Spiroplatanoxylon densiradiatum (Petrescu) Süss, 2007

Iamandei, Stanila, Iamandei, Eugenia & Stoia, Tiberiu, 2020, Fossil Woods From Bozovici (Sw Romania), Acta Palaeontologica Romaniae 16 (1), pp. 3-17 : 9-13

Spiroplatanoxylon densiradiatum (Petrescu) Süss, 2007

Fig. 6 , a-i.

Macroscopic description

A decimetric sized sample of petrified trunk ( Fig. 3 ), displaying whitish color, fibrous texture and regular growth rings, was submitted to palaeoxylotomical study. From it, thin sections standard oriented were realized and a microscopic study was performed on them. The studied material originates in Bozovici depression, coming from mid-Badenian sediments (upper part of Valea Slătinicului Member of Lăpușnicul Mare Formation), and is deposited in the National Geological Museum Collection ( NGM Col., Bucharest) under the number 27,622.

Microscopical description

The growth rings are clearly distinct and the wood structure is diffuse-porous to semi-ring-porous. The multiseriate rays present typical dilatations at ring boundaries.

The vessels in cross section seen appear as solitary pores or as tangential pairs or diagonal or in clusters. Thay have angular section, slightly rounded or even deformed, and have moderately thick walls, of 7-10 μm the double wall. The woody fascicles, may have tangentially, between two rays, 1-6 vessels. In the early-wood the vessels are meansized, their diameters ratio (r/tg) is of 40-95/30-63 μm. Their relatively uniform size is gradually diminishing, in the late-wood, to (20)30-40/(15)20-30 μm, so defining a diffuse-porous wood, tending to semi-ring-porous. The density of the vessels is 115–130 per square mm. In radial section, scalariform perforations can be seen, on tilted plates, with 18-24 thin bars, quite poorly preserved. The walls of the vessels present bordered pits, opposite, horizontaly elongated, to scalariform by apertures' coalescence, and are rather badly preserved. Helical thickenings are also present, visible especially in tangential section. The length of vessels elements is difficult to measure. No tyloses were observed inside the vessels. Sometimes clusters of vascular tracheids appear, with similar pits, like on vessels.

The wood parenchyma apears in cross section of apotracheal type, diffuse, and appear scattered cells among the fibres, or as uniseriate short lines. In longitudinal view appear as strands of rectangular parenchyma cells, empty or with crystals inside, probably with simple pits on walls, quite poorly preserved. Vessels’ elements length is difficult to measure.

The rays, in cross section appear two sized, show linear trajectory, and are formed by rectangular cells. The multiseriate rays have typical platanoid dilatations at growth ring boundaries. In tangential view they appear clearly twosized, of 1-4 and 8-12 cells wide, the multiseriate are fusiform and compact, have cell-sheath and are usually tall, having up to 30 cells in height, or more. The body ray cells are slightly polygonal, and unequally sized and relatively thick-walled, and the uniseriate ray-endings have 1-5 slightly polygonal cells. The ray-density is of 7- 12 rays on tangential horizontal mm. In radially view the rays are heterocellular, having procumbent body-cells and marginals square or taller. In cross-fields, poorly preserved glyptostroboid pits appear. Sometimes, secretory cells appear, often bearing crystals.

The fibres are angular in cross section, with rounded lumina and thick walls. Poorly preserved pits on the longitudinal walls were observed.

Affinities and discussions

After evaluating the microscopical features observed in our specimen regarding the diffuse-porous pattern of the pores in cross-section, typical dilated rays at growth-ring boundary ("platanoid dilatations"), typical pitting on vessels and on vascular tracheids and scalariform perforations and, also, the presence of helical thickenings, even poorly preserved, we found some generic similitudes with the platanaceous structure ( Greguss, 1969).

There are three fossil genera created for platanaceous fossil woods: Plataninium , Platanoxylon and Spiroplatanoxylon .

The genus Plataninium has a long history: it was created in 1842 by Unger (published in Endlicher, 1842) as a "pocket taxon" for platanoid broad-rayed fossil wood, not illustrated but with a very comprising diagnosis based on the type-species P. acerinum Unger , described on a fossil wood coming from an unknown horizon and locality (see Crawley, 1989). It was emended by Vater (1884), unifying the diagnoses of Phegonium (or Fegonium ) Unger, and Plataninium Unger . It was emended again by Felix (1894) and, later, by Page (1968), in order to comprise members of the families Fagaceae (Fagus type), Platanaceae ( Platanus type), Eupteleaceae ( Euptelea type) and Icacinaceae ( Citronella, Ottoschultzia types), for specimens difficult to be assigned. After few times its diagnosis was emended again by Brett (1972) who admitted it as morphogenus, attributing to it four species: Plataninium decipiens Brett, 1972 ; P. europeanum Prakash, Březinova et Bůžek, 1971 ; P. haydenii Felix, 1896 ; P. brettii Crawley, 1989 . Some years later, Süss and Müller-Stoll (1977) consider Plataninium an inutile, ambiguous name that has to be invalidated.

Therefore, this name was several times subject of revision or dispute, some species being redescribed as belonging to other genera, for exemple: Plataninium subaffine Vater, 1884 , that became Hedycaryoxylon subaffine (Vater) Süss, 1960 ; or Plataninium aceroides (Goeppert) Windisch, 1886 , that became Fagoxylon acerinum (Windisch) Süss, 1986 ; or have been replaced by names as Platanus , Platanoxylon or Spiroplatanoxylon (see Edwards and Jogmans, 1931; Süss, 1960, 1971, 1986, 2007; Süss and Müller-Stoll, 1977).

The name Plataninium is still used by some American, Asian and even European scientists, and considered as a primitive type. This supports the Bayley concept regarding the evolution of the xylotomic features (see Süss, 1960) from Plataninium , which has scalariform perforations, spiral thickenings, marked heterocellular rays, to Platanoxylon , as an evolved type, which has simple perforations, no spiral thickenings and homocellular rays ( Selmeier, 1996). Anyway, now Plataninium is usually described from sites with platanoid leaves from Cretaceous to Eocene of North America, Paleogene from Japan and even Upper Cenozoic in Europe (see Taylor & Taylor, 1993). We cite here some species described in the last decades under the genus name Plataninium Unger (in Endlicher) emend. Vater, 1884:

- P. obbruxellense Stockmans, 1936 , described from the Eocene of Belgium. Specimens from Apuseni Mountains described as Platanoxylon cf. obbruxelense (Stockmans) Petrescu, 1969 and by Petrescu and Lazăr (1970) as having a slightly disordered structure, small pores, scalariform perforation plates and relatively thin rays.

- P. californicum Page, 1968 and P. platanoides Page, 1968 described from Cretaceous deposits of California.

- P. decipiens Brett, 1972 , is also described by Crawley (1989), and by Poole et al. (2002) from similar Paleogene deposits (Eocene London Clay) of south-east England, and by Meijer (2000) from the Late Cretaceous of Belgium.

- P. brettii Crawley, 1989 , is described on a badly preserved specimen, from Paleocene of Great Britain - Inner Hebrides.

- P. piercei Wheeler, McClammer et LaPasha, 1995 , described from deposits of terminal Cretaceous to Paleocene of San Juan Basin, New Mexico, North America.

- P. jezoensis Takahashi et Suzuki, 2003 and P. ogasawarae Takahashi et Suzuki, 2003 , described from Cretaceus of Japan.

The morphogenus Platanoxylon Andreánszky, 1951 (ex Prakash et al. 1971) was also subject of revision and dispute, at least in the last time ( Süss & Müller-Stoll, 1977; Süss, 2007, with references). After all these, a clear diagnosis has to separate the three admitted genera of fossil plants. Hence, after the revision of Süss (2007), some already described species were assigned to this morphogenus ( Selmeier, 1996 considered it as organ-genus!) as it follows:

- Plataninium haydenii Felix, 1894 , P. pacificum Platen, 1908 and P. crystallophilum Platen, 1908 were revised to Platanoxylon haydenii (Felix) Süss et Müller-Stoll, 1977 , Platanoxylon pacificum (Platen) Süss et Müller-Stoll, 1977 , and respectively to Platanoxylon crystallo-philum (Platen) Süss et Müller-Stoll, 1977 .

- Moreover, six species were validated by Süss et Müller-Stoll (1977) as correctly attributed to Platanoxylon : P. andreanszkyi Süss et Müller-Stoll, 1977 , P. hungaricum Süss et Müller-Stoll, 1977 , P. palibacsii Süss et Müller-Stoll, 1977 , P. sarmaticum Süss et Müller-Stoll, 1977 , P. catenatum Süss et Müller-Stoll, 1977, also described by us in the South Apuseni Mountains (Iamandei & Iamandei, 2000), and P. cohenii (Schuster) Süss, 1980 , adding other generic identification as Platanoxylon sp. , described from Hungary, Czech Republic and Germany by Andreánszky (1952), as belonging to Platanoxylon , according to Selmeier (1996, 2001) and Gottwald (2000). Also, the specimen described by Petrescu & Vasiliu (1987) from Romania is a Platanoxylon .

After a special interest accorded to this type of fossil wood by Süss (2007, with references), a new taxon was created, „aus dem Tertiär von Europa und Vorderasie“, Spiroplatanoxylon Süss 2007 , which is characterized, beside the plane-like wood structure, by two aspects considered more primitive, but characteristic to the genus: the scalariform perforation plates and the helical thickenings on vessels. The type-species was chosen Plataninium porosum Felix, 1887 - which was redescribed and transferred to Spiroplatanoxylon porosum (Felix) Süss, 2007 as comb. nova, and two new species of Spiroplatanoxylon were described:

- S. gregussii Süss, 2007 , which has bigger vessels, mostly solitary and less frequent per square mm, very abundant tyloses, broad multiseriate rays and crystals in the axial parenchyma present as chambered cells or, rarely, as idioblasts.

- S. mueller-stollii Süss, 2007 , quite similar, with mostly solitary vessels, less frequent per square mm, very abundant tyloses, generally thinner rays, which can even be aggregated and also broad multiseriate rays and crystals in the axial parenchyma present as chambered cells or, rarely, as idioblasts; the same species was identified by Továrková et al. (2011) from Brno area, Czech Rep.

Also, several species of Icacinoxylon , Platanoxylon and Plataninium are reidentified and placed into the genus Spiroplatanoxylon . It is very of note that even Page (1968) also emended once the genus Plataninium , to comprise broader-rayed structures, including members of the Icacinaceae (equivalents of Citronella, Ottoschultzia ). - Plataninium europeanum Prakash, Březinová et Bůžek, 1971 found in northern Bohemia, was revised by Süss (2007) to Spiroplatanoxylon europeanum (Prakash, Březinová and Bůžek) Süss , having smaller vessels with scalariform perforated plates, spiral thickenings, crystals and low vessel frequency.

- Plataninium porosum Felix, 1887 , found in Apuseni Mountains, at Almaşu Mare, was revised to Spiroplatanoxylon porosum (Felix) Süss, 2007 , having spiral thickenings on vessels and long chains of crystalliferous parenchyma. The same species of Plataninium was described also by Petrescu and Nuţu (1970), from Boiu, South Apuseni Mountains, so it becomes Spiroplatanoxylon porosum (Felix) Süss, 2007 too.

- Platanoxylon bohemicum Prakash, Brezinova and Buzek, 1971 was revised to S. bohemicum (Prakash, Březinová and Bůžek) Süss, 2007 , having high scalariform perforation plates, abundant tyloses and shorter rays. However, more recently, Sakala et al. (2010) revisiting the original site from where Prakash et al. (1971) have described Platanoxylon bohemicum , Plataninium europeanum and Dryoxylon bohemicum , have made microscopical observations on new slides and confirmed the presence of spiral thickenings and rare prismatic crystals in axial parenchyma cells in Plataninium but not in Platanoxylon . So, they decided to use the denomination Spiroplatanoxylon only for the Plataninium type of wood in which are observed spiral thickening and crystals. As a precaution, they kept the three morphospecies as separate units, even if they thought they all may represent the same botanical species. Also, it is of interest that Greguss (1967, 1969) have described some specimens as Icacinoxylon sp. seu Platanoxylon sp. , or Icacinoxylon / Platanoxylon (see also Gryc & Sakala, 2010). In fact, 18 badly preserved specimens studied by Greguss (1967), were identified as Icacinoxylon sp. seu Platanoxylon sp. , having scalariform perforation plates, with 6-10, 18-20, or 30-35 bars, crystals inside vessels, parenchyma or rays, sometimes radial or axial canals, or typical platanoid aspect, and were, or must be attributed to Spiroplatanoxylon sp. :

- Andreánszky (1951) described Platanoxylon sp. with helical thickenings on vessels and scalariform perforations that could be Spiroplatanoxylon sp. ( Süss, 2007).

- S. citronelloides (Shilkina) Süss, 2007 , was described by Shilkina (1956) as a species of Icacinoxylon , and then, was found again and described by Greguss (1967) as having vessels high scalariform perforation plates with numerous bars (30–50), spiral thickenings abundant tyloses and shorter rays.

- Spiroplatanoxylon hortobagyii (Greguss) Süss, 2007 , (former Icacinoxylon ) has oval vessels in cross-section, and shorter rays, beside the generic features.

- Spiroplatanoxylon shilkinae (Greguss) Süss, 2007 , (former Icacinoxylon ) has vessels with high scalariform perforation plates with numerous bars, and lower rays (to 8 cells broad), which can even be aggregated.

- Spiroplatanoxylon platanoides (Greguss) Süss, 2007 , (former Icacinoxylon ) has heterocellular rays with several rows of upright marginal cells. The species of Greguss was also described from the Apuseni Mountains by Petrescu (1978) even if on badly preserved specimens.

- Spiroplatanoxylon densiradiatum (Petrescu) Süss, 2007 , (former Icacinoxylon ) has smaller angular vessels, solitary or in clusters, 75 per square mm., and scalariform perforation plates with 20-35 bars, spiral thickenings, two-sized rays with high ray-frequency and ray-cells with crystals.

- Spiroplatanoxylon densiporosum (Petrescu) Süss, 2007 , (former Icacinoxylon ) has small and numerous vessels that show scalariform perforation plates with more numerous bars and spiral thickenings, long chains of crystalliferous parenchyma, high rays and terminal fibrous band.

- Spiroplatanoxylon romanicum (Petrescu) Süss, 2007 , (former Icacinoxylon ) have rather numerous vessels (50- 70 per square mm) with high scalariform perforation plates with numerous bars (up to 30-35), spiral thickenings, secretory parenchyma cells, rays of 11-16 cells broad and very high.

Also, there are other described European species of Icacinoxylon that must be translated to Spiroplatanoxylon :

- Icacinoxylon crystallophorum Greguss, 1967 a species having high scalariform perforation plates, up to 55 bars and broad compact rays of 20 cells wide, found also by Petrescu (1978) and by Iamandei and Iamandei (1997) in the Apuseni Mountains, and must also be assigned to Spiroplatanoxylon cystallophorum (Greguss) Iamandei , comb. nova.

- Icacinoxylon sylvaticum (Tuzson) Greguss, 1969 , and I. aff. sylvaticum (Tuzson) Greguss, 1969 , described from Hungary and found again by Dupéron (1978) from the Stampien of Agenais, France, must become Spiroplatanoxylon sylvaticum (Tuzson) Iamandei , comb. nova, and respectively Spiroplatanoxylon aff. sylvaticum (Tuzson) Iamandei , comb. nova.

Discussing the attribution of a French wood to Icacinoxylon aff. sylvaticum (Tuzson) Greguss, 1969 , Duperon (1976) wrote: "la seul espèce actuelle présentant des eléments de vaisseaux spiralés est Citronella mucronata , et la plupart des bois décrits sous le nom d' Icacinoxylon possèdent des éléments de vaisseaux spiralés. Faut-il en conclure d'emblée que l'évolution de la famille tend vers la disparition de cet ornement? Il serait bien hardi de l'affirmer. Cependant, Metcalfe and Chalk (1950) constatent que les bois à perforations de vaisseaux scalariformes ont plus fréquemment des spirales que les bois à perforations de vaisseaux simples et à structures étagées. Les perforations scalariformes étant un caractère d'autant plus primitif que le nombre de barres est élevé, on est tenté de penser que la plupart des bois fossiles d'Icacinaceae sont à caractères plutot archaïques".

- Another species of Icacinoxylon described from Georgia as I. goderdzicum Shilkina, 1958 , has to be translated to Spiroplatanoxylon goderdzicum (Shilkina) Iamandei , comb. nova.

Some fossil woods described from the Romanian Carpathians and identified as Icacinoxylon both at species and genus level, must be discussed and correctly assigned to the valid accepted genus:

- Icacinoxylon tyleradiatum Iamandei et Iamandei, 1997 , described from the latest Cretaceous deposits of Apuseni Mountains, having vessels with scalariform perforation plates with numerous bars, 20 or more, tyloses, spiral thickenings on vascular tracheids, or on small vessels, secretory parenchyma cells with dark content, two-sized rays, fine of 2-3 and broad of 12-16, dissected, high heterogeneous and heterocellular, 1-5 marginal square, or upright ray-cells, with gum and crystals inside, must become Spiroplatanoxylon tyleradiatum (Iamandei et Iamandei) Iamandei , comb. nova.

- Icacinoxylon grambastfessardi Petrescu, 1972 (in Petrescu & Dragastan, 1972) from the Oligocene of Telega (Prahova county) must be redescribed as S piroplatanoxylon grambastfessardi (Petrescu) Iamandei , comb. nova, even if no spiral thickenings were observed, but high scalariform perforation plates, with up to 25 bars, and twosized rays 1-4 seriate and fusiform, of up 16-20 cells broad, ray-cells with crystals.

- Numerous other specimens identified as Icacinoxylon sp. , described from the late Miocene of Prăvăleni ( Petrescu & Nuţu, 1971, 1972), from the Oligocene from NW Transilvania ( Petrescu, 1978) and from the Oligocene of Carpathians’ Curvature ( Petrescu et al., 1989) on specimens quite badly preserved, having semi-ring-porous wood, often diffuse, not too large solitary vessels or grouped in 2-3, with scalariform perforations, two-sized rays, fine of 1-4 cells wide and of 6-(10-12)-18 cells broad, either compact or aggregate, or dissected, heterogeneous with polygonal ray-cells, unequally sized and with crystals and few other details. They have obvious platanoid wood structure, and must be also assigned to Spiroplatanoxylon , as valid taxon.

Since the wood structure of the studied specimen from Bozovici resembles to many described specimens of Spiroplatanoxylon , especially to the forms described by Petrescu & Dragastan, 1972) from the Oligocene of Telega and also by Petrescu and Nuţu (1972) from Prăvăleni, we tried to find very specific details to discriminate between them and we found a close identity with the species Spiroplatanoxylon densiradiatum (Petrescu) Süss, 2007 , by having diffuse-porous to semi-ring porous distribution of the polygonal and not to large vessels (under 100 μm in diameter), numerous two-sized rays, with platanoid dilation at ring boundary in cross-section, and scalariform perforations with 18-24-35 thin bars, with bordered pits, opposite, horizontally elongate to scalariform, with helical thickenings, with crystals in parenchyma, two-sized rays of 1-4 and 8-12 cells wide, high, fusiform, compact heterocellular, ray-cells with crystals, thick-walled fibres - angular in cross section. In these conditions, we assign our studied specimen to the species Spiroplatanoxylon densiradiatum (Petrescu) Süss, 2007 .

Figures

Fig. 6 a-i. (graphic scale, μm). Spiroplatanoxylon densiradiatum (Petrescu) Süss 2007, Bozovici, Romania. a-c. (cross section) – distinct boundary of growth ring, diffuse-porous wood structure, ray dilatations at boundaries. d-f. Tangential section – two-sized rays - fine and multiseriate (d), spiral thickenings on vessels (f). g-i. Radial section – badly preserved scalariform perforation plates (h), round simple pits in cross fields.

Fig. 3 Two of the studied samples (nos. 27621, 27622).