Pinuxylon pineoides (Kraus) Koeniguer, 1967

IAMANDEI, STĂNILĂ, IAMANDEI, EUGENIA, VELITZELOS, DIMITRIOS & VELITZELOS, EVANGELOS, 2022, Palaeoxylotomical Studies In The Cenozoic Petrified Forests Of Greece. Part Two - Conifers, Acta Palaeontologica Romaniae 18 (1), pp. 65-111 : 89-93

Pinuxylon pineoides (Kraus) Koeniguer, 1967

Fig. 9 , a-i.

Material

From all the studied samples of petrified wood collected from the Aegean area, some of them presented a pinaceous xylostructure, marked by the presence of axial resin canals of Pinus type. From them, 11 samples were grouped here as presenting a very similar xylotomy. They were collected from Lesbos Island - the samples numbered with Lsv 334, 349, 365, 371, 380, 445, 451, 537, 558, 578 and from Limnos island - the sample Li.228. The studied material is registered as “Velitzelos Collection” and stored in the Collection of the Faculty of Geology and Geoenvironment , of the NKUA .

Microscopic description

Growth rings – are relatively wide, of 20-40 cells or more, show gradual transition or sometimes quite abrupt and distinct ring-boundaries in cross section. The early-wood is more or less developed, with relatively thin-walled tracheids, sometimes compressed and deformed, usually gradually diminishing in size up to the late-wood. Axial resin canals of pinoid type are present, with thin-walled epithelial cells sometimes destroyed, detailed described below.

Tracheids – have a polygonal cross section, with rounded corners and relatively thin walls in the early-wood, of 3-5 μm the double wall, and the radial / tangential diameter is of 25-45 / 15-35 μm. The transitional wood is represented by thicker walled polygonal rounded tracheids, of 6-7 μm the double wall and with diameters of 15-25 / 10-15 μm (r/tg d.). The final wood which is represented by 6-8 rows of radially flattened thick-walled cells, having 8-10 μm the double wall, and smaller sized of 10-15/5-10 μm (r/tg d.). Between two succesive rays, 2-11 radial rows of tracheids can be counted. Also, intermingled between them, radial rows of tracheids with narrower-lumened tracheids can be n, having smaller tangential diameter. Angular intercellular spaces between tracheids also appear. The density is 1150-1640 tracheids per mm 2. The tracheidal pitting appears usually poorly preserved. On the radial walls, abietineous pits of 16-20 μm in diameter appear, with round apertures of 5-7 μm, predominantly uniseriate arranged, less biseriate, spaced or contiguous and without crassulae. Sometimes, on the tangential walls, small round to oval pits of (5-)8-12 μm in diameter appear, with small round aperture, spaced or contiguous and arranged in a single row. Organic deposits as resin inside the tracheids sometimes are present. Tracheid length could not be measured, due to poor preservation. The late-wood tracheid are thick walled. Helical thickenings absent, but sometimes inclined striations can be guessed.

Axial parenchyma – is absent.

Rays – appear in tangential view as uniseriate and multiseriate fusiform rays, with 1-2(3) resin canals sometimes with remains of epitelial cells. The uniseriate rays have of 2-15(-20) cells in height, so the average height is of medium type and have sometimes some biseriate storeys. The fusiform rays bear 1-2(3) resin canals, are of 2-3 cells wide and of 12-20 cells in height, having long uniseriate endings of 3-6 cells. Lateral intercellular triangular spaces are common. The ray-density is of 12-18 rays on tangential millimeter. In radial view the rays appear heterogeneous, with parenchymal ray-cells of 17-20 μm tall, and transversal ray-tracheids which appear either inside the ray-body or marginally arranged. Often, they are difficult to separate from the parenchymal body-cells, since

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they have smooth walls also, or slightly rough, maybe with some thickenings suggesting small denticles. The parenchymal ray-cell walls are smooth or slightly rough and unpitted, not too thick (2.5-3.5 μm double wall), while the tangential walls (or end-walls) are relatively thin, of 1.5-2 μm the simple wall, or slightly nodular, but usually poorly preserved. The ray-tracheids are slightly taller, up to 25-35 μm or more, show wavy and rough outer wall, and are devoid of spiral thickenings. The normal cross fields show poorly preserved pitting as 1-2(-3) pinoid pits, often with taxodioid to cupresoid aspect of 8- 10 μm with large inclined lens- like apertures, arranged in a single row, or as 4-6 pits in two superposed rows or slightly irregular. The pits on the ray-tracheids are slightly smaller and more rounded that the normal ones, but usually poorly preserved. Indentures absent.

Radial resin canals – are present and appear intercellularly inside the ray-body, as 1-2(3) canals of 15-30 μm in diameter, often with resin grains inside and are lined by thin epitelial cells, most often destroyed.

Axial intercellular resin canals – are in circular or radially-oval shape, are surrounded by thin-walled epithelial parenchyma cells sometimes with small thickenings and are usually destroyed. The canals appear isolated or grouped in the growth ring, sometimes in the transitional wood, usually in the late wood but not close to the ring boundary. In cross section the canals are often narrow, having (30-) 100-150 (-200) μm tangential diameter. Traumatic canals are absent.

Mineral inclusions – are absent.

Affinities and discussions

In the first stage of the palaeoxylotomical study of a part of “Velitzelos Collection” there were selected the specimens showing coniferous structure with typical axial resin canals lined by thin-walled epithelial cells, that clearly indicate secondary wood of Pinus type. Even if often present poorly preserved, 11 specimens of them were grouped and studied here as having very similar xylotomy.

Thus, the studied petrified wood samples show a homoxylous structure in cross section, with typical normal axial resin canals not too large, lined by thin-walled epithelial cells usually destroyed, and appear isolated or grouped in the transitional wood and late-wood, but sometimes not only there, so defining a pinaceous secondary wood of Pinus type. This wood-type is different from other pinaceous genera, which have a very specific number of epithelial thick-walled cells lining the resin canals: Pseudotsuga has 5-7 cells, Picea has 8-10 cells and Larix has 10-14 cells, but they are thin-walled in Pinus , and often destroyed (Greguss, 1955, p.121-123; Ickert-Bond, 2001).

Also, the presence of ray-tracheids having rough or slightly dentate walls is observed, an important taxonomic detail which leads to the pines of Diploxylon - type, the hard pines, classified under subgenus Pinus L. (Gernandt et al., 2005). This important detail distinguishes them from the pines of Haploxylon - type, the soft pines, classified under the subgenus Strobus Lemmon (Gernandt et al., 2005; Mantzouka et al., 2019a), which are characterized by smooth-walled ray-tracheids. The cross-fields show fenestriform or pinoid pits, which appear more or less numerous in both types of pines, so this detail could be of specific taxonomic value. Further details regarding the characteristics of the Pinus wood type derive from the papers of Bailey (1910), Phillips (1941), Jane (1956), Greguss (1955, 1962), Hudson (1960), Esau (1965), Patel (1971), van der Burgh (1973), Panshin & De Zeeuw (1980), Süss (1989), Ickert-Bond (2001), Schoch et al. (2004), Farjon (2005b).

As a principle, we suppose that the extant European pines had mostly European Cenozoic ancestors and we enumerate here all the current European (many of them Mediterranean) pines of Diploxylon type (subgenus Pinus ): Pinus halepensis , P. brutia , P. pinaster , P. pinea , P. canariensis , P. nigra , P. sylvestris , P. heildrichii , P. uncinata and of Haploxylon type (subgenus Strobus ): Pinus peuce and P. cembra (Gernandt et al., 2005; Earle, 2020). In this context it was interesting to consult the paper of Barbero et al. (1998) who present the ecological status of the few current species of Pinus and their biogeography in the Mediterranean Basin.

As for the xylotomy of the European species, they have usually 1-2(3-6) pinoid (or piceoid) pits in cross fields, superposed or paired or in other arrangement, the radial traheidal pitting shows 1(-2)-seriate pits, and the parenchyma is usually absent or appear only exceptionally (Greguss, 1955, 1972; Dolezych et al., 2011).

The fossil genus Pinuxylon was defined by Gothan (1905), with Pinuxylon succiniferum Goeppert (Kräusel) as type-species which was emended later (Dolezych et al., 2011) after revising the original material of the bazionim Pinites succinifer Goeppert (in Goeppert & Berendt, 1845), which was identified as a pine of Haploxylon - type. During the time, a large number of European species were referred to this fossil genus (Dolezych et al., 2011).

In order to identify the studied specimens, we have comparatively analyzed the European fossil pines described and published till now, specifying, when possible, their current equivalents, trying to find similarities with our studied material:

• Kräusel (1920) described Pinuxylon taedioides Kräusel and P. paxii Kräusel as pines of Diploxylon - type and, some years later, also Kräusel (1949) described a P. zobelianum (Göppert) Kräusel a fossil pine of Haploxylon - type, from cembra-group, giving also a synthesis of all the previous described forms, reported as fossil equivalents of the extant genus Pinus L., so, m to be different from our specimens.

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• Rössler (1937) described the species Pinuxylon cembraeforme Rössler , similar to extant Pinus cembra - Strobus subsection (= of Haploxylon type); also, he described Pinuxylon vateri (Platen) Rössler which has similarities with the current Pinus taeda L., or with P. banksiana Lamb. , and Pinuxylon paxii Kräusel - similar to Pinus pinaster Aiton , as pines of Dipoxylon - type, all from Germany (also, by van der Burgh, 1973). All of them m to be different from our specimens.

• Some forms of Cenozoic fossil pines with taxodioid or round pits in cross fields, were described by Greguss (1954, 1967) as Pinuxylon haploxyloides Greguss , and also P. albicauloides Greguss , P. tarnocziense (Tuzson) Greguss and Pinuxylon sp. , most probably all of them of Diploxylon type. The presence of spiral thickenings in P. albicauloides Greguss , described also by Roy & Hills (1972), m to represent only shrinkage fissures due to diagenesis, as Dolezych & Reinhardt (2020) observed. Anyway, there is no similarity with our specimens.

• Huard (1966, 1967) has described Pinuxylon arjuzanxianum Huard and P. landensis Huard both with pine wood of Diploxylon type and similar structure of pines from Pinaster subsection, but are different.

• Lecointre & Koeniguer (1965) and Koeniguer (1967) described from Maroc a Pinuxylon pineoides (Kraus) Koeniguer , emmending a pine initially described by Kraus (1886), from Sicily, as species of Pityoxylon . This species has xylotomical details similar to the extant Pinus pinea L. and is also very similar to our here studied specimens, in regard to axial resin canals and cross field pitting.

• Some other pines characterized by small pinoid and even fenestriform pits in cross fields were identified only as Pinuxylon sp. by Petrescu & Nuţu (1970) or by Comble et al. (1973); they are, probably, of Diploxylon type, but are not very similar to our studied specimens.Also, van der Burgh (1973) described two species also slightly different: Pinuxylon ponderosoides van der Burgh and P. halepensoides van der Burgh , both of Dipoxylon - type, similar to the current species of Pinaster subsection, based on the similarities of the aspect of the resin canals with their thin-walled epithelial cells. The last has a very similar wood structure with the current Pinus halepensis Miller (=Aleppo pine) in regard to the radial biseriate pits on tracheids and the ray-cells with nodular end-walls and pitted horizontal walls. Both species are different from our studied specimens.

• Privé-Gill & Watelet (1980) described a Pinuxylon sp. , quite similar to the P. parryoides (Gothan) Kräusel emend. van der Burgh , of Haploxylon - type, so it is different from our specimens.

• Iamandei & Iamandei (2000) have described a species of pine of Haploxylon - type – Pinuxylon marinasii Iamandei et Iamandei – quite similar to P. parryoides (Gothan) Kräusel emend. van der Burgh , having large

92 resin canals, radial uniseriate pitting on tracheids, sometimes irregularly paired or already biseriate, spaced or contiguous, and small taxodioid pits, therefore different from our specimens.

• By the study of some fossil wood remains collected from Miocene volcano-sedimentary sediments of Lemnos and Lesbos, Süss & Velitzelos (1993, 1994, 2009) have described some species of Pinoxylon Knowlton emend. Read. But this is a Mesozoic genus of “protopinaceous” type, with internal wood structure of Pinustype, without fusiform rays with radial canals, as it is specified in the original diagnosis of Knowlton (1900, p.420). The genus Pinoxylon was considered by Medlyn & Tidwell (1979) and by Stewart & Rotwell (1993) as equivalent to Protopiceoxylon Gothan , so this taxonomic identifications of Süss & Velitzelos remained problematic regarding the equivalence to an extant genus. Therfore, the authors preferred to create a new genus, Lesbosoxylon Süss et Velitzelos 2010 , describing a new species and designating as type-species Lesbosoxylon ventricosuradiatum Süss et Velitzelos 2010 , instead of Pinoxylon , described as having “idioblast- cells” in rays, and reattributing to this genus all the species described before, as follows: Lesbosoxylon parenchymatosum - with parenchyma; L. paradoxum - having spiral thickenings on tracheids; L. pseudoparadoxum - having very high rays; L. diversiradiatum - with 1-2-seriate rays; and L. graciliradiatum - having also “idioblast- cells” in rays (Süss & Velitzelos, 2010). Recently, a new species was described by Akkemik et al. (2020) as Lesbosoxylon kemaliyensis Akkemik & Mantzouka , having a Pinus wood structure, with typical axial and radial resin canals, heterogeneous rays with smooth-walled ray-tracheids ( Haploxylon type?), and pinoid cross fields. However, we consider the genus Lesbosoxylon Süss et Velitzelos as identical with Pinuxylon Gothan , so that a revision of the original material of all these species is still required. Anyway, all these species are different from our studied specimens.

• Petrescu & Bican-Brişan (2004) described a Pinuxylon sp. , studying a charcoalified material collected from the salt-mine of Ocna Dej ( Romania). It has a pine structure, with resin canals, cross fields with 2-3 small pinoid pits in horizontal row, and ray tracheids which suggest Diploxylon - type of pine, slightly different of our material.

• Iamandei et al. (2011, 2016) also described Carpathian pines of Diploxylon type, characterized by large normal axial resin canals with moderately thick-walled epithelial cells, which was assigned to Pinuxylon sp. cf. Pinus sylvestris L., slightly different of our studied specimens.

• Dolezych et al. (2011) redescribing the original material on which Goeppert (1883) has described Pinites succinifer Goepp. (transfered by Gothan, 1906 - to Pinuxylon , as nov. comb.) and later confirmed by Kräusel as type-species for the genus Pinuxylon ( Kräusel 1919, p.248-249; 1949, p.135), but without giving a diagnosis for it. Thus, after the original material of the bazionim was restudied, a diagnosis was offered by Dolezych (in Dolezych et al., 2011) for this type-species, correctly named Pinuxylon succiniferum (Goeppert) Kräusel emend. Dolezych , identified it as a pine of Haploxylon - type, combining features of pines from subgenus Strobus , with pinoid and ooporoid pits in cross fields, ray cell walls smooth or with thikenings and with thin-walled epithelial cells lining the axial resin canals. It is clearly different from our specimens.

• More recently Dolezych & Reinhardt (2020) described Pinuxylon selmeierianum Dolezych & Reinhardt from the Paleogene of northern Ellesmere Island, Canada, but anatomically similar to the current species Pinus lambertiana , an American pine of Haploxylon - type, also, different from our specimens.

• Mantzouka et al. (2019) described from Alonissos island, Greece, a new fossil species named Pinuxylon alonissianum Mantzouka et Sakala , of Diploxylon - type, considered as similar to some current species of Pinus especialy from Central America as Pinus arizonica Engelmann ex Rothrock, P. arizonica var. cooperi (C.E. Blanco) Farjon , P. engelmannii Carrière , P. jeffreyi Greville et Balfour , so, are different from our specimens.

• Recently, Akkemik et al. (2021) have identified also in the Anatolian area a Pinuxylon cf. P. tarnocziense (Tuzśon) Greguss , having subsidiary parenchyma cells around the resin canals, so, it is different of our specimens.

After this comparative analysis, we consider that the xylotomy of our studied specimens resembles well the structure of the current Pinus pin ea L., as is described and figured by Greguss (1955), Schweingruber (1990), Akkemik & Yaman (2012), and Crivellaro & Schweingruber (2013). Pinus pinea L., known as the ‘Stone pine’, or ‘Umbrella pine’ is native to the Mediterranean region, occurring in Southern Europe, and Middle East (Earle, 2020). Xylotomically, the secondary wood of this pine has growth rings with gradual to abrupt transition from earlywood to latewood and axial resin canals of Pinus - type, with thin epithelial cells partially destroyed and usually located in the late wood. The tracheids have rectangular rounded cross-section in earlywood, smaller and radially flattened in latewood and, on the radial walls has uniseriate pits. The axial parenchyma is absent. The rays are uniseriate and low, of 1-8 cells, and fusiform, of 2-3 cells wide, and up to 15 cells tall, with 1-2(3) resin canals lined by thin-walled epithelial cells. The end walls of ray parenchyma cells are nodular, the horizontal walls pitted. Cross-fields with 1-2 cupressoid pits (as vertical pair) or more numerous, in 2-3 rows in the marginal cross-fields of ray-tracheids (heterogeneous rays). The ray tracheids have usually smooth walls or slightly rough, suggesting tiny denticles. The xylotomy of our studied specimens shows relatively wide growth rings, with gradual transition, sometimes quite abrupt, and distinct ring-boundaries. The normal resin canals are relatively narrow, and are located sometimes in the transitional wood, but usually in the late-wood, having thin walledepithelial cells, sometimes destroyed. The tracheids have polygonal-rounded cross section, not too large, with relatively thin-walls in the early wood, thicker in late-wood. The radial pits are predominantly in uniseriate arrangement, less biseriate, and without crassulae. Sometimes, tangential smaller pits appear. Parenchyma is absent. The rays, of medium height, are uniseriate and 2-3-seriate and fusiform, with long uniseriate endings and have 1-3 resin canals. The cells from the ray-body have the end-walls relatively thin or slightly nodular, and the ray-tracheids have smooth or rough wavy outer walls, rarely with some thickenings suggesting small denticles, and no spiral thickenings. Cross fields with 1-3 pinoid pits of 8-10 μm with cupressoid to taxodioid aspect, with inclined lens-like apertures, horizontally arranged, in a single row, or 4-6 pits in two superposed rows. Sometime, organic deposits appear inside of tracheids.

All these details, synthetically described for the studied specimens, suggest a pine of Diploxylon - type, classified under subgenus Pinus L. (Gernandt et al., 2005) and are similar to those described to the extant Pinus pinea L., as well as to those of the fossil species as it was described by Koeniguer (1967), so we decided to assign them, to the species Pinuxylon pineoides (Kraus) Koeniguer, 1967 .

Figures

Fig. 9 a-i. (graphic scale). Pinuxylon pineoides (Kraus) Koeniguer, 1967; (Specimen Lsv.365). a-c: Cross section - gradual transition, distinct growth rings; parenchyma absent; resin canals of pinoid type, with thin walledepithelial cells, partially destroyed; d-f: Tangential section - tracheids, uniseriate rays, axial canal (4), fusiform ray with ray-canal (5-6); g-i: Radial section - tracheidal pitting 1-2-seriate, opposite (8); rays homocellular, cells all procumbent, ray-tracheids weakly dentate; poorly preserved cross-field with 1-3(4) pinoid pits.