-sized vessels and, tracheids, and libriform fibers and parenchyma as diffuse, as short tangential uniseriate bands. Two-sized rays: short, uniseriate rays, sometimes as false broad rays; and real broad rays, compound, compact and aggregate".

The genus was slightly emended by Müller-Stoll & Mädel (1957) and by Gros (1983, 1988). This is the last diagnosis of Quercoxylon given by Gros (1988): "Porous or ring-porous structure, usually simply perforated solitary vessels, alternate vascular pitting; pitted parenchyma, banded or diffuse; libriform tracheids and small vessels in groundmass; and two-sized rays".

Later Suzuki & Ohba (1991), have described some quercineous species from Japan, and made a revision of fossil woods of Quercus - type and of Lithocarpoxylon genus. They did not commented the validity of the genus Lithocarpoxylon , created by Petrescu (1978), which was considered later invalid by Selmeier (1997) who considered the diagnosis of Quercoxylon as including references to similar details. For more details on the adventures of these morphogenera see Suzuki & Ohba (1991), Gregory et al. (2009, pp.46-54).

The analysis of the xylotomy of our studied specimens shows that they should belong to “Weisseichen” type, from Quercus section, since they present in cross-section ring porous structure with large vessels in the earlywood and small, polygonal, thin-walled vessels in the latewood, as Hadziev & Mädel (1962) said.

But the distribution of the vessels may have many variations, as some other authors previously have been shown (see Selmeier, 1971; Privé-Gill, 1975). Thus, Privé-Gill (1975) observed 5 types of latewood vessels' distribution: 1. typical dendritical distribution as radial complexes separate by libriform bands; 2. diffuse distribution; 3. irregular distribution of few vessels or absence; 4. the vessels of the earlywood separated by a fibrous zone; 5. as radial complexes of vessels with gradual diminishing and separated by libriform fibers. The same author observed that the different types of distribution of the latewood vessels may correspond to some ecological variation that affected the trunk growing. Well-developed latewood indicates irrigated soil and reduced latewood is determined by a dry climate. The localization of the sample in the tree is told by the proportion of latewood, which can be bigger in the trunk than just under the canopy or at the periphery and, the growth rings are more reduced in the branches that are in the trunk. Reviewing many extant described species of “white oak”, the same author observed that there is a big intraspecific variation (Privé-Gill, 1975, p. 125)

Selmeier (1996) also observed that, generally, the vessel diameter, ray size, ray frequency and ray distribution are variable and, even in the same described fossil species, the biometric values may be different. For this reason, in the xylotomical description of extant wood, we find no actualized measurements. So the accurate measurements made by palaeoxylologists cannot be always useful to identify an unknown.

Much more, the rules from “IAWA List of Microscopic Features for Hardwood ” (Wheeler et al., 1989) impose different ways to express the older measurements.

Thus, because the current genus Quercus L., having over 500 species, shows a great interspecific xylotomic homogeneity, there is very difficult to delimitate the fossil species, and they have only a descriptive value, as formspecies, otherwise very numerous (see Gregory et al., 2009, pp.46-54).

After this discussion, and reviewing the description of our specimens, we observed that the arrangement of the vessels of the early, transitional and latewood is very similar to the current white oak group (Sect. Quercus ), spread in Europe and partially in western Asia.

It appears that such an oak-type has dominated the European Miocene, since such similar forms, even identical (see Petrescu, 1976) have been described in the Pannonian space by Felix, Andreánszky, Hofmann, Greguss (in Müller-Stoll & Mädel, 1957), usually described as equivalents to the current Quercus robur L., which belong to Quercus section, identified later as Quercoxylon bavaricum Selmeier 1971. In fact, many similar species were described in Carpathians, even from Huşi- Avereşti- Soleşti area, an area slightly northward of the site of origin of here studied material, which we suppose it was transported from that area, and we cite them: Quercoxylon sarmaticum, Q. compactum, Q. macarovicii, Q. kersonianum and Q. solesticum described by Starostin & Trelea (1969, 1984, 1987). Also, some other quite similar Miocene oak wood were described from the Carpathian area by Iamandei et al. (2001 a,b,c; 2006; 2008a,b; 2010; 2017; 2020).

Some of the wood remains collected from Solești area by Iamandei & Iamandei (2010) were attributed to Quercoxylon bavaricum Selmeier, considered as the nearest living relative to the current Quercus robur L., maybe to Q. petraea (Matt.) Liebl., as a white oak too, and very similar (Greguss, 1959; Schweingruber, 1990; Akkemik & Yaman, 2012). However, this species described by Selmeier (1971) from Germany has a correspondent within the Carpathian area in Quercoxylon sarmaticum described by Starostin & Trelea (1969), and this species must be revised, because, if Petrescu (1976) was right, the Romanian fossil species could have priority versus the German one, defining the same type of wood.

Therefore, after the above discussion and taking into account the structural details observed in our here studied specimens, regarding the vessels arrangement in the growth ring, the aspect of the ground tissue and of cross-field pitting which are similar to the form described by Selmeier (1971), we assign them to the species Quercoxylon bavaricum Selmeier 1971.

Figures

Fig. 4 - Quercoxylon bavaricum Selmeier, 1971. Specimen Pb58. a, b, c – cross section: ring-porous wood with two-sized vessels, two-sized rays, growth-ring boundary marked by the abrupt size change of early wood; d, e, f - tangential section: broad fusiform rays, high, sometimes dissected, and thin rays 1-2 seriate; g, h, i - radial section: poorly pitted vessels with simple perforations and alternate pitting on vascular tracheids (i), poorly preserved cross-fields (g, i), probably with rectangular pits ”in palisade” (h)