Brachyoxylon cristianicum, 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 : 17-21

Brachyoxylon cristianicum sp. nov.

Fig. 7 a-i; Fig. 8 a-i.

The studied material

The studied material is represented by 8 pieces of silicified wood, four of them found in the Cristian area, on the Fabricii valley, and four in the Holbav area, on the Maiului brook, in the Lower Jurassic continental deposits with pyroclastic character considered of Sinemurian- Pliensbachian age, most probably Pliensbachian. All the studied samples are trunk or thick-branch fragments, having decimetric size, dark to black color and, by magnifying glass or even by naked eye, regular fibrous structure without vessels can be seen, clearly suggesting a conifer wood. The specimens having field-numbers: 1015, 1016Cr, 1018Cr, 1019Cr, 1020Cr, 1027, 1042, 1043 belong to "Grădinaru Collection", and are deposited now in the collections of the Geological Institute of Romania ( G. I. R. Collections), under the inventory numbers: 27636 –27643 respectively, at the National Geological Museum, in Bucharest.

Microscopic description

All the eight studied specimens present s e c o n d a r y w o o d with tracheidoxylic structure with quite distinct growth-rings boundary marked by the late wood which has smaller tracheids than the early wood. The structure is devoid of any normal or traumatic resin canals.

T r a c h e i d s - in cross section viewed are polygonal with slightly rounded corners, sometimes determining intercellular spaces (specimens 1020, 1027), unequal in size, with quadrangular to rounded large lumina with radial/tangential diameters of 20-42(50-60)/20-35-(45-65) μm (smaller in the late wood), and with relatively not too thick walls: (4)6-9 μm the double wall in the early and in the transitional wood, up to 9-13 μm the double wall in the late wood. The interradial bundles have 1-10(12) radial regular rows of tracheids and density is 992- 1444(1518) tracheids per mm2. Tangentially seen the tracheids are usually unpitted, or rarely pitted (in specimen 1019) with small round pits (d=8-12 μm) with small apertures (d=4-5 μm), in a single row arranged, slightly irregular, locally as two opposite pits. The radial pitting is of mixed type, with almost abietinean pits, arranged either in a single vertical row as round pits, spaced, or contiguous separated by a horizontal line, with round or slightly elliptic apertures, or biseriate, round, alternate or opposite, contiguous on short rows often continued by uniseriate portions, not occupying all the length of the wall. In fact, generally, the pitted areas don't cover integrally the tracheidal wall. The pits have round borders of 18-22 often flattened when contiguous (d/D=0.65-0.88), and round or tilted to lying elliptic apertures of 7.5-10/2-3 μm (in specimen 1019). None crassulae or helical thickenings have been seen, but sometimes a brown content is present .

A x i a l p a r e n c h y m a - in cross-section view is present, few, dispersed, often difficult to identify amongst the tracheids. It has relatively thin vertical walls relatively similar to those of the tracheids and sometimes with simple pits, irregularly arranged on 1-2 rows. The horizontal (terminal) walls are thin and smooth or slightly nodular and, inside the cells, resin content or plugs or granules, and sometimes small polygonal crystals are present (specimens 1015, 1019).

M e d u l l a r y r a y s - in cross section appear usually uniseriate, rectilinear, and with smooth horizontal cellular walls. In tangential view the rays have 1-20 (sometimes up to 25) cells in height, i.e. 30-570(650) μm high. The taller uniseriate rays have biseriations, sometimes giving aspect of fusiform rays (specimens 1015, 1016). The ray cells are polygonal rounded to oval sometimes having dark content and relatively thin walls. Ray density is 5-11 rays per horizontal tangential millimeter. Radially the rays are homocellular, cells all procumbent of 20-30 μm tall, moderately thin walled, of 4-5(7) μm the double wall. Within the marginal rows the cells are taller, of 25- 35(40) μm, and have corrugated outer wall. The crossfields of araucarioid type have 1-6 oculipores, hexagonal or slightly rounded to oval, of cupressoid type, tending to podocarpoid, with 15-19/8-12 μm in diameters, having circular or short elliptic tilted apertures of 10-14/5-7 μm. The pits arrangement is in 1-2 rows in the ray body fields, in the marginal fields on two horizontal rows, always alternate to slightly irregular when on two rows .

Affinities and discussions

After the microscopic study of each sample and the synthetic evaluation of their generic characters, a number of 8 samples of fossil wood were grouped all of them having tracheidoxylic structure, uniseriate rays, radial pitting of mixed type on the tracheids and araucarioid cross-fields, suggesting a Brachyoxylon type of wood slightly differebt of the above described form (see Table 1).

The discussion on the status of the genus Brachyoxylon Hollick et Jeffrey , and of the species described under it along the time is presented in detail above. Even if the type species ( Brachyoxylon notabile Hollick et Jeffrey, 1909 ) has a very elliptic diagnosis, referring only to "a tracheidoxyl devoid of normal secretory ducts, with mixed radial pitting, araucarioid cross-fields and other ray walls integer" (see Philippe, 1993), it is interesting that traumatic canals were described, considered as a result of injury or freezing ( Taylor & Taylor, 1993, p. 689; Philippe, 1995). Anyway, the specimens described here are devoid of any kind of canal.

In our studied specimens, apart from the known characters of the genus, there are some specific xylotomic details regarding the size of tracheids or their behavior insi- de the growth ring, their pitting type, aspect and size of the pits on the tracheids and in the cross fields. The accurate description of these very specific features of the studied material can be useful for the comparison with the already published and valid species (see Table 1).

Anyway, it seems that the presence or the absence of the axial parenchyma and of the crassulae, and also the ray heights and the pits' number in the cross-fields, are important to separate the species of Brachyoxylon (Philippe, 1993, 1995).

- Two forms of Brachyoxylon preserving primary structure and pith were described from the Lower Cretaceous in southeastern of Romania as B.avramii and B. dobrogiacum (Iamandei & Iamandei, 2005) as having a tracheidoxylic structure with few parenchyma and uniseriate low rays, with mixed radial pitting and araucarioid cross-fields (sensu Philippe, 1993), bearing cupressoid to podocarpoid pits in an araucarioid arrangement, but are slightly different of the here studied material which has high rays with biseriate stories, even with aspect of biseriate fusiform rays.

- Also, Vera & Césari (2012) have described an interesting material represented by five specimens of Aptian fossil woods from Argentina and identified as Brachyoxylon sp. cf. Brachyoxylon boureaui Serra, 1966 having radial pitting of a mixed type, predominantly uniseriate, with circular to polygonal bordered pits, sometimes biseriate and triseriate, even if less common, and numerous pits in the cross-fields (8 up to 26 small circular pits arranged in 2-4 rows, with slit-like pit apertures, obliquely oriented), details which are very different of our specimens.

- The Jurassic new species of Brachyoxylon currumilii described by Bodnar et al. (2013) from Argentina, has also some details which differ of our material. But in that paper there is a very well documented discussion on the status of Brachyoxylon genus and its already described species, and a comparative table of the valid species of Brachyoxylon in which we added, for comparison, the essential xylotomical characters of our studied material ( Table 1).

- Interesting for comparison seems to us the more recently reidentification of Philippe et al. (2018) from "Lignier collection" of a sample initially identified as Cedroxylon blevilense by Lignier, as Brachyoxylon blevillense (Lignier) Philippe et al., 2018 based on the mixed type of radial pitting and the araucarioid cross-fields. Also the identification of the “Échantillon n°150” from the same collection, which showed typical araucarioid cross-fields with up to six crowded oculipores and traumatic(?) resiniferous axial parenchyma is similar to Brachyoxylon liebermannii Philippe, 1995 , a species previously described from the Liassic of northeastern France.

But, considering the xylotomic similarities and dissimilarities of our studied material with the already published species from this comparative table ( Table 1) and the comparison with the original description of the species, and also, the discussions from above, it can be observed that our material is special.

Thus, it presents a secondary wood structure without resin canals, with mixed radial pitting, uniseriate spaced or contiguous, or biseriate, alternate or opposite, contiguous, with round to elliptic apertures, with axial parenchyma, with uniseriate rays or even biseriate, relatively high, and cross fields with cupressoid oculipores in an araucarioid arrangement.

These xylotomical details are slightly different of the other species of Brachyoxylon described in this paper and, based on this diference well exprimed in the Table 1 from above, we considered we have described here a new species that we named Brachyoxylon cristianicum sp. nov., after the name of the region from where it was fo- und (Romanian spelling for Christian is Cristian). From the studied material a holotype was designated the specimen with field number 1019 in "Grădinaru Collection", (inventory number 27639), and a paratype the specimen with field number 1020 in "Grădinaru Collection (inventory number 27640), from a population of 8 specimens of fossil wood with similar to almost identical xylotomic, all material now deposited in GIR Collection, in the National Geological Museum, Bucharest, as we specified above.

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

Tracheidoxyl with distinct growth rings, devoid of any canal. Tangentially the tracheids are usually unpitted, rarely with small pits, uniseriate. Radial pitting of mixed type, of pits abietinean but with araucarian arrangement, uniseriate, spaced or contiguous, with round or slightly elliptic apertures, or biseriate, alternate or opposite, contiguous and flattened, in short rows often continued by uniseriate portions, not occupying all the length of the wall. Axial parenchyma present, dispersed, in longitudinal view presenting thin and smooth or slightly nodular horizontal walls and resin content or even crystals. Rays uniseriate, relatively tall, with biseriations, sometimes with fusiform aspect, homocellular, having typical araucarioid cross-fields with 1-6 oculipores alternate to slightly irregular on 1-2 rows, hexagonal ronded to oval, of cupressoid type tending to podocarpoid, with round or short elliptic tilted apertures.

Figures

Fig. 7 Brachyoxylon cristianicum sp. nov. - holotype - inventory no. 27639 (field no. 1019cr). a-c Cross-section: distinct growthring boundary, thick-walled tracheids, polygonal with rounded corners, and linear rays. d-f Tangential section: unpitted tracheids, parenchyma with pits and crystals; uniseriate rays. g-i Radial section: uniseriate radial piting on tracheids of mixed type, cross fields with 1-2 rows of oculipores.

Fig. 8 Brachyoxylon cristianicum sp. nov. - paratype - inventory no. 27640 (field no. 1020cr). a-c Cross-section: distinct growth-ring boundary, polygonal tracheids with rounded corners and thick walls; linear rays. d-f Tangential section: unpitted tracheids, parenchyma with pits and uniseriate rays. g-i Radial section: radial piting on tracheids of mixed type, uniseriate and biseriate, cross fields with 1-2 rows of oculipores.