Rhysocaryoxylon madsenii Sakala et Gryc, 2011

Rhysocaryoxylon madsenii Sakala et Gryc, 2011

Fig. 12 View Fig , photos a-i.

Studied material. A similar special juglandaceous xylostructure of Juglans type was observed in 33 samples of petrified wood, collected from early Miocene volcano-sedimentary deposits, of Limnos and Lesbos Islands . They are registered and kept in the Collections of the Faculty of Geol. & Geoenviron. of NKUA, under these field numbers: Li192, Li277; and from Lesvos Island : Lsv10, Lsv11, Lsv12, Lsv15, Lsv 19, Lsv25, Lsv49, Lsv58, Lsv62, Lsv69, Lsv76, Lsv79, Lsv81, Lsv84, Lsv86, Lsv94, Lsv102, Lsv104, Lsv110, Lsv350, Lsv382, Lsv383, Lsv384, Lsv396, Lsv397, Lsv416? Lsv423, Lsv430, Lsv446, Lsv452 Lsv544 ?.

Microscopic description. Growth rings – present, with quite indistinct ring boundaries, poorly marked by a few rows of smaller and flattened cells of ground tissue as final wood, which are suddenly followed by larger normal cells of the early wood, where larger vessels also appear. However, in some specimens, such a sudden passage from late-wood to early-wood is often poorly preserved and so, the ring boundary is less distinct.

The vessels – usually thick-walled, are so arranged that define a wood structure with porosity sometimes of diffuse porous type, but most usually of semi-ring porous type, since larger vessels appear in the early wood, gradually diminishing in the late wood. Between two successive rays they appear in radial pattern arranged, but in all the structure with multiple neighbor fascicles, a diagonal pattern is visible. The vessels appear chiefly solitary (60-90%) and in radial multiples of 2-3(-5) vessels. The large solitary vessels have the lumina size of 60-150 / 45-90 μm the radial / tangential diameter, diminishing to 15-50 / 10-36 μm in the late- wood. Their shape is round to radial oval, often more or less deformed, and has a wall thickness of 7-10 μm the double wall. In the longitudinal view, the vessels show exclusively simple perforation plates. The intervessel pits - are polygonal of bordered type and have a contiguous, alternate arrangement, sometimes opposite to subopposite. The pits are polygonal and mean-sized, of around 10-12 µm in diameter. The vessel-ray pits are similar to intervessel pits, having much reduced borders. The cross-field pits, usually difficult to observe due to poor preservation, are described below. Thin helical thickenings in vessel elements sometimes appear (Lsv15, Lsv62). Tyloses inside the vessels' lumina rarely were seen but, usually, a blurry brown to dark content is present. The mean tangential diameter of vessel lumina is around 70 µm. Vessels' density is between 20 - 40 vessels/mm 2. The mean vessel element length is between 300-800 µm.

Tracheids – vascular fibres or vasicentric tracheids were not observed.

The fibers – represent the major part of ground tissue, and the vertical walls seem to have small bordered pits. Sometimes, they are septate.

The axial parenchyma – in cross-section, appears paratracheal few, but usually apotracheal banded, visible as long tangential bands 1-3-seriate, in a reticulate arrangement. In the longitudinal view, the parenchyma appears as rows of vertical rectangular cells. Some parenchyma cells are chambered and crystalliferous, with a single big prismatic crystal in each chamber, visible in the longitudinal sections as long vertical chains of 8- 11(15) successive chambers.

The rays – are present as fine rays, usually 1-3 seriate, low to high. The analysis of ray-cellular composition defines rays of heterocellular type, since the body ray cells are all procumbent, with 1(-2) rows of upright and/or square marginal cells, which sometimes have polygonal-rounded big crystals inside and also, dark content. In cross-fields with vessels, small quadrangular to elliptic horizontal elongate simple pits, more numerous in the marginal cross fields, as superposed pairs are often poorly visible because of the dark content or of numerous crystals, or simply, of bad preservation. Sheath cells or tile cells, in rays, are not present. The ray density is between 4-12 rays/mm.

Storied structures – absent. Secretory elements – as oil/mucilage cells absent. Intercellular canals – absent. Cambial variants – as included phloem, absent. Mineral inclusions – prismatic crystals appear in chambered axial parenchyma cells often as long series and this is a special diagnostic character: the presence of big crystals in enlarged chambered cells, in long chains of 8-11 or more large crystals. Also, small prismatic crystals or crystal sand could appear in ray cells, in upright and / or square ray cells, but also in procumbent ray cells.

Affinities and discussions. From the studied petrified wood remains to show juglandaceous structure we remarked some of them with similar features, showing a diffuse porous structure in cross-section, and the axial parenchyma which appears in cross-section as long tangential bands 1-3-seriate, in a reticulate arrangement and longitudinal view, as chambered and crystalliferous parenchyma, with long vertical chains of 8-11 hypertrophied chambers with a singular crystal inside. Other details, like the size and arrangement of the vessels, in cross-section, with exclusively simple perforations and numerous alternate bordered pits, in vertical view, are typical for the juglandaceous wood, as is presented in Greguss (1954), Dupéron (1988), Schweingrüber (1990), Schoch et al. (2004), Wheeler et al. (2011) and Akkemik & Yaman (2012).

Thin helical thickenings in the vessel elements, sometimes, are present. Taking into account the discussions at the above-identified species, especially on the specific xylotomical details of the main extant juglandaceous genera, as summarized by Dupéron (1988), who discussed the presence or absence of the crystalliferous parenchyma, as well as the thickness of the vascular wall, we observe the possible similitude of the xylotomy of our specimens with the current species of Juglans L., most probably of Black Tropical Walnut type, which have quite thick-walled vessels and present variations of the crystalliferous parenchyma that could appear as long vertical chains of chambers (more than 5), bearing solitary crystals and apotracheal parenchyma as long tangential bands (see Duperon, 1988), similar to our fossil specimens .

Blokhina (2007), doing a new revision of the knowledge on Juglandaceous xylotomy and palaeoxylotomy, presented some questions on taxonomy, evolution and phylogeny of this group and gave a key of identification of wood anatomy of modern and fossil Juglandaceae . Using this key, but also the other information from the paper, it is clear that „a wood structure with diffuse or semi-ring-porous structure, crystals in axial parenchyma and thin to thick-walled vessels“ define a Rhysocaryoxylon structure, completed by „solitary vessels or in radial multiples, vessel-ray and vesselparenchyma pits with significantly reduced borders and large apertures, apotracheal parenchyma in bands of 1– 2(4) cells wide, and rays are 1-3(5) - seriate, homocellular to slightly heterocellular, with or without crystals“.

Originally, juglandaceous fossil wood was described as Juglandinium schenki Felix, 1884 , revised later as Caryojuglandoxylon schenkii (Felix) Müller-Stoll & Mädel, 1960 . Later, it was revised again as Rhysocaryoxylon schenkii (Felix) Dupéron, 1988 , and it was designated as a type-species for the genus Rhysocaryoxylon Dupéron, 1988 .

But, in 2011, describing a new species of Rhysocaryoxylon, Sakala & Gryc (2011) , suggested that is necessary to be prepared a proposal for TAXON (the Journal of the IAPT), to conserve the name Rhysocaryoxylon against Caryojuglandoxylon and this, because of the presence in these xylostructures of crystalliferous idioblasts similar to Carya type, but more numerous, and because of the presence of smaller vessels and narrower rays tending to be uniseriate in the majority, features that do not correspond to the accepted diagnosis based on the type-species Rhysocaryoxylon schenkii (Felix) Dupéron.

However, using the generic key for juglandaceous fossil wood of Dupéron (1988), we observe that the affinity of our studied specimens with Rhysocaryoxylon genus is valid, especially by the aspect of the crystalliferous axial parenchyma in cross sections and the longitudinal ones. Thus, Dupéron (1988) characterized this genus like this: „porous to semi- ring-porous structure with thick-walled vessels, solitary or in small multiples, having simple perforations and intervascular pitting polygonal, alternately arranged; by banded apotracheal parenchyma, as 1-2(4) seriate bands, long, regular, paratracheal less abundant and, in longitudinal view obviously crystalliferous appearing as vertical chains of cells a little bit bigger than the ordinary ones; also, by rays 1-3(5)- seriate and heterogeneous and septate pith“, features that appear in our here studied specimens too.

We compared the described features of our specimens with other European species already described as: Rhysocaryoxylon schenkii (Felix) Dupéron, 1988 ; R. triebelii (Caspary) Dupéron, 1988 ; R. fryxellii (Prakash & Barghoorn) Dupéron, 1988 ; R. caucasicum (Gaivoronsky) Dupéron, 1988 ; R. tertiarum (Prakash & Barghoorn) Dupéron, 1988 ; Rhysocaryoxylon pilinyense (Greguss) Dupéron, 1988 ; R. pravalense Iamandei & Iamandei, 2002 ; R. ocii Iamandei & Iamandei, 2002 ; R. transylvanicum Iamandei & Iamandei, 2003 (and in Iamandei et al., 2013), and R. madsenii Sakala et Gryc, 2011 , also described by us from Rhodopes, Bulgaria (Iamandei et al., 2016). Almost all these cited taxa were considered to represent fossil equivalents of Black Tropical Walnuts, having parenchyma with vertical chains that are longer than 5 successive crystalliferous chambers, like in our specimens. Only R. transylvanicum seems to be similar to J. nigra , a species of Black Temperate Walnuts type (Iamandei & Iamandei, 2003; Iamandei et al., 2013), having short vertical chains of no more than 5 chambers with solitary crystals, so this type is different of the here studied specimens.

However we found a very close similarity, up to identity with the species described by Sakala & Gryc (2011) from Czech Rep., and by us from Rhodopes Mts., as having semi-ring-porous structure, indistinct ring boundary, thick-walled vessels, with simple perforations and alternate pitting; banded and reticulate, typically crystalliferous parenchyma, with long vertical chains of more than 5 barrel-like cells with large, solitary crystals; rays 1-3(5)-seriate, heterocellular rays with 1-4 marginal crystalliferous cells. Accordingly, based on the similarities listed above, we assign our specimens described here, to the species Rhysocaryoxylon madsenii Sakala et Gryc, 2011 , representing a fossil correspondent of the Black Tropical Walnut type.