Brachyoxylon holbavicum Iamandei, Iamandei and Grădinaru, 2018

Iamandei, Stanila, Iamandei, Eugenia & Grădinaru, Eugen, 2022, Contributions To The Study Of The Early Jurassic Petrified Forest Of Holbav And Cristian Areas (Brașov Region, South Carpathians, Romania), 2 Part., Acta Palaeontologica Romaniae 18 (1), pp. 25-48 : 31-33

Brachyoxylon holbavicum Iamandei, Iamandei and Grădinaru, 2018

Fig. 3 , a-i.

Material

The studied material is represented by 7 samples of silicified wood collected from the Holbav locality, on Maiului brook. All the studied samples are fragments of trunk or thick branches, with decimetric size and dark to black colored. By hand lens or even by naked eye, the regular fibrous structure without vessels suggests a conifer wood. The specimens, with the following field numbers: 1089, 1090, 1093, 1094, 1096, 1102, 1105, belong to "Grădinaru Collection", and are stored at the National Museum of Geology , in Bucharest, under the inventory numbers: 27711, 27712, 27713, 27714, 27715, 27716, 27718 .

Microscopic description

Growth rings – well developed, with tracheidoxylic structure, abrupt or gradual transition, distinct boundaries, no resin canals.

Tracheids – with polygonal cross section, usually quadrangular with rather thick and slightly corrugated walls in the early-wood, diameters of 20-40(60) μm and wall thickness of 5-7(8) μm (double wall), quite similar in the late-wood. Between two successive rays, 1-12 radial regular rows tracheids appear and intercellular spaces are present. Density is 836-1520 tracheids per square mm. On tangential walls the pitting is usually absent. The radial pits are of mixed type, either uniseriate, with round pits, spaced or contiguous and variably flattened (flattening index: d/D=0.79-0.88), or biseriate, with round to oval pits, of 19-24 μm the diameter, with round to oblique-elliptic apertures of 7-10/3-5 μm, in opposite or alternate arrangement, sometimes contiguous, combined with short uniseriate portions. There are no crassulae or helical thickenings, but sometimes striations are present.

Axial parenchyma – is absent.

Rays – in cross-section appear thin, with linear trajectory, from rectangular cells radially elongated, with smooth unpitted horizontal walls. Tangentially they appear exclusively uniseriate, with 2-17(-25) cells in height, sometimes more. And, sometimes, the taller rays have 1-2 short biseriate storeys of the same thickness as a single cell. Lateral empty spaces commonly appear. Ray-density is of 7-12 rays per tangential horizontal millimeter. The rays are homogeneous, with parenchymal cells all procumbent, of 20-24 μm in height, in the marginal rows taller, of 28-35(-50) μm. The horizontal walls are moderately thick walled, 4-6 μm for the double wall (sometimes thicker), and the outer wall of the marginals is slightly corrugated. Tangential wall is relatively thick. Indentures are not visible. The cross-fields have 1-6 cupressoid pits, tending to podocarpoid, usually hexagonal-rounded to oval, with diameters of 9-15(18) / 6-8(10) μm and with tilted elliptic apertures of 5-8 / 3-5 μm. In normal fields the pits arrangement is 1-3(-4) pits in horizontal row and, in the taller rays, 4-6(-9) pits in 1-2(3) horizontal rows, alternately or slightly irregularly arranged.

Radial or axial resin canals – are absent.

Mineral inclusions – are not present.

Affinities and discussion

All the 7 samples of fossil wood studied here present the next xylotomy: typical tracheidoxylic structure with thick-walled tracheids, having radial pitting of mixed type, with rays mostly uniseriate showing cupressoid cross-fields (tending to araucarioid). All these xylotomical details are consistent with those of the genus Brachyoxylon Hollick and Jeffrey.

This genus was created by Hollick and Jeffrey (1909), having as type species Brachyoxylon notabile Hollick et Jeffrey , with a very elliptic diagnosis: “tracheidoxyl devoid of normal secretory ducts, with radial pitting of mixed type, araucarioid cross-fields and other ray walls, integer” (see Philippe, 1993; Philipe and Bamford, 2008). The occasionally presence of traumatic canals is considered as a result of injury, or of freezing (see Phillipe, 1995).

In order to identify the specimens studied here at species level, we evaluated other fossil forms previously described. We noted that some species are no longer considered to be Brachyoxylon species, or are doubtful, so they should be excluded from discussion (see Iamandei et al., 2018).

There are many forms of Brachyoxylon described showing a large spread of this Mesozoic genus on the planet and we have quoted and discussed, in detail, some of them (in Iamandei et al., 2018). They were described from Japan ( Iijima et al., 1989), from Mexico ( Cevallos-Ferriz, 1992), from USA ( Hickey et al., 2011), from Argentina ( Vera and Césari, 2012; Bodnar et al., 2013), all of them having various specific xylotomical details, partially similar to our material.

But the European species Brachyoxylon saurinii described by Boureau and Serra (1961), B. avramii and B. dobrogiacum described by Iamandei and Iamandei (2005), B. serrae described by Phillipe et al. (2011) and, especially B. holbavicum , described by us from the same area ( Iamandei et al., 2018) are closer to our studied specimens. And, related to this, the debate on Mesozoic palaeoenvironment of large European forests made by Garcia et al. (1998) is very interesting and, also, the complex biogeographic analysis of Gondwanian Mesozoic forests of Philippe et al. (2004).

In summary, taking into account the comparative table (see Table 1, in Iamandei et al., 2018) and the synthetic description of the specimens studied here (mixed radial pitting on tracheids, axial parenchyma absent, uniseriate rays medium-tall, cross-fields with cupressoid pits, tending to podocarpoid, in quassi-araucarioid arrangement), we consider our samples to perfectly fit to the diagnosis of Brachyoxylon holbavicum Iamandei, Iamandei and Grădinaru , a species previously identified in the same area (see Iamandei et al. 2018).

Figures

Fig. 3 Brachyoxylon holbavicum Iamandei, Iamandei and Grădinaru, 2018. (inventory no. 27711). a-c: Cross section - tracheids polygonal with rounded corners, and linear rays; d-f: Tangential section - pitting on tracheids - absent; rays uniseriate; g-i: Radial section – radial pitting of mixed type on tracheids, pits hexagonal and, when biseriate, alternate; cross-fields with 2-3(4) cupressoid to araucarioid in 1 row or, when more numerous, in 2-3 superposed rows.