Brachyoxylon holbavicum, Iamandei & Iamandei & Grădinaru, 2018

Iamandei, Stănilă, Iamandei, Eugenia & Grădinaru, Eugen, 2018, Contributions To The Study Of The Early Jurassic Petrified Forest Of Holbav And Cristian Areas (Brașov Region, South Carpathians, Romania). 1 Part, Acta Palaeontologica Romaniae 14 (2), pp. 3-34 : 12-17

Genus Brachyoxylon Hollick & Jeffrey, 1909 Brachyoxylon holbavicum sp. nov.

Fig. 5 a-i; Fig. 6 a-i.

The studied material

The studied material is represented by 24 decimetric-sized samples of silicified wood found around Holbav locality area (South BraȘov County), on Maiului brook, in the Early Jurassic continental pyroclastic deposits considered of Pliensbachian age. All the studied samples are trunk or thick-branch fragments, with decimetric size, dark to black color and by magnifying glass or even by naked eye, regular fibrous structure without vessels suggesting a conifer wood. The specimens with field numbers: 1006, 1007, 1008, 1009, 1010, 1012, 1013, 1014, 1021, 1022, 1024, 1025, 1026, 1028, 1030, 1031, 1033, 1034, 1035, 1039, 1044, 1045, 1046, 1047 - belong to "Grădinaru Collection", are deposited now in the collections of the Geological Institute of Romania ( G. I. R. Collections), under the inventory numbers 27612 - 27635 respectively, at the National Geological Museum, in Bucharest.

Microscopic description

In a synthetic description, all the studied specimens present s e c o n d a r y w o o d with tracheidoxylic structure devoid of normal resin canals, with a slight gradual diminishing of tracheids' size from the early to the late wood (very obvious in the specimens 1012, 1033), with relatively less distinct up to clearly distinct growth rings boundaries, marked by few rows of smaller flattened tracheids of the late wood. However in a specimen (i.e. 1010), an isolated canal rounded by a parenchyma cells was observed in cross section, most probably of traumatic origin and keeping inside some granular dark content. In some other specimens the thinner walled early and transitional wood is slightly compressed, or crushed or even collapsed, like in the charcoal (in the specimens: 1012, 1013, 1014, 1026, 1028). Sometimes, few details can be seen due to bad preservation during fossilization processes leading to vitrinite (as in the specimen 1029 which was excluded from study since it has no any structure kept, or only partially in the specimen 1031). Only in the sample 1044 appear the axial zone with primary wood that in cross section shows also arthropods galleries with coprolites, visible especially in the longitudinal sections.

T r a c h e i d s with polygonal, mainly quadrangular cross section with rounded corners are displaying quadrangular slightly rounded lumina, or with curled walls often giving a "star-like" aspect of the lumina, often with discontinued walls due to the fossilization processes that often lead to deformed lumina or all of them are crushed by lateral compression (like in the specimen 1047). In a normal structure the tracheids, unequal in size, i.e. gradually diminishing from the early to the late wood, with radial/tangential diameters of 25-50 (60)/20- 40(60) μm and with thinner walls (3.5)5-7 μm (double wall) in the early wood and thicker in transitional wood, of 7-9 μm, to very thick in the late wood, of 10-13 μm the double wall. Between two successive rays there are bundles usually of 1-12 radial regular rows of normal tracheids, sometimes with obvious inter-cellular spaces (in the specimen 1008, 1034), and the density is of (550)836- 1520 tracheids per square millimeter, even if sometimes is difficult to appreciate their number or to count them. Tangentially the tracheidal walls are usually unpitted, only sometimes seem to be uniseriately pitted with round or slightly flattened pits (smaller than on the radial walls), of 14-17 μm in diameters, contiguous or slightly spaced (in the specimens: 1026, 1030, 1047). The radial pitting is of mixed type, both uniseriate, with round pits, spaced or contiguous in a single vertical row and variably flattened (flattening index: d/D=0.56-0.79-0.88), or, (less) biseriate, round to oval, opposite to/or alternate, sometimes contiguous, combined with short uniseriate portions (in 1012). Generally, the pitted areas don't cover integrally the tracheidal wall. Also, when biseriate, round to oval pits appear, opposite or alternate, sometimes contiguous, combined with short uniseriate portions (in the specimens 1008, 1010). The pits have round to slightly oval borders of 19-24 μm in diameter to horizontal elliptic shape (when flattened), of 15.5-22 μm in diameters, and with round to oblique-elliptic apertures of 7-10/(2)3-5 μm. There are no crassulae or helical thickenings. In the specimen 1034 the tracheids seems to show striations on the walls. Rarely (in specimen 1010), a solitary isolated canal was seen, rounded by epithelial parenchyma cells, probably of unknown traumatic origin, situation observed only in cross section. Inside the canal, some granular dark content appear.

A x i a l p a r e n c h y m a - is absent.

M e d u l l a r y r a y s - in cross section seen are thin, have a linear trajectory and are constituted from rectangular cells radially elongated, with smooth unpitted horizontal walls. Tangentially the rays are exclusively uniseriate, having 1-25 cells in height, or more, i.e. 25-600(650) μm. Sometimes the taller rays have 1-2 short biseriate storeys of the same thickness that one cell, more numerous in specimen 1046. The ray-cells are polygonal-rounded to oval (in height), usually having relatively not too thick walls and, sometimes, obvious lateral empty spaces (in specimens 1007, 1013, 1014, 1024, 1026, 1031, 1034). The ray-density is of 7-12 rays per tangential horizontal millimeter. In radial section the rays are homocellular, constituted by cells all procumbent of 20-24 μm in height. In the marginal rows, the cells are taller, of 28-35(- 50) μm. The ray cells have moderately thick horizontal walls, of 4-6.2 μm the double wall, sometimes more, and the outer wall of the marginals is slightly curled. The tangential wall is thick and there are no visible indentures. The typical araucarioid cross-fields have 1-6 oculipores of cupressoid, tending to podocarpoid, usually hexagonal-rounded to oval, with diameters of of 9-15(18) / 6- 8(10) μm, with tilted elliptic apertures of 5-8 / 3-5 μm. The pits arrangement is usually alternate as 1-6(9) pits on 1-2(3) horizontal rows in the body ray cells and 2-9 pits on 2-3 horizontal rows, alternate or slightly irregular, in the marginal fields

Affinities and discussions

All the here studied 24 samples of fossil wood present tracheidoxylic structure and mostly uniseriate rays, pitting of mixed type on the radial tracheidal walls and araucarioid cross-fields, xylotomical details that send to Brachyoxylon Hollick et Jeffrey , a type of Mesozoic tracheidoxyl.

The type species for this genus, Brachyoxylon notabile Hollick et Jeffrey, 1909 has also radial tracheidal pitting of mixed type and araucarioid cross-fields. The presence of traumatic resin canals is considered as a result of injury (see table 1). Otherwise, in a synthesis of Taylor & Taylor (1993, p. 689) is noted that no "cheirolepidiaceous" wood has normal resin canals but traumatic canals can be present sometimes in this genus, as a result of injury or freezing (see also Phillipe, 1995).

So, the description of our studied structures agrees with the diagnosis of the genus Brachyoxylon Hollick et Jeffrey , even if the original diagnosis, after the designated generotype ( B. notabile Hollick et Jeffrey, 1909 ) is very elliptic, defining "a tracheidoxyl devoid of normal secretory ducts, with mixed radial pitting, araucarioid cross-fields and other ray walls integer" (see Philippe, 1993; Philipe & Bamford, 2008).

To do a comparison with fossil forms already described and published, it is important to remind that some species are no more considered as Brachyoxylon species, so must be excluded from a discussion, as for example:

- Brachyoxylon nipponicum Nishida , that according Vozenin-Serra & Pons (1990) is synonymous with Protopodocarpoxylon orientale Serra, 1969 (see also Machhour & Pons, 1992);

- Brachyoxylon brachyphylloides (Torrey) Kräusel , according Philippe (1993) is in fact a species of Telephragmoxylon : T. brachyphylloides (Torrey) Philippe ;

- Brachyoxylon notabile Hollick et Jeffrey, 1909 , B. woodworthianum Torrey, 1923 , B. libermanii Philippe, 1995 preserve also primary structure and pith and such structures were described as Telephragmoxylon (by Torrey, 1921) or as Brachyoxylon sp. (by Holden, 1913a; Shimakura, 1937), having mostly uniseriate radial pitting and low rays (according to Machhour & Pons, 1992);

- Brachyphyllum desnoyerisii as initially was described by Lemoigne (1968) was renamed as Agathoxylon desnoyersii (Lemoigne) Philippe, 1995 , and now considered as a junior taxonomical syonym (Philippe et al., 2018).

- Also, Brachyoxylon urkutense Greguss according Philippe & Barbacka (1997) it is synonymous with Simplicioxylon hungaricum Andreánszky, 1949 .

Two older described taxa from North America as Paracedroxylon scituatense Sinnot and Voltzioxylon dokumense Torrey were sent into synonymy with Brachyoxylon , as B. scituatense (Sinnot) Philippe, 1993 and B. dokumense (Torrey) Philippe, 1993 - respectively, having typical characters of this genus.

Anyway, we tried to describe more accurately the very specific features of this big group of specimens (24) which could define a new species. All these characters were introduced into a comparative table of other described species of Brachyoxylon ( Table 1), taken from Bodnar et al. (2013), and we observed that our described material is slightly different, by the rare apparition of traumatic canals, the absence of the axial parenchyma, and the presence of the exclusively uniseriate rays, relatively tall, of up to 25 cells.

There are numerous already described forms of Brachyoxylon , from different regions of the Earth, with various very specific details. However, we observed that B. saurinii of Boureau & Serra (1961), B. avramii and B. dobrogiacum of Iamandei & Iamandei (2005) and B. serrae of Phillipe et al. (2011) have high rays of more than 20 cels and uniseriate radial pitting, quite similar to our studied material. The both species described by Iamandei & Iamandei (2005) from Romania are branch fragments preserving also primary wood, like the specimen 1044 studied here which also shows arthropods galleries with coprolites, but the characters of the secondary wood are not similar.

Many other forms of Brachyoxylon were described in the world, showing a large spread of this Mesozoic genus on the planet, and we only quote some of them:

- Iijima et al. (1989) described a species of Brachyoxylon , from Far East , from the uppermost Triassic - Lower Jurassic , in the Adoyama Chert from Kuzuh, Japan, by the study of a drifted piece of wood with this type of structure .

- Cevallos-Ferriz (1992) described some other species of Brachyoxylon from the Upper Cretaceous from north of Mexic.

- Philippe et al. (2004a) cited above, made a complex biogeographic analysis of Jurassic–early Cretaceous wo- od assemblages, Brachyoxylon included, from Gondwana.

- Hickey et al. (2011) described a typical Brachyoxylon from the Upper Triasssic from Connecticut, USA.

- Also, Vera & Césari (2012) have described five specimens of Aptian fossil woods from Argentina, identified as Brachyoxylon sp. cf. Brachyoxylon boureaui Serra, 1966 characterized by radial mixed pitting as predominantly uniseriate rows of circular to polygonal bordered pits. The biseriate and triseriate pitting pattern is also present, but is less common. The cross-field regions show 8-26 small circular pits arranged in 2-4 rows, with slit-like pit apertures, obliquely oriented, in rays exclusively uniseriate, and relatively low.

- Bodnar et al. (2013) described a Brachyoxylon currumilii from the lower-middle Jurassic of Argentina and made a comparative table of already published species of Brachyoxylon ( Table 1), in which we added the xylotomical characters of our here studied material, for the comparison of the essential xylotomical features.

- Garcia et al. (1998) made a debate on the Jurassic palaeoenvironment of large European forests of Agathoxylon and Brachyoxylon described on the France territory.

- More recently, Philippe et al. (2018) reidentified from "Lignier collection" a sample initially identified by Lignier as Cedroxylon blevilense , as new combination: Brachyoxylon blevillense (Lignier) Philippe et al., 2018 - based on the mixed type of radial pitting and the araucarioid cross-fields. Also they identified the “Échantillon n°150” as having typical araucarioid cross- fields, with up to six crowded oculipores and resiniferous axial parenchyma (traumatic?), similar to Brachyoxylon liebermannii Philippe , a species previously described by Philippe (1995), from the Liassic of northeastern France.

So, considering the xylotomic matches of our studied material and the already published species from the comparative table (see Table 1) and the comparison with the original description of other species, we concluded that our material (which presents a secondary wood structure usually devoid of resin canals, even if rarely of traumatic type can appear, with mixed radial pitting, with axial parenchyma usually, with uniseriate rays or partially biseriate, relatively high, up to 25 cells and and cross-fields with oculipores of cupressoid type, tending to podocarpoid, in an araucarioid arrangement) has a structure of Brachyoxylon of a special type.

Here, in our paper, other very similar form of Brachyoxylon was described (i.e. B. cristianicum ), slightly similar and coming almost from the same areal (see Table 1). So, taking into account these observations and discussions, we consider we have described here a new species of Brachyoxylon , that we named Brachyoxylon holbavicum sp. nov., after the name of the locality area (Holbav) from where it was found.

From the studied material it was designated as holotype the specimen with inventory number: 27614 (field no. 1008) and a paratype with inventory number: 27635 (field no. 1039) from a population of 24 specimens of fossil wood from "Grădinaru Collection"with similar xylotomic characters to almost identical. The mentioned types and all the studied material are now deposited in G. I. R. Collection, in the National Geological Museum in Bucharest ,

Diagnosis of the new species Brachyoxylon holbavicum sp. nov.:

The secondary wood of tracheidoxylic type has distinct growth rings, sometimes with rare, isolated, traumatic canals, thick-walled tracheids, polygonal in cross-section gradually diminishing in size to the late-wood. Tangentially the tracheids are usually unpitted, rarely with smaller pits contiguous or slightly spaced. The radial pitting is of mixed type, usually uniseriate with round pits, spaced or contiguous and variably flattened and, less, biseriate, with round to oval, opposite to alternate, sometimes contiguous pits, combined with short uniseriate portions. Pits with round to oblique-elliptic apertures. Axial parenchyma is absent. Rays exclusively are uniseriate, tall, with biseriate storeys. Rays exclusively uniseriate, tall, with biseriate storeys, with lateral empty spaces and relatively thick walls. Homocellular, with curled outer wall, and without indentures. Cross-fields typical araucarioid, 1-6(9) oculipores hexagonal-rounded to oval of cupressoid type tending to podocarpoid in 1-3 horizontal rows arranged, alternate to slightly irregular, in the marginal fields on 2-3 horizontal rows.

Figures

Fig. 5 Brachyoxylon holbavicum sp. nov. - holotype - inventory no. 27614 (field no. 1008). a-c Cross-section: distinct growth-ring boundary, thick-walled tracheids polygonal with rounded corners; linear rays. d-f Tangential section: unpitted tracheids, parenchyma, uniseriate rays. g-i Radial section: radial piting of mixed type on tracheids, araucarioid cross fields with 2-3 superposed rows of cupressoid pits.

Fig. 6 Brachyoxylon holbavicum sp. nov. - paratype - inventory no. 27635 (field no. 1039). a-c Cross-section: quite distinct growth-ring boundary, thick-walled tracheids polygonal with rounded corners, and linear rays. d-f Tangential section: unpitted tracheids, uniseriate rays with biseriate storeys. g-i Radial section: radial piting on tracheids of mixed type, araucarioid cross fields with 2-3 superposed rows of cupressoid pits.