Quercoxylon intermedium Petrescu et Velitzelos, 1981

Iamandei, Stănilă, Iamandei, Eugenia & Ursachi, Laurențiu, 2023, Late-Miocene Moldavian Petrified Forest, Acta Palaeontologica Romaniae 19 (1), pp. 61-85 : 72-74

Quercoxylon intermedium Petrescu et Velitzelos, 1981

Fig. 5 , a-i.

Material code: Pb39, Pb41, Pb42, Pb50, Pb55, Pb56, Pb57, Pb61 and Pb62.

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 big vessels – visible even by the naked eye, typical for a dicot.

Microscopic description

The growth rings are quite distinct in cross-section, the wood structure appears as semi-ring-porous, with vessels in radial bundles, usually gradually diminishing from the earlywood to the latewood one. Often, in some specimens, a two-sized aspect of the vessels is visible. The ring boundaries are marked by the abrupt change between the latewood with small vessels and the suddenly starting earlywood with big vessels. Also, two-sized rays are usual.

The vessels appear exclusively solitary, arranged in radial bundles, giving a slightly irregular to diffuse general aspect. The cross-section of the earlywood vessels is round to oval, often deformed or radially elongated. Their lumina varies between 200–350 µm in diameter, while the smaller vessels diminishing to the latewood are usually rounded polygonal or star-like, and their diameters vary between 40–150 µm. The vessels are moderately thick-walled: 3–5 µm the simple wall. The vessels’ density is variable, as 3–5 vessels per square millimeter in the earlywood, and between 7- 14 in the latewood. Simple perforations on tilted plates are present and numerous bordered pits are visible on the vessel walls, corresponding to those of the vasicentric parenchyma. The pits are slightly small oval, of 4-5 µm in diameter, opposite, suboppositely to slightly alternately arranged, and crowded. Helical thickenings are not visible, also tyloses or content, since poorly preservation. Mean vessel elements, difficult to measure, range from 350 to 800 µm.

The ground tissue is constituted from libriform fibers and parenchyma, which are often difficult to identify from each other, due to bad preservation.

The libriform fibres, in cross-section seen, constitute the major part of ground tisue, are relatively thick walled and, longitudinally viewed, are pitted and non-septated. Fibrotracheids and vasicentric tracheids are also present, and difficult to identify in cross-section, but on their vertical walls, pitting can be observed.

The axial parenchyma appear in cross-section of apotracheal type, either diffuse, scattered among the fibers, or diffuse-in-aggregates. Also, paratraheal parenchyma appears, less visible in cross-section, closely appressed to the vessels and pitted in longitudinal view, sometimes chambered and crystalliferous, difficult to observe, due to the poor preservation.

The rays are of two sizes and, in cross-section, appear quite linear. The fine rays, usually uniseriate, are numerous and low. The broad rays occur, in cross-section, at relatively uniform and large intervals. They are multiseriate, of 13– 20 cells wide (i. e. up to 300–350 µm in width), are often taller than 1 mm and are usually dissect- ed by libriform fibers, giving them a typical aspect of compound-aggregate or even aggregate rays. In the tangential section, the ray cells appear rounded to polygonal cells, unequal in size (8– 15µm) and rather thin-walled (2–3 µm the width of the double wall). The ray-density is variable, varying between 10–20 rays on a tangential millimeter. In radial view the rays are slightly heterocellular, showing procumbent cells in the median part, followed by squared to upright cells in the external part (marginally). In the cross fields, numerous rectangular or vertical elliptic large pits are arranged “in palisade”. Sometimes, gum remains and solitary crystals are present inside the ray cells, often difficult to observe, due to poor preservation.

Special details - as storied structures, secretory elements, intercellular canals, cambial variants, included phloem not observed or are absent.

Mineral inclusions seems to be present as rounded crystals in chambered axial parenchyma cells and in ray parenchyma cells.

Affinities and discussions

The studied specimens show, in cross-section, a semi-ring porous wood-structure with broad rays and this indicates them as possible fagaceous woods (see Petrescu, 1976). The aspect of the vessels, the two-sized rays with the broad rays situated at relatively uniform and large intervals and usually dissected by libriform fibers giving aspect of compound-aggregate or aggregate rays and their cross fields with pits “in palisade”, all clearly suggest an oak-tree type structure, as resulted from consulting Greguss (1954), Hadziev & Mädel (1962), Schweingruber (1990), the site of Schoch et al. (2004) - onwards and Wheeler et al. (2011 - InsideWood – onwards) (see discussion above).

However, their structure could be also diffuse-porous, which is characteristic of the evergreen species, while the ring-porous structure characterizes the deciduous species of Quercus (and the most septentrional species of Lithocarpus ). In the root wood, the deciduous species often tend to lose their ring-porousness and to become similar to the evergreen species, and the broad rays become divided into false rays, i.e. aggregate rays (Privé-Gill, 1975).

For quercineous fossil wood (see discussion above), the accepted valid name is Quercoxylon Kräusel 1939 (emend. Müller-Stoll & Mädel, 1957; em. Gros, 1988), and based on the cited keys of identification, it appears that the here studied specimens suggest a structure of white oak type, close to the extant Mesobalanus group.

Thus for comparison we used several fossil forms of Quercoxylon , described by Müller-Stoll & Mädel (1957), Greguss (1969), Nagy & Petrescu (1969), Hadžiev & Mädel (1962), Huard (1966), Privé-Gill (1975, 1984, 1990), Privé & Brousse (1976); Petrescu (1976, 1978), Petrescu et al. (1969, 1970, 1971, 1972, 1978, 1980, 1981), Starostin & Trelea (1969, 1984), Selmeier (1971, 1997), Suzuki & Ohba (1991), Selmeier et Velitzelos (2000), Iamandei et al. (2008a,b, 2010, 2011, 2014).

Taking into account the great intraspecific anatomic homogeneity inside the current Quercus genus and the variability of the measurable characters (see Prive-Gill, 1975 and Selmeier, 1996), and comparing the xylotomy of our specimens with that found in the above-cited studies, at other fossil oak described above as Quercoxylon bavaricum , we admit that, at the first sight, the wood structure of the here studied specimens seems to be similar to the current Quercus borealis L. (in Schweingruber, 1990), a synonymous of the current Quercus rubra L., a species of the red oak group ( Quercus , section Lobatae ), living now in North America, in the northeastern United States and southeast Canada (and locally named Northern Red Oak, or Champion Oak).

However, we consider that the Miocene ancestors of the extant oak species should not be searched on the American continent, but in the Mediterranean-European area.

Thus, we selected Petrescu et al. (1980, 1981), which have described from the Oligocene of Trakia (North-East of Greece) a Quercoxylon helladae (Petrescu, Velitzelos & Stavropodis, 1980) Selmeier 1997 , which has some similarities with our specimens, and also the species Quercoxylon intermedium Petrescu & Velitzelos 1981 which is very similar to mediterranean evergreen oak species and to our studied specimens by the distribution of vessels in cross-section, by the aspect of rays of compound-aggregate and the aggregate type and by the cross-field pitting with palisade arrangement. This species was also described by Iamandei et al. (2014) from Rhodopes mts., Bulgaria. Reviewing the current mediterranean Quercus ilex L., an evergreen oak, and also the current Q. cerris L., a deciduous form native to southeastern Europe and Asia Minor (Akkemik 2012; see also InsideWood), we observed that a lot of structural details are similar with those described in our specimens.

Thus, the tall broad rays usually dissected by libriform fibers giving a typical aspect of compound-aggregate or even aggregate rays ( Fig. 6 e-f) suggest a similarity up to identity with the fossil species Quercoxylon intermedium Petrescu et Velitzelos 1981 , to which we assign the studied specimens.

Figures

Fig. 5 - Quercoxylon intermedium Petrescu et Velitzelos, 1981. Specimen Pb55. a, b, c – cross section: semi-ring-porous wood with gradually diminishing vessels, in radial bundles, and two-sized rays and growth-ring boundary difficult to guesss; d, e, f – tangential section: broad fusiform rays, dissected by libriform fibers giving them a typical aspect of compound-aggregate or even aggregate rays (e, f), and thin rays 1-2 seriate; g, h, i – radial section: vessels with simple perforations and alternate pitting on vessels, bad preserved cross-field (g, h) but clear rectangular or vertical elliptic large pits arranged “in palisade”(i).

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.