Zosterophyllum undefined-a

McSweeney, Fearghus R., Shimeta, Jeff & Buckeridge, John St J. S., 2022, Lower Devonian Zosterophyllum-like plants from central Victoria, Australia, and their significance, Memoirs of Museum Victoria 81, pp. 25-41 : 29-36

cf. Zosterophyllum sp. A

Gen. et sp. indet.

Figures 4–9

Material examined. NMV P256740.1 and P256740.2, part and counterpart, respectively.

Locality. Frenchmans Spur Track, ∼ 10 km west of Matlock , central Victoria .

Horizon and age. Wilson Creek Shale, middle Pragian– Emsian, L. Devonian ( Carey and Bolger, 1995; Mawson and Talent, 1994).

Description. The specimen consists of a longitudinally elongate lax spike with its apical region missing. The basal half of the spike contains about a third of the total sporangia in two rows (figs 4, 5a), and distally, the sporangia are more closely arranged (?helically) but the insertion points are not clear. The naked fertile axis is unbranched, 1.3–2.6 mm wide, curving basally, the spike slightly decreases in width acropetally. The lax spike is 10 mm wide and up to at least 45 mm long, consisting of 20 sporangia arranged on long vascularised stalks up to 2.0 mm long and 1.0– 1.3 mm wide, at acute angles 15°–45° to the vertical, before the stalks reorientate towards the apex of the spike just beneath each sporangium. There is very little vertical overlap of sporangia. Some fine protuberances and depressions emanating from the vascular trace (fig. 6) are interpreted here as representing insertions of further stalks. The junction between sporangium and stalk is unknown. The sporangia are circular to reniform in face view, 0.95–3.7 mm wide and 0.5– 2.3 mm high, with weakly developed lobes (fig. 7) and a narrow border visible on distal margin of some sporangia, such as sporangia two, 10 (figs 7, 8) and 12, are 0.13–0.15 mm wide and is interpreted as likely pertaining to dehiscence. Sporangia one and two are longitudinally elliptical and are interpreted to be infolded, such that half the abaxial valve is visible (fig. 8). Vascular trace 0.34–1.3 mm in the fertile axis, 0.17–0.21 mm on the stalks. There are two sporangia in close proximity to the spike, but they are clearly orientated at an angle to indicate they may come from another axis in their vicinity (fig. 5a).

The vascular trace is conspicuous in that it is preferentially preserved compared with cortical tissue (fig. 4), the cortex being preserved as a grey film in the surrounding matrix. The stalk of sporangium one (fig. 8) is inserted almost perpendicular to the fertile axis and is bent such that most of the stalk is parallel to the fertile axis before curving upwards, just beneath the basal region of the sporangium. Several poorly preserved axes lie beneath the spike but are too poorly preserved and lack direct connection to warrant further consideration. However, the subtending axis to the spike aligns with an axis 4 mm wide (figs. 8, 9) and is suggestive of derivation from the same spike. This suggests the linear aerial extent of cover of the plant was at least 45 mm wide. Furthermore, the horizontal orientation of this axis to the spike is suggestive of a rhizomatous system, but it remains equivocal due to the absence of reticulum axes and H- and K-branching (sensu Hao et al., 2010: fig. 3; Walton, 1964: fig. 1). To the right of the apical region of the spike on the part, there are at least four axes that do not possess any attached sporangia, and beneath these axes towards the middle of the spike, a poorly preserved axis is visible with two sporangia (fig. 9) not directly attached to it but with their sporangial stalks orientated towards it, suggesting it was once attached. The alignment of these axes with the spike may indicate a tuft habit, but without clear evidence of additional spikes, its habit remains inexplicit.

Remarks. The description is based on one specimen – 45 mm high, part and counterpart with one spike (fig. 4) with sporangia laxly arranged on vascularised long stalks – preserved as a fine film of carbonaceous material lacking anatomy. The specimen occurs with three specimens of Yarravia sp. Lang and Cookson, 1935, on the same plane ( McSweeney et al. 2021a: fig. 5a–d). The limits of the fertile axis and stalks are defined by grey film on each side of a much darker vascular trace. The fine slender nature of the darkened linear structures below the stalks are too narrow to support a sporangium, such that it seems parsimonious for the original widths of the axes to be defined by these ghosted grey areas. Lele and Walton (1961: 471), when describing axes prepared from acetate transfers, found the xylem to appear as a preferentially preserved dark bands (and to be about one sixth the axial width) and noted the vascular traces were often displaced from their central position. This, they postulated, was likely due to decay of the cortex prior to burial during early digenesis. This would help explain the convoluted nature of the vascular trace herein (figs. 4–6), indicating the structure of the axes had already started to break down before becoming fully fossilised.

The specimen possesses depressions and protuberances along parts of its fertile axis, which is especially noticeable midway along the spike (fig. 6). These are interpreted as likely insertion points for some axes of sporangia and follows Edwards’ (1975: 255) interpretation of a similar feature on Z. myretonianum . Xue (2009: 507), in describing Z. minorstachyum , suggested that small conical protuberances along the axes may reflect parasitism. This possibility was considered, but the irregularities on the vascular trace are primarily depressions in areas noticeably lacking sporangia, and in some cases appear to be the basal-most attachment of the stalk to the fertile axis’ vascular trace. Additionally, we did not consider areas lacking in sporangia to be indicative of a deciduous spike, as seen with Z. deciduum from the Emsian, Lower Devonian of Belgium ( Gerrienne, 1988). While it is plausible that once the more mature proximal sporangia has dehisced and subsequently abscised, plants would be better served by losing some sporangia in this region to concentrate energy on immature sporangia in the distal region of the spike. However, the specimen still possesses large proximal sporangia and only some sporangia appear to be missing, suggesting that they may have been lost, most likely as a result of excision due to the biostratinomy phase ( Jackson, 2010: 5) of fossilisation. The absence of a junction at the axial–sporangial interface does not mean it never existed because it may have been destroyed during fossilisation. When examining Llanover specimens of Zosterophyllum from the Old Red Sandstone of South Wales, Edwards (1969a: 924) found organs could be superimposed and amalgamated into the surrounding tissue during preservation, resulting in them been indistinguishable.

The specimen described herein is atypical in comparison with most Zosterophyllums because of paucity of folded sporangia seen in lateral view with only two proximal sporangia so preserved. Furthermore, the sporangia rarely overlap each other, with one instance occurring in the proximal region of the spike where sporangium two has been pushed onto the basal region of sporangium three (fig. 4b, arrow at stalk of sporangium two) and distally for sporangia 19 and 20 (fig. 4).

Comparison with other taxa. The sporangia of the specimen are borne alternatively in two rows on opposite sides of the axis, akin to Platyzosterophyllum , and so the specimen was compared to Platyzosterophyllum first. However, some Platyzosterophyllum possess sporangia emanating from two rows on one side of the axis, such as Z. cf. fertile in Wellman et al. (2000: 181) and are noticeably more compact. The stalks of Z. fertile are perpendicular to the fertile axis, before sharply turning towards the apex, such that they are borne in an upright to slightly recumbent position ( Wellman et al. 2000: 181). This characteristic of recurved stalks perpendicular to the fertile axis is also seen in Z. spectabile Schweitzer, 1979 , according to Gensel (1982: 662). However, the specimen clearly differs from these taxa because the stalks are orientated at acute angles of 15°–45° without any noticeable change in orientation, other than immediately below each sporangium, where they sharply reorientate upright and parallel to the fertile axis (figs 5a, 6, 7a). Furthermore, the sporangia of Z. fertile are oblate ( Wellman et al. 2000: 183), being almost linear along the margins, while the specimen’s sporangia are rounded to reniform. The dimensions of both taxa also differ slightly, with Z. fertile possessing stalks that are much narrower than the 1.0– 1.2 mm width for the specimen, with Z. fertile at most reaching 0.5 mm wide ( Edwards, 1972) but generally (including for Z. cf. fertile ) 0.3–0.4 mm wide ( Edwards, 1969a; Leclercq, 1942; Wellman et al., 2000). The sporangial dimensions for Z. fertile are, in part, similar to the specimen, with the sporangia of Z. fertile up to 2.3 mm wide ( Edwards, 1972), and for Z. cf. fertile specimens the sporangial dimensions were 2.0– 2.3 mm wide ( Edwards, 1969a; Leclercq, 1942; Wellman et al., 2000). The specimen’s sporangial widths vary more greatly on the same spike and range between 0.95–3.7 mm wide, suggesting the plant was not mature. In comparison with the Welsh specimen, Z. llanoveranum , sporangia are arranged in 1–2 alternative rows but differs from the specimen with sporangia borne close together and in the distal region of the spike, sometimes helically arranged ( Edwards, 1969b). This could not be confirmed here because the stalk insertion points are lacking.

Edwards (1975: 263) cautioned against the use of the arrangement of the sporangia on the spike as a definitive characteristic with which to delineate species. Edwards noted bilateral symmetry basally in the spike with the distal part helically arranged in some specimens of Z. myretonianum and attributed it to the compression of widely spaced spirally arranged sporangia, giving this misleading appearance ( Edwards, 1975: 261). Furthermore, Gerrienne (1988: 328) made similar observations, adding that the difference may also be due to different ontogenetic stages of individual spikes, and Gensel (1982) noted for Z. divaricatum , sporangia bending and twisting of sporangia to one side.

In comparison with species within the subgenus Zosterophyllum with reniform sporangia, the specimen is closest to Z. bifurcatum Li and Cai, 1977 ; Z. deciduum ; Z. myretonianum Lang, 1927 ; Z. ramosum ; Z. rhenanum ; Z. yunnanicum Hsü, 1966 ; and Z. shengfengense , all of which have sporangia in approximately the same size range.

Zosterophyllum myretonianum is one of the best studied Zosterophyllum to date ( Edwards, 1975; Lang, 1927; Lele and Walton, 1961). Edwards (1969: 261) noted when examining Z. myretonianum from Aberlemno, Scotland, that they possessed spikes with different levels of sporangial packing, such that the specimens could be divided into compact, intermediate and laxly arranged spikes. The sporangial–stalk interface of Z. myretonianum , according to Edwards (1975), possesses a dome-like region at the point of insertion on the sporangium of some of the specimens, which produces its reniform shape. It is noticeable that in Z. myretonianum , despite different stages in development, the orientation of the sporangial stalk remains largely constant, with the sporangial stalk inserted at almost 90° ( Edwards, 1975) to the fertile axis before curving upwards immediately with the sporangium held erect. This clearly differs from the specimen where the sporangial stalks extend from the fertile axis, curving upwards only just beneath the sporangium and in some cases attached to the sporangium at an angle, thus producing a splayed appearance. Zosterophyllum bifurcatum possess well-developed lobes and much narrower stalks than the specimen reaching up to 0.6 mm wide according to Li and Cai (1977) and Hao and Xue (2013). Zosterophyllum rhenanum also possess well-developed sporangial lobes and has a noticeable junction between sporangium and stalk, and a large border of 0.6 mm ( Hao and Xue, 2013; Schweitzer, 1979). Z osterophyllum deciduum has weakly developed sporangial lobes ( Gerrienne, 1988: 322), similar to the specimen, but the sporangial stalks were wide (0.4–0.75 mm) near the fertile axis and narrow (0.1–0.3 mm) near the sporangium ( Gerrienne, 1988: 320), with the contact between the sporangium and subtending stalk producing a clear junction with no evidence of widening beneath the sporangia ( Gerrienne, 1988: 331). These characteristics are at odds with what is observed with the specimen where sporangial stalks remain parallel in width before widening into the base of the sporangium. Furthermore, Z. deciduum bifurcates both below and within its fertile parts ( Gerrienne, 1988).

In comparison with Zosterophyllum from the South China plate with similar sporangial dimension, Zosterophyllum shengfengense from the Lochkovian, Lower Devonian of Xitun Formation, Yunnan, China, differs from the specimen in not possessing any sporangial basal lobes, and shorter stalks, 0.5–0.8 mm wide and 0.8–1.6 mm long ( Hao et al., 2010; Hao and Xue, 2013). Furthermore, Zosterophyllum shengfengense ( Hao et al., 2010: 222) , like Z. myretonianum ( Lele and Walton, 1961: 471) , possess tubercles proximally on the plant, unlike the specimen ( Hao et al., 2010: fig. 2a). Zosterophyllum yunnanicum from the Xujiachong Formation, Yunnan, possess crowded spikes with up to 50 sporangia circular to elliptical in face view, dehiscence zone up to 0.5 mm wide, stalks 0.3–0.9 mm wide and 0.6–3.0 mm long inserted an acute angle to the fertile axis and widening into the bases of sporangia ( Edwards et al., 2015: 223). The stalks emanate perpendicular to the spike, based on Edwards et al. (2015: pl. 4, figs 1, 2) and immediately reorientate producing 30°–40° to the fertile axis ( Wang, 2007: 528). This reorientation of the stalks near the fertile axis differs significantly from the specimen, where the stalks reorientate only just beneath each sporangium. Furthermore, Z. yunnanicum produces a dome-like structure at the stalk– sporangium interface ( Edwards et al., 2015).

Comparison with known Victorian zosterophyll taxa. Only four zosterophylls have thus far been described from Victoria. These include Z. australianum Lang and Cookson, 1930 ; Z. ramosum Hao and Wang, 2000 ; Parazosterophyllum timsiae McSweeney et al., 2020; and Gippslandites minutus McSweeney et al., 2020. Both Z. australianum and Z. ramosum occur in the Norton Gully Sandstone Formation of Victoria and are younger than the specimen, which is currently only known from the underlying Wilson Creek Shale. Zosterophyllum australianum occurs at North Road Quarry, Walhalla, Victoria, and Yunnan (Posongchong Formation), China ( Hao and Xue 2013; Lang and Cookson 1930). Zosterophyllum australianum possess sporangia that are noticeably larger than the specimen and are longitudinally elliptical or fan-shaped, 2.8–8.0 mm wide and 2.2–5.0 mm high, with short stalks inserted on the fertile axis at 90° ( Hao and Xue, 2013; Lang and Cookson, 1930). Zosterophyllum ramosum occur at Mount Pleasant and Halls Flat Road, Alexandra ( Cookson, 1935; Hao and Wang, 2000). Mount Pleasant Road is the type locality of Yarravia (Hedeia) corymbosa Cookson, 1935 , and cf. Baragwanathia longifolia , cf. Yarravia oblonga , cf. Hostinella and Pachytheca sp. have been found by Cookson (1935) to occur with Z. ramosum ( McSweeney et al., 2021a, b). Zosterophyllum ramosum was originally called Z. australianum by Cookson (1935: pl. 10, figs 9–12), but was later reinterpreted by Hao and Wang (2000: 31) to be a new species Z. ramosum , which also occurs in Yunnan (Posongchong Formation), China. Zosterophyllum ramosum possess circular to reniform sporangia similar to the specimen, but the sporangia are larger, being 1.6–6.0 mm wide and 1.9–5.5 mm high, on stalks up to 5.0 mm inserted on the fertile axis at 15°–35° ( Hao and Wang, 2000; Hao and Xue, 2013). Both Z. ramosum and Z. australianum , according to Hao and Xue (2013: fig. 6.5), possess apple-shaped Za-type sporangium with extended thickened margins, a character not found in the specimen. Parazosterophyllum timsiae is from Ghin Ghin Road, Yea, in the base of the Humevale Formation and based on Rickards & Garratt (1990) Pridoli, upper Silurian–Pragian, Lower Devonian, and may be either coeval or older than the specimen and differ significantly from the specimen with its spike terminating lateral branch ( McSweeney et al., 2020). Gippslandites minutus is from an outcrop of the Boola formation (Lochkovian–Pragian, L. Devonian) near Boola Quarry, Tyers, Victoria ( Tims, 1980; McSweeney et al., 2000). The Boola formation is slightly older than the Wilson Creek Shale, which overlies the Boola formation at Coopers Creek according to Edwards et al. (1997: 39). Gippslandites minutus differs from the specimen because its sporangia are much smaller, 0.6–2.6 mm wide and 0.3–1.9 mm high, and differ significantly from Zosterophyllum spp. with anisovalvate sporangia ( McSweeney et al., 2020).

The defining characteristic of the specimen is primarily the angle of insertion of the vascularised stalks and no overlap between vertical adjacent sporangia. As noted by Edwards (1975: 264), the most useful characters in species delimitation within Zosterophyllum are stalk and sporangial characters. It is clear that the specimen differs from zosterophylls from Victoria primarily on sporangial morphology and symmetry. As the sporangial stalks were likely longer in life when turgid and prior to degradation resulting in convoluted vascular trace, the lack of vertical overlap of sporangia and vascularisation of the stalks, and clear demarcation of insertion points on the fertile axis means the specimen cannot be readily put into the subgenus Platyzosterophyllum , and is thus assigned to cf. Zosterophyllum sp. A . until better material becomes available to allow for further assessment of its phylogenetic and taxonomic position.

Figures

Figure 4. cf. Zosterophyllum sp. A. from the Wilson Creek Shale Formation on Frenchmans Spur, 10 km west of Matlock: a, line drawing, dotted lines are from faint remains of compression. S=Sporangium; Q=Poorly preserved sporangium or sporangium likely not belonging to same spike; b, part NMV P256740.1. Arrow at stalk, which has been pushed across fertile axis; c, counterpart, NMV P256740.2 with part of basal region of spike missing. Note. Counterpart image reversed to be in the same orientation as the part.

Figure 5. cf. Zosterophyllum sp. A. (part, NMV P256740.1): a, two isolated sporangia at arrows (pre-dégagement) with basal region of both sporangia orientated away from the spike; b–d, on the reverse of the slab, isolated sporangia with much similar dimensions and weakly developed sporangial lobes. Image a taken by Rodney Start © Museums Victoria.

Figure 6. cf. Zosterophyllum sp. A. (part, NMV P256740.1): a, line drawing of medial region of the spike seen in B; b, the cortex (Ctx) is represented by a tincture of light grey in comparison to a darker coloured vascular trace (Vt). Some perturbations and depressions (*) likely represent additional stalks. Note small proximal sporangia (at arrow) in a central position on the fertile axis; c, distal part of spike – lower arrow at stalk that has been pushed across the fertile axis. At the upper arrow sporangium with no stalk, partially behind another sporangium. Smaller sporangia more centrally located suggests distal part of spike may be helically inserted. Images taken by Rodney Start © Museums Victoria.

Figure 7. cf. Zosterophyllum sp. A. (part, NMV P256740.1). Interpretative line drawings: a, c, sporangia in b, d, respectively. The sporangia have weakly developed lobes (Lb), and a lack of a clear junction between the stalk and sporangium. A fine border (Br) is visible only alone distal margins.

Figure 8. cf. Zosterophyllum sp. A. (part, NMV P256740.1): a, proximal region of spike; b, interpretative line drawing. Sporangia one and two (S1–2) appear infolded, with both abaxial (Abx) and partial adaxial (Adx) valves visible in sporangia and two perturbations and depressions (*) possibly representative of additional stalks that were not preserved. Q1, shadowing of possible sporangium, and upper arrow shows change in orientation of stalk beneath sporangium four.

Figure 9. cf. Zosterophyllum sp. A. (part, NMV P256740.1). Proximal region of spike with sporangia one (S1) and two (S2) visible and fertile axis curving into?rhizomatous region (arrow 1) and possibly extending out towards axes (arrows 2 and 3). Isolated sporangium (S) in same orientation as sporangia in spike. Image taken by Rodney Start © Museums Victoria.