While we recognize that the taxonomy of Paleozoic woods and isolated stems is complex and that some morphotaxa are now considered to be invalid (such as Araucarioxylon, see Philippe, 1993 and Philippe, 2011), it is beyond the scope of this article to discuss the validity of previously described taxa. In the comparison section of the paper, we therefore use the generic and specific names as proposed by the original authors.

The specimen is 4 cm in length and represents half of a decorticated trunk with about 8 cm of preserved radius (Fig. 2). The central part (i.e., pith and primary xylem) is present in the basal-most part of the specimen. A well-preserved branch about 13 × 1 mm in diameter is included in the outer part of the wood cylinder.

Pith. The pith of the trunk is circular and 4 mm in diameter in the trunk (Fig. 2; Fig. 3A, B), slightly oval and 3 × 5 mm in the branch. It is heterogeneous and contains parenchyma and clusters of sclereids forming sclerotic nests (Fig. 3A–C). Parenchyma cells are polygonal in shape both in transverse and longitudinal section, and have an average diameter of 53 μm. In the region of the clusters of sclerotic cells, parenchyma cells are often more rectangular in shape and sometimes elongated, up to 150 μm in diameter (Fig. 3C). Sclerotic nests are 90 to 400 μm in diameter in transverse section and up to 900 μm in height (Fig. 3 A, B). Individual sclereids are polygonal and generally similar in shape and size to the parenchyma cells, except for their thickened walls (Fig. 3C, D). There is evidence of a gradient in cell wall thickness among the pith cells, probably indicating the ongoing differentiation of sclereids from the parenchyma. The periphery of the pith also contains lacunae of various dimensions that seem to be associated with primary xylem strands and departing leaf traces (Fig. 3A–C, E). Some of the largest lacunae, however, contain coprolites (Fig. 3B, C, arrows) and the spaces may have been caused by arthropods.

Primary xylem and traces to lateral organs. The primary xylem is difficult to distinguish in cross section. Small groups of primary xylem tracheids are, however, visible at the pith-secondary xylem boundary (Fig. 3E). In longitudinal section, the anatomy of the strands appears more conspicuous and maturation is endarch (Fig. 3F); the walls of protoxylem and metaxylem tracheids contain helical to reticulate thickenings (Fig. 3F). Two different types of traces to lateral organs are present, leaf traces and a single branch trace. Leaf traces depart in pairs at the margin of the pith (Fig. 3C), with one trace slightly more advanced developmentally than the other (Fig. 3A, C). These vascular traces are truncated relatively close to the margin of the pith, indicating that the leaves were short lived. The branch exhibits anatomy comparable to that of the trunk (Fig. 4A) although sclerotic nests in the pith are less prominent. The branch trace extends out from the sympodia at an angle of about 45°.

Secondary xylem. The secondary xylem of the trunk and branch is composed exclusively of tracheids and small parenchymatous rays. About 20 growth rings are preserved in the trunk (Fig. 4B). These are accompanied by a few false rings, each characterized by three to four rows of tracheids with reduced radial diameters. Although some parts of the secondary xylem have been distorted, it is possible to estimate that the width of the growth rings ranges from about 1 mm towards the center of the axis to about 5 mm in the outer part. This is comparable to other woods described from the Permian of Antarctica (e.g., Taylor and Ryberg, 2007 and references therein). Tracheids are rectangular to polygonal in cross section, and their radial diameter varies between 5 μm (in the latewood) and 55 μm (earlywood). Rays are uniseriate, rarely partly biseriate (Fig. 4C). They are one to ten cells high, occasionally up to 15 cells high. The radial walls of the tracheids contain one to three rows of bordered pits about 10 to 12 μm in diameter, each with a circular aperture (Fig. 4D–F). In tracheids with uniseriate pitting, the pits are circular in shape and do not occupy the whole width of the wall (Fig. 4D). Tracheids with bi- or triseriate pitting have the pits arranged alternately (Fig. 4D–F); pits are circular but can also appear hexagonal when crowded (Fig. 4E). Cross-field pitting consists of one to four oval, simple pits about 9 × 6 μm in diameter (Fig. 4G, H). The most common arrangement is four pits organized in two rows.

A comparison with previously described Permian axes from Antarctica shows that the trunk from Coalsack Bluff probably represents a new morphotaxon for this region. A summary of the comparison for some anatomical characters of the primary and secondary vascular system is presented in Table 1. The new trunk differs from stems of G. skaarensis (Pigg and Taylor, 1993), which have a smaller pith (1 mm in diameter) composed entirely of parenchyma cells. Some of the stems from the Skaar Ridge permineralized peat observed by Pigg and Taylor have cells with opaque contents in their pith but none (n > 150) have sclereids, and this is also true for branches produced by two putative glossopterid trunks from the same locality (Decombeix et al., 2010a). G. skaarensis also has smaller rays than the new trunk (two cells high on average, n = 176), although this might be a developmental difference.

Two other genera reported from Antarctica by Maheshwari (1972), Polysolenoxylon and Damudoxylon, differ from the new trunk by their lack of sclereids in the pith. Polysolenoxylon has a pith with secretory cells and canals. Damudoxylon species have either a homogeneous parenchymatous pith or, as is the case in the Antarctic specimen Damudoxylon sp., a pith with interspersed secretory cells. The new trunk is also easily distinguished from other Antarctic taxa by anatomical characters of the secondary xylem. In a piece of wood assigned to Dadoxylon weaverense from Mt Weaver, up to 75% of the rays are biseriate (Maheshwari, 1972), a condition found in a small number of Permian gymnosperm axes and clearly different from the new trunk. In Megaporoxylon and Protophyllocladoxylon, cross-field pitting consists of a single, rarely two, large pits, versus several small pits in the Coalsack Bluff specimen. In addition, Megaporoxylon is characterized by a large pith up to 3 cm in diameter that can contain secretory cells but has no sclereids.

Maheshwari (1972) also described three species of Araucarioxylon based on wood fragments with no pith preserved. In one of them, A. bengalense, the radial pitting of the tracheids is opposite to subopposite with a tendency for the pits to form groups, as in the genus Australoxylon (Marguerier, 1973). This is different from the arrangement in the Coalsack trunk and more similar to what is observed in some Vertebraria roots and in two putative glossopterid trunks from Skaar Ridge (Decombeix et al., 2009 and Decombeix et al., 2010a). Whether these specimens and A. bengalense represent the same taxon is, however, difficult to assess given the lack of information on primary structures in the A. bengalense specimen. The two other species of Araucarioxylon described by Maheshwari, Araucarioxylon sp. cf. ningahense and A. allanii, have secondary xylem anatomy comparable to that of the new trunk but again it is difficult to assess their affinities with certainty in the absence of preserved primary tissues.

A significant number of Permian stem genera have been described from other regions of Gondwana, in particular India and South America. Among those that have secondary xylem anatomy generally comparable to that of the new trunk, several can be distinguished by a lack of sclereid clusters in the pith, including Atlanticoxylon (Mussa, 1986), Barakaroxylon (Surange and Maithy, 1961), Brasilestiloxylon (Mussa, 1978), Paulistoxylon (Mussa, 1986), Petalopitys (Mussa, 1986), Solenobrasilioxylon (Mussa, 1978), and Solenoxylon (Kräusel, 1956). Piracicaboxylon from the Permian of Uruguay (Mussa, 1986) has groups of sclerified cells in the pith but they are fibers and not isodiametric sclereids as in the Antarctic trunk.

Morphogenera that have both secondary xylem and pith anatomy most similar to the trunk from Coalsack Bluff are Austroscleromedulloxylon Mussa et al., 1980 and Kaokoxylon Kräusel (1956). These two taxa show a number of similarities and might represent the same morphogenus. The best-known species of Austroscleromedulloxylon is the type species, A. geraldini, from the Permian of Brazil (Mussa, 1986) and Uruguay (Crisafulli, 1998). The Brazilian specimen is characterized by a pith up to 4 mm in diameter, while in the Uruguayan specimen it is 1.5 × 0.8 cm. Both have lacunae on the periphery of the pith and sclerotic nests. The general wood anatomy of the Brazilian specimen also agrees with that of the Coalsack Bluff trunk, with tracheid pitting consisting of two to three rows of araucarioid pits, uniseriate rays usually one to five cells high, and several small pits in the cross field. A difference, however, is that primary xylem maturation is mesarch in A. geraldini versus endarch in the Coalsack Bluff specimen. The Brazilian specimen assigned to A. geraldini differs from both the Uruguayan example of this species and the Coalsack Bluff trunk by having secondary xylem with rays that are often biseriate (Crisafulli, 1998).

Kaokoxylon Kräusel is a widespread taxon known from Permian and Triassic localities of South Africa, Australia, India, Brazil, and Argentina. The genus was established to include Gondwanan taxa with Araucarioxylon-type wood that have a heterogeneous pith with groups of sclerenchymatous cells. Seven species have been described and all of them have a small pith (< 1 cm in diameter, usually around 5 mm) and endarch primary xylem maturation. Two species, K. reuningi from the Permian of Namibia (Kräusel, 1956) and K. rioclarense from the Permian of Brazil (Mussa, 1982, as cited in Crisafulli and Dutra, 2009), differ from the Coalsack trunk in having secretory canals in the pith. In addition, the sclereids in K. rioclarense are located only at the periphery of the pith, usually in association with the secretory cells. Kaokoxylon sclerosum, the type species from the Triassic of Namibia (Kräusel, 1956), also differs from the Antarctic trunk by the arrangement of its medullar sclerenchyma cells, which form large columns in the pith. Based on pith anatomy, the Antarctic specimen is thus closer to the species that have spherical to lenticular sclerotic nests, i.e., K. zalesskyi from the Permian of India and Argentina (Herbst and Crisafulli, 1997 and Maheshwari, 1967) and the Triassic of Brazil (Crisafulli and Dutra, 2009), K. durum from the Permian of Namibia (Kräusel, 1956), K. farleyense from the Permian of Australia (Maheshwari, 1972), and K. pseudotrimedullaris from the Permian of India (Prasad, 1982). All of these species also agree with the trunk from Coalsack Bluff in terms of secondary xylem anatomy, except for K. durum, which has numerous cross-field pits (Maheshwari, 1972). It is also interesting to note that Prasad (1982) reported the presence of double vascular traces to lateral organs in K. pseudotrimedullaris, although he described them as branch traces while we have interpreted them as leaf traces in the Coalsack Bluff specimen. The mode of emission of lateral organs in other species of Kaokoxylon remains unknown.

Finally, it is necessary to provide a comparison of the Coalsack Bluff trunk with Antarctic gymnosperm axes described from the same area but occurring in Triassic deposits. Two genera with preserved primary vascular structures have been described from the Fremouw Formation of the central Transantarctic Mountains, Kykloxylon (corystosperm) and Notophytum (conifer). Both are found preserved in silicified peat from Fremouw Peak, about 30 km from Coalsack Bluff. They represent young stems with attached leaf bases, which make it possible to determine their affinities with much more accuracy than in the case of decorticated stems. Notophytum is easily distinguished from the Coalsack Bluff trunk by characters such as the lack of sclerotic nests in the pith, simple leaf traces, and podocarp type of pitting with one or rarely two large cross-field pits (Meyer-Berthaud and Taylor, 1991). Kykloxylon, on the other hand, shows some significant similarities with the new trunk, including the presence of sclerotic nests in the pith, endarch maturation of primary xylem, and double leaf traces. Interestingly, in a preliminary description of Kykloxylon, Meyer-Berthaud and Taylor (1992) noted that if found without the attached Dicroidium leaf bases, this type of stem would probably be assigned to Kaokoxylon; however, Kykloxylon has typical secretory structures in the pith that are not found in the Coalsack Bluff specimen. It is possible that these structures were only present in young axes (Decombeix et al., 2010b), but they are also missing in the branch of the new trunk. The pith of Kykloxylon is also smaller, only 0.05 to 0.26 cm in diameter, compared to 0.5 cm in the new trunk. Another difference is that cross-field pitting in Kykloxylon consists of oval-to-elongated pits that range from 7 × 10 μm to 10 × 25 μm (Meyer-Berthaud et al., 1993) while there is no evidence of such elongated cross-field pits in the new trunk. Thus, while both Kykloxylon and the Coalsack Bluff trunk would fit in the concept of Kaokoxylon, they apparently represent distinct stem morphotaxa.