Kloster and Gnaedinger—Agathoxylon wood anatomy from La Matilde Formation, Argentina Materials and methods
The specimens described come from the La Matilde Formation, SantaCruz Province, Argentina, (Fig. 1.1) and were collected from the Bajo El Puma locality of the central and south-western sector and another four localities from the Gran Bajo de San Julián sector: Laguna La Guadalosa (Estancia El Mineral), Cerro Conito, Barda Blanca and the northern part of Estancia Meseta Chica (Estancia Meseta Chica), and the south-east border of Laguna del Carbón (Estancia La Silvita) (Figs. 1.2, 3). The in situ petrified woods and rolled trees reach diameters of up to 2.4m (Fig. 2). Fragments of silicified wood showing good cellular pre-
servation were analyzed. Standard petrographic thin-sections were prepared for the wood fragments oriented along three planes: radial longitudinal (RLS), tangential longitudinal (TLS), and transverse (TS). In addition, techniques based on acetate peels and dissociated material (Jones and Rowe, 1999) also were employed. Thin-sections were studied in detail under a Leica microscope (DM500) and microphotographs were taken by digital camera (Leica ICC50) with Leica Application Suite EZ 3.2.3 software and a scanning electron microscope (SEM Jeol 5800LV) at the Universidad Nacional del Nordeste (Cor- rientes, Argentina). The wood has been described in accordance with the list
549
1964 Dadoxylon agathioides Kräusel and Jain, p. 59, pl. 1, figs. 1–3, pl. 3, figs. 14, 15, text-figs. 1, 4.
1974 Araucarioxylon agathioides (Kräusel and Jain); Bose and Maheshwari [seen in Yadav and Bhattacharyya, 1996, p. 59].
Holotype.—24288/255 B.S.I.P. from Mandro, Rajmahal Hills, Bihar, India (Jurassic).
of microscopic features for softwood identification (García Esteban et al., 2002, 2003; Richter et al., 2004). Greguss’s (1955) glossary of terms and the standard measurements established by Chattaway (1932) are followed. At least 20 individual measure- ments of the various anatomical elements were recorded, giving values for means, maxima and minima, and occasionally the maximum number established (e.g,. 42µm[25–66µm] or 3–14 [35] cells). Systems of nomenclature and nomenclatural reviews are
those found in Bamford and Philippe (2001) and Philippe and Bamford (2008).
Repository and institutional abbreviations.—Specimens and microscopic slides are deposited in the paleontological collec- tion of the Universidad Nacional de Nordeste (UNNE) “Dr. Rafael Herbst” Paleobotany Section (CTES-PB) and (CTES-PMP), Corrientes, Argentina.
Systematic paleobotany Order Coniferales Engler, 1897
Family Araucariaceae Henkel and Hochstetter, 1865 Genus Agathoxylon Hartig, 1848
Type species.—Agathoxylon cordaianum Hartig, 1848, p. 188.
Remarks.—According to the criteria of Bamford and Philippe (2001), Philippe and Bamford (2008), and Rößler et al. (2014), the genus Agathoxylon Hartig has nomenclatural priority over the genera Araucarioxylon and Dadoxylon; hence, the La Matilde Formation specimens are assigned to Agathoxylon Hartig.
Agathoxylon agathioides (Kräusel and Jain) new combination Figures 3, 4.1, 4.2.
Description.—Several fragments of silicified wood found in situ in the Barda Blanca, Cerro Conito, and Laguna La Guadalosa localities from the Gran Bajo de San Julián sector and Bajo El Puma locality (Figs. 1, 2) of the central and south-western sector were analyzed. The logs found in situ measure up to 1.50m in diameter, and one of the logs reaches up to 4mlong by 1.10min diameter. Specimen CTES-PB 10647 has a wavy external appearance due to the effects of pressure during fossilization, because in making the conventional cuts it is observed that in the same section it shows a mixture of longitudinal radial and tan- gential and cross-sections. All specimens correspond to pyc- noxylic wood, decorticated and with good preservation of the secondary xylem. The description is based on the specimen from the Barda Blanca locality (CTES-PB 10659), although examination of the other specimens supports it (Tables 1, 2). This specimen, in TS, does not show growth rings nor are they observed with the microscope, even though at first glance they are apparently well marked, which is due to the presence of “shearing zones” sensu Erasmus (1976) (Fig. 3.1). In TS, the tracheids have rectangular-quadrangular to polygonal outlines. The radial diameter of the tracheids is 31 μm(15–37 μm) and the tangential is 33 μm (22–45 μm). The average number of rows of tracheids separating the rays is 4, with a range of 1–11. The rays are presented in a non-continuous way through the section (Fig. 3.1). In RLS, the wood type (tracheid radial pitting) is araucarian (100% of the contiguous pits are araucarioid). Pits are bordered, uniseriate, and flattened (35% [32–38%]) biseri- ate, hexagonal, and alternate (32% [29–37%]), and occasionally triseriate or bi-triseriate, hexagonal, alternate, or opposite (0.4% [0–1%]). In addition, they are uniseriate, with biseriate portions (32% [30–36%]), flattened, and hexagonal, alternate (23% [21–25%]) and opposite in a single pair (8% [6–12%]) or in several pairs (1% [0–2%]). Rare groups of 6–10 pits are recog- nized; they start and end with a single pit and have portions without pitting in the walls of the tracheids. The size of the biseriate pits is 15 ×15 μm and the uniseriate pits are 15× 11 μm. The aperture is cross-like (internal and external elliptical aperture), measuring 15 ×4 μmor11 ×4 μm, or cir-
cular measuring 7.5 μm(Figs. 3.2, 3.3, 3.4, 3.5, 4.1, 4.2). Cross- fields show an araucarioid type arrangement, with 2–6 arau- carioid pits (circular aperture) or cupressoid pits (elliptical aperture) whose frequency range is 4–6, arranged in groups or in horizontal or vertical rows. In some marginal cross-fields, 7–8 pits can be observed (Fig. 3.6, 3.7). In TLS, the radial system is homogeneous, with homocellular, uniseriate rays (84%), and some partially biseriate rays with 1–3 cells (16%). The height ranges from 1–22, 37 cells, with an average of seven cells. The radial end cells are elliptical and the central ones are rectangular- ovoid, and they measure 26 ×22 μm, 30 ×26 μm, or 34× 30 μm in width and height. In tangential and radial section, the
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