Quercoxylon sp.

Iamandei, Stanila, Iamandei, Eugenia & Todea, Petru, 2020, Palaeoxylotomical Study Of A New Collection From Zarand Miocene Petrified Forest, South Apuseni Mts., Acta Palaeontologica Romaniae 16 (1), pp. 19-34 : 28-31

Quercoxylon sp. cf. Quercus frainetto Tenore

Fig. 6 , a-i.

Material

From a sample of petrified wood collected from South Apuseni mts, Ociu area, Bisericii Book , from Mid-Miocene deposits (Late Badenian-Early Sarmatian), kept within “ Todea Collection ”, in Crișcior- Brad locality, under the indicative OB-3 , with standard prepared thin sections that were submitted to the paleoxylotomical study.

Microscopic description

The growth rings in cross-section are distinct, but with not well marked boundaries by a special tissue; the wood structure is ring-porous and shows an abrupt change between the early and the late-wood: from few solitary, round, big, early-wood vessels, to smaller, solitary, round or slightly oval late-wood vessels, in radial pattern arranged, to slightly diagonal or as composed fascicles arranged, from dendritic to almost porous arrangement, the pores’ size gradually diminishing to the final wood.

The vessels in the early wood, are exclusively solitary, round, large sized of 200-350 µm in diameter and thick-walled (of 5-7 µm the simple wall), in 1-3 tangential rows arranged. In the transitional to late wood, round to radial oval, small sized vessels appear (of 90-150/70-130 µm r/tg diameters), also exclusively solitary and thick-walled. The vessels’ density is variable, of 2-3 pores per square millimeter in the early-wood, more numerous (11-14) only in the late-wood (mean density is between 7-11 pores, variably-sized, per square millimeter). In longitudinally view, the vessels have simple perforation plates, quite difficult to see due to bad preservation. On vessels and on vasicentric tracheids, alternate bordered pits with horizontal apertures appear.

The axial parenchyma is less visible in cross section, must be diffuse, of apotracheal type, scattered among the libriform or tracheids. In longitudinal sections can be seen as vertical strands of rectangular chambered cells among the elements of ground-mass, or even paratracheal. Due to bad preservation no other detail is visible.

The rays, in cross-section seen, appear linear, slightly wavy in the early wood and are two-sized: fine rays, mostly uniseriates, short and numerous and broad rays, compact multiseriates of 8-15 cells, often taller than 1 mm. Tangentially they show polygonal cells, unequal in size - of 12-25 µm in diameter, and relatively thin-walled (2-3 µm the double-wall). The ray-frequency is variable, of 10-14 rays on tangential millimeter, the multiseriates being rare and, at relatively uniform intervals. Radially, the rays are homocellular showing cells all procumbent, and the cross fields have vertical-rectangular bordered pits, in palisade arrangement.

The fibers, transversally seen, appear as thick-walled cells with point-like lumina. In longitudinal view, they are unpitted and unsepted. The fibro-tracheids are relatively similar to the fibers and difficult to separate in all sections, due to bad preservation.

Affinities and discussions

The cross-section of the studied specimen shows a typical aspect of wood of Fagaceae , i.e. obvious ring-porousity, rounded pores and two-sized rays, finer, mostly uniseriate and multiseriates broad, typical features of some quercineous structures. After Hadziev & Mädel (1962) for the extant oaks could be separated four types of wood structures:

• Type white oaks (Weisseichen), comprising most of the species of Lepidobalanus sectio – having ring porous structure and small, polygonal, thin-walled vessels in the late wood.

• Type red oaks (Roteichen), comprising the species of Eritrobalanus sectio and some species of Lepidobalanoideae section, with ring-porous structure and relatively large, round, thick-walled vessels in the late wood.

• Type sempervirent oaks, comprising species of Quercus and of Lithocarpus with (diffuse)-porous or semi-ring-porous structure, and relatively small and spaced vessels often radially arranged in the late wood.

• Type oak root-wood, present in all the Quercineae, having diffuse-porous structure and crowded large pores.

Privé-Gill (1975), observed that the oak-wood is characterized by vessels in radial rows, more or less dendritically distributed, and simple perforated, specifying that two-sized rays (fine, 1-2-seriates and broad, multiseriates) are present in Quercus L. and in Lithocarpus Bl. (= Pasania Oerst. ) with some tens of species from temperate to warm regions of north hemisphere, islander also. The structures with diffuse pore distribution characterize the evergreen species. The ring-porous structure is typical to 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 becoming similar to the evergreen species, and the broad rays become divided in false rays, i.e. aggregate rays ( Privé-Gill, 1975).

Also, for a correct generic identification, other two keys of identification were consulted:

• The key of Shimanji (in Suzuki & Ohba, 1991) who made a comparative synthesis of the essential xylotomical characters of the extant fagaceous taxa, based on the anatomic studies and the phylogenetic relations between the genera of Fagaceae , comprised in a big table presented. Using this key we observed that having ring porous structure, round thick-walled vessels in latewood, simple perforations and compact thick rays, our structure can be similar to a Quercus of white type from Quercus section.

• The key of Petrescu (1976), systematizing the extant and fossil Fagaceous wood-types ( Table 1 ), and separating them by ring structure, perforated plates and ray-type, key slightly emended here by us, since Lithocarpoxylon is considered invalid genus ( Selmeier, 1997). Using at least these keys, the generic identification as Quercoxylon of our specimen is correct. Consulting, also, the atlas of Schweingruber (1990), the onward site of Schoch et al. (2004), the site InsideWood 2004 -onwards, and the paper of Wheeler (2011), we also found a perfect similarity of our studied specimen with the quercineous structure.

This fossil genus Quercoxylon was initially created by Elise Hofmann (1929), when she quotes the species Quercoxylon cerris , but without Fig.s or description, so, it was considered not correctly published. Ten years after, it was Kräusel (1939) who correctly defined and imposed the genus name Quercoxylon , describing a new species, Q. retzianum Kräusel, 1939 , and establishing it as typespecies, giving a correct genus’ diagnosis, as it follows: Secondary wood porous or ring porous, with more or less obvious growth rings, usually solitary vessels with simple perforations, with big alternate pits, bigger to parenchyma or to ray cells, from irregular oval to polygonal, usually vertical; specific two-sized vessels and tracheids; libriform fibres and parenchyma difusse, or as short tangential uniseriate bands; two-sized rays, short uniseriate, sometimes as false broad rays, and real broad rays, compound, compact and aggregate.

This genus diagnosis was emended by Müller-Stoll & Mädel (1957), and again by Gros (1983, 1988), to comprise new specifications on the porous or ring-porous structure, on the usually solitary vessels simple perforated and with alternate vascular pitting, on the pitted parenchyma, banded or diffuse, on the libriform, tracheids and small vessels from groundmass and on the two-sized rays.

Another revision and emendation was made by Suzuki & Ohba (1990), they proposing to get out from the confusion with many other older validly published generic names of oak wood-structures as Kloedenia, Quercinium, Quercites , Quercoxylon , by sending them in synonymy, and replacing them with the Linnaean name Quercus , but this proposal not respecting the current ICBN rules was not followed by any paleoxylologist (see Selmeier, 1997). They also emmended the genus Lithocarpoxylon Petrescu, 1978 which was invalidated by Selmeier (1997).

Reviewing many extant described species of “white oak”, Privé-Gill (1975) conclude that there is a big intraspecific variation. This is true for the entire genus Quercus L., which shows a great interspecific anatomic homogeneity and in such a situation, it is very difficult to delimitate a fossil species and, often, any xylotomic observation has only a descriptive value for the studied specimen ( Privé-Gill, 1975).

Selmeier (1996) observed also that vessel diameter, ray size, ray frequency and ray distribution are variable, and even in the same described fossil species the biometric values may differ. For this reason, in the xylotomical description of extant wood we find no actualized measurements, and any accurate measurements made by paleoxylologists cannot be always useful to identify an unknown. Much more, the rules from " IAWA List of Microscopic Features for Hardwood" (Wheeler at al., 1989) impose different ways to express the older measurements. A lot of other oak fossil woods described from the European Miocene were usually identified with “white oak-type ”, as Quercoxylon bavaricum Selmeier, 1971 , a perfect equivalent of the extant Quercus robur L. which belong to Quercus section.

This species described by Selmeier (1971) have a correspondent within Carpathian area in Quercoxylon sarmaticum Starostin & Trelea, 1969 which seems to have priority, defining the same type of wood which was found again by Petrescu (1976), Lupu (1984), Iamandei et al. (2001, 2011).

The xylotomical analysis of here studied specimen shows that it should belong to "Roteichen" type, from Erytrobalanus sectio, since they present in cross section, ring porous structure and small, round, thick-walled vessels in late wood, with radial or tending to dendritic pattern distribution of the vessels, or to already porous even if they may have many variations, as some other authors previously have been shown (see Selmeier, 1971; Privé, 1975).

At a first sight, the here studied wood structure seems to correspond to the extant Quercus borealis L. (in Schweingruber, 1990), known now as synonym of extant Quercus rubra L., which belong to the red oak group ( Quercus , section Lobatae ), living now in North America, in the northeastern United States and southeast of Canada, locally named “Northern Red Oak” or “Champion Oak”. Also, similarities of the structure of our specimen with the extant Quercus suber (cork oak) and with the fossil Quercoxylon compactum (Starostin & Trelea, 1969) were observed.

However, the cross-fields with vessels show in a homocellular ray structure numerous quadrangular rounded vertically aranged bordered pits, in palissade, very similar to extant species Quercus frainetto Ten. (see Schweingruber, 1990), known as Hungarian Oak or Italian Oak (synonyms: Quercus conferta Kit. , Quercus farnetto Ten.) classified in Quercus sect. Mesobalanus , and is a species of oak native to Southeastern Europe and Turkey.

A similar specimen was described by us, from the Middle Miocene from Bala area, Mehedinţi County, Romania ( Iamandei et al., 2011). In these circumstances we prefer to give to the here studied specimen the name Quercoxylon sp. cf. Quercus frainetto Ten., and to search for a better fossilized specimen, or specimens, which could support the description maybe of a new species.

Figures

Fig. 6 a-i (graphic scale, μm). Quercoxylon sp. cf. Quercus frainetto Ten., Ociu, Bisericii brook, South Apuseni, Romania, sample OB-3. a-c. (cross section) – indistinct boundary of growth ring, ring-porous wood structure, two-sized solitary thick-walled vessels, round late-wood vessels two-sized rays. d-f. (tangential section) – two-sized rays, fine and multiseriate, short vascular elements, parenchyma cells. g-i. (radial section) – bordered pits with horizontal apertures on vessels, simple perforation plates, rectangular pits "in palisade" in cross fields

Tables

Table 1 Key of identification of Fagaceous wood (after Petrescu, 1976, slightly modified)

	Vessels, perforations	Rays	Extant taxa	Valid fossil taxa
Type A	Structures with solitary vessels or grouped (2-3 or more), with simple and scalariform perforations on vessels	Fine rays (1-3-seriate) and compact multiseriate.	Fagus Nothofagus	Fagoxylon, Nothofagoxylon
Type B		Exclusively uniseriate rays	Castanea Castanopsis	Castanoxylon
	Structures with solitary vessels, simple perforated, sometimes some scalariform, with few bars	Uniseriate and compact multiseriate rays, sometimes compact-composed or, partially aggregate rays Uniseriate and multiseriate aggregate rays	Quercus Lithocarpus	Quercoxylon