Permineralized axis

5 Specimen: NSM004GF013.001 (Figs. 3– 4)

6 Description: The first specimen is an unbranched woody axis, 18– 26 mm in diameter, with a preserved length of 19 cm. It was located within a fine- to medium-grained, multistorey channel body at 138 m in the section of Davies and Gibling (2003). The specimen lay prostrate in lag deposits at the base of the second channel storey, together with abundant indeterminate woody material, much of it preserved as charcoal. Given its facies context the studied specimen is clearly allochthonous. It is unclear whether this axis was permineralized at a different site and subsequently reworked into the channel, or whether permineralization took place in the host channel itself. However, given that permineralized cordaitalean wood is mostly restricted to the well-drained alluvial plain association at Joggins (Falcon-Lang 2003b), it is a reasonable inference that the allochthonous axis described in this paper originated within the seasonal dryland environment in general.

7 The specimen exhibits exceptional anatomical preservation permitting a very detailed description. The axis, which is partially flattened, probably as a result of sediment compaction, has a total diameter of 18– 26 mm. It consists of a central parenchymatous pith, 7.1– 13.3 mm in diameter, surrounded by a pycnoxylic xylem cylinder, 5.1– 6.9 mm wide (Fig. 3A). A small amount of extraxylary tissue adheres to the outer margin of the axis in one region, but is too poorly preserved to warrant description.

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8 The pith is septate with horizontal diaphragms spaced 1.4– 2.3 mm apart (Fig. 3B). At the margins of the pith, tissue consists of vertically-oriented, rectangular, resin-filled parenchyma (70– 80 µm high, 20– 30 µm wide) (Fig. 4A– D). Similar, horizontally-orientated parenchyma or larger (up 110 µm diameter), irregular parenchyma compose the septa.

9 The primary vasculature is non-sympodial, and primary xylem only occurs as the downward extension of mesarch leaf traces (Fig. 4A). In TS, leaf traces adjacent to the pith are clearly paired (Fig. 4B), and when followed down the stem axis, fuse into a single trace. Leaf traces are helically arranged, and after branching, depart from the margin of the pith at an initial angle of 30– 40° from the vertical, before becoming sub-horizontal in the secondary xylem (Fig. 4D). They are typically 210– 300 µm in diameter, contain protoxylem and metaxylem, and over their lower surface are continuous with the secondary xylem. In proximal regions, a layer of profusely pitted parenchyma, continuous with the pith, covers the upper surface of the leaf trace (Fig. 4D– E). Protoxylem tracheids, 6– 12 µm in diameter, are characterised by annular and spiral thickening with scalariform and transitional pitting seen on the 9– 15 µm diameter metaxylem tracheids (Fig. 4C, F).

10 Secondary xylem is composed of densely arranged tracheids cells typical of coniferopsids (i.e. pycnoxylic). Tracheids, 12– 36 µm in diameter, are characterised by 1– 2-seriate, contiguous, alternate, bordered pitting on their radial walls (Fig. 4H). Bordered pits are circular to hexagonal (11 µm) with circular to slit-like, crossed apertures (4– 5 µm). Tangential tracheid walls are unpitted. Rays are exclusively uniseriate, 1– 8 cells high (mode: 1– 2), up to 1.35 mm long, and locally continuous with the pith (Fig. 4G). They are composed of rectangular parenchyma cells (12– 21µm wide, 24– 35 µm high, 42– 63 µm long) which exhibit 1-6 araucarioid pits (5 µm diameter) in each cross-field (Fig. 4I). Viewed in TS, tracheids are arranged in files of interspersed wide (29 µm) and narrow (10 µm) cells as seen in extant Ginkgo (Srivastava 1963) (Fig. 4J– K).

11 Growth rings are absent (Fig. 4J); however, two extremely weak growth interruptions characterised by diffuse zones of 12– 18 µm diameter tracheids are present (Figs. 3A, 4K). Locally associated with one growth interruption are small zones of traumatic tissue. These contain irregularly spaced tracheids, and resin-filled, profusely-pitted parenchyma (Fig. 4L). A zone of tracheid cells with opaque cell walls in this region may suggest that part of the surface of the growth interruption was partially charred.

12 Interpretation: This specimen is cordaitalean as indicated by its septate pith. In the past, two types of cordaitalean axis have been identified. Axes showing endarch maturation and a sympodial vasculature have been classified as Cordaixylon, while those with mesarch leaf traces and a non-sympodial vasculature, as seen in the Joggins specimen (Fig. 4A), are classified in the genus Mesoxylon (Scott and Maslen 1910; Trivett and Rothwell 1985). However, discoveries of axes with intermediate and overlapping characters has precipitated a crisis in traditional cordaitalean taxonomy (Trivett and Rothwell 1988, 1991). Nevertheless, as no unified taxonomic system has yet arisen to properly characterise contemporary knowledge of cordaitalean diversity (Trivett and Rothwell 1991), the traditional generic concepts of Mesoxylon and Cordaixylon currently remain acceptable.

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13 The Joggins specimen is closely similar to, if not identical with, Mesoxylon sutcliffi i from Marsdenian-Langsettian marine roof shales in northern England (Scott and Maslen 1910; Maslen 1911; Scott et al. 1997). However, given that extraxylary tissue, a key feature of the Mesoxylon sutcliffi i concept, is not well-preserved in the Joggins specimen (Fig. 3A), this taxonomic assignment is made with reservation. Specimens of Dadoxylon recentium described from alluvial plain units at Joggins (Falcon-Lang 2003b) are identical to the secondary xylem of Mesoxylon cf. sutcliffi i, and consequently, most, if not all, wood of this type at Joggins is cordaitalean.

Permineralized stump

14 Specimen: NSM004GF013.002 (Fig. 5)

15 Description: The second specimen is a woody tree stump preserved in growth position. It occurs within a 7-m-thick succession of fine-grained sheet sandstone beds between 276– 283 m in section of Davies and Gibling (2003). This interval lies a few metres below the ‘ Hebert beds' recently described as a seasonal dryland waterhole deposit (Falcon-Lang et al. 2004; Hebert and Calder 2004). The stump is rooted in thin red/grey mudstone beds between individual sheet sandstone units, and overlain by fine-grained sandstone. Specifically, it is located at the base of the cliff line, where it is partially buried by talus deposits (Fig. 5).

16 The stump is 17 cm in diameter above its flared base and has a preserved height of 11 cm. Most of the stump has evidently rotted away, resulting in a wide, irregular, central cavity infilled by fine-grained sandstone and siltstone. The stump outline is defined by a very thick (up to 7 cm wide) organic rind which preserves several narrow, radially-orientated, buttresses on the external surface. Although mostly coalified, isolated patches of the stump have been calcite-permineralized and retain poorly preserved anatomical details.

17 Interpretation: Preserved detail is sufficient to identify the material as pycnoxylic gymnosperm wood of Dadoxylon-type, the preserved rind comprising the secondary xylem cylinder, and the central void, marking the probable position of the decay-expanded pith. Given that all three species of Dadoxylon known from Joggins (Falcon-Lang 2003b) are now considered to be the cordaitalean woods, the fossil stump represents the first cordaitalean tree found in growth position at Joggins.