Genus Abietopitys Kräusel, in Kräusel and Range, 1928
Type species. Abietopitys perforata Kräusel, in Kräusel and Range, 1928.
Abietopitys petriellae (Brea and Césari) nov. comb.
Figures 2.A-L
Basionym. Phyllocladopitys petriella Brea and Césari, 1995
Holotype. BAPb 244 (with slides 244a to 244m).
Other material. BAPb 12905 to 12909 (with their slides).
Emended diagnosis. Parenchymatous homocellular pith. Primary xylem prominent to the pith, adjacent to the secondary xylem with mesarch protoxylem. Small leaf traces diverge as single bundles. Secondary xylem and tracheids with bordered pits exclusively in radial walls, usually biseriate and with an alternate arrangement. Rays homocellular, mostly uniseriate with a medium average height and with thickenings on the cell walls. Cross-field pits with usually 1 to 5 circular simple pits. Growth ring boundaries distinct.
Description. Because of the similar characteristics of the specimens here described and those of the original specimen, they are regarded as a single taxonomic unit. The maximum width of the specimens is 14 cm and the maximum length is 25 cm, in a secondary xylem fragment (BAPb 12909). The wood is pycnoxylic and resembles that of gymnosperms with tracheids aligned in radial rows (figure 2.G). Only two of the five new specimens have the pith preserved (figure 2.A). The phloem and cortex were not preserved, and there were only two leaf traces observed in one specimen (BAPbPm 449).
The woods present distinct growth ring boundaries (figure 2.B). Ring width varies between specimens from 1.5 to 5 mm. However, the ring widths in the same specimen do not vary very much. The transition from earlywood to latewood is gradual.
The pith is cylindrical, not septated, and has a diameter of 2 cm in the specimen BAPb 12906; minerals usually replace the contact zone with the xylem. In the specimen BAPb 12907 the diameter is 2.5 cm and presents moderate lobes. It is homocellular and composed of polygonal to circular parenchymatic cells, with a diameter in transverse section of 121 µm (55-75 µm). There are intercellular spaces between cells. The cell wall is thin, less than 4 µm of width (figure 2.D).
The primary xylem differentiation is mesarch; individual bundles are prominent into the pith and adjacent to the secondary xylem (figure 2.F). There are up to 11 rows of metaxylem tracheids with a radial diameter of 33 µm (25-45 µm) showing scalariform pits. This is followed by 2 or 3 rows of transition tracheids with similar radial diameter and opposite two-three seriate pits (figure 2.E). Four primary xylem bundles were observed in specimen BAPb 12906, while BAPb 12907, that has a slightly bigger diameter, presents eight. Both specimens have the pith and primary xylem very well preserved.
The secondary xylem is composed of tracheids of rectangular shape with a radial diameter of 30 µm (22-45 µm) and a tangential diameter of 28 µm (17-40 µm) (figure 2.G). Bordered pits are present exclusively in the radial walls. The apertures are elliptical to circular with a diameter of ca . 10 µm. The tracheid pitting is usually biseriate and is arranged in an alternate pattern (figure 2.I). Uniseriate or triseriate pitting is also present. Pitting is always continuous and fills completely the radial wall.
One specimen (BAPb 12906) has an anomalous growth having tracheids in a radial longitudinal view in transverse section (figure 2.C). The anomaly is 9 mm long by 2 mm wide mith centripetal development, and it starts in the second ring (figure 2.C).
The rays are homocellular, exclusively uniseriate and have a medium average height within 1 to 19 cells tall (figure 2.H). There are 4 (3-6) rays per mm. The ray cells are radially very long, which precludes the observation of their endings; when these are discerned, they are oblique (figure 2.K). The ray cells have peculiar thickenings; in the radial walls they have regularly spaced protrusions arising from the upper and lower walls showing a nodular appearance (figures 2.K, 2.L). In the tangential (figure 2.H) and horizontal walls (figure 2.G) the thickenings extend from wall to wall.
The cross-fields have 1 to 5 simple pits, with a mean diameter of ca . 10 µm (figure 2.J) and apparently without any arrangement.
Comparisons
Kräusel, in Kräusel and Range (1928) described the morphospecies of fossil wood Abietopitys perforata from the Permian of the Karoo Supergroup. Later, Kräusel (1956) and Bangert and Bamford (2001) found new specimens of this species, but they did not add details and only reported its presence in a different locality. Archangelsky (1960) added two new species to the genus from the Permian of Chubut, Argentina, A. patagonica and A. crassiradiata . He assigned these two new species to the genus based on the particular thickenings of the ray cell walls. However, following Lepekhina (1972) proposal, these two species should be assigned to a new morphogenus of exclusively secondary xylem of the Abietopitys type, which Lepekhina named ?Type A?. Lepekhina proposed different morphogenera to include those specimens with secondary xylem with pith and primary xylem, and those without it. Her reasoning was that is not unusual to find specimens with the same secondary xylem and different primary xylem or pith and vice versa.
The most particular diagnostic feature of this genus is the presence of thickenings in the ray cell walls, and pits in the tangential ray cell walls (Kräusel, in Kräusel and Range, 1928) which distinguish it from other morphogenus of secondary xylem such as Protophyllocladoxylon Kraüsel with short ray cell walls with smooth walls. The thickenings in the tangential ray cell walls were interpreted by Kräusel (in Kräusel and Range, 1928) as pits, but Archangelsky (1960) suggests that they do not reach such grade of specialization. These thickenings could be localized thickenings of the primary wall, as in the transverse walls of axial parenchyma in some Cupressaceae (Richter et al ., 2004), and not due to pitting. However, the ray cells are not the typical parenchymatic ray cells, and apparently they have some degree of specialization (Archangelsky, 1960). The thickenings on the radial wall with a nodular appearance, slightly resemble the dentate wall of ray tracheids, which are common in some extant conifers of the genus Pinus , among other genera. In the transverse walls, where the thickenings extend from wall to wall, they resemble the ray tracheids of some Callixylon Zalessky (see Chitaley and Cai, 2001, plate I, figures 7 and 8), a Devonian and Carboniferous progymnosperm wood. In the tangential walls the thickenings also extend from wall to wall. Whether they are pits or just thickenings of the tangential walls is difficult to determine. Kräusel (in Kräusel and Range, 1928) compared these thickenings to the pits of the tangential walls of the ray elements of the extant Pinaceae genera Abies and Larix . Based on the similarities he found with Abies , he named the fossil specimens Abietopitys . However, this could lead to confusion because Abies has opposite pitting arrangement on the radial walls of the tracheids and Abietopitys has an alternate arrangement.
The comparison of this new combination with the other species of Abietopitys (table 1) allows to conclude that this is a new species for this genus. The most similar is Abietopitys perforata but the woods described here have different ray cell height, different shape of primary xylem in transverse sections (subtriangular in A. perforata and rounded in A . petriellae ) and different cross-field pitting. They also differ in the age (Permian in A. perforata ).
All features described for the new specimens from Quebrada de la Mina are also present in Phyllocladopitys petriellae Brea and Césari (1995); although in the original diagnosis and description the thickenings of the ray cell walls were neither mentioned nor illustrated. The original material (BAPb 244 and slides) was revised, and although this is not as finely preserved as the new material, the thickenings of the ray cell walls were observed (figures 2.G, 2H). Locally biseriate rays were mentioned for the holotype and not observed in the new material. As a result, a new combination to the genus Abietopitys is proposed because of the thickenings that the genus Phyllocladopitys lack. In addition, original and new specimens show pitting of the tracheids exclusively in the radial walls (figure 2.I).
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