Zosterophyllum undefined-b

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 : 36-39

cf. Zosterophyllum sp. B .

Gen. et sp. indet.

Figures 10 , 11

Material. NMV P256742.1 and P256742.2 (P4-5 field note identifier), part and counterpart, respectively.

Locality. P4 is equivalent to Loc. 1 in Garratt (1978: fig. 2), and occurs on Ghin Ghin Road, 8 km northwest of Yea township, central Victoria.

Horizon and age. Humevale Siltstone, Pragian–Emsian, Lower Devonian ( Edwards et al., 1997; Garratt, 1978; Rickards, 2000; Rickards and Garratt, 1990; VandenBerg et al. 2000; VandenBerg pers. comm. June 2021).

Description. Single specimen, comprising compact spike, with part and counterpart preserved in semi-relief as an iron oxide coated impression and cast, only gross morphological features are visible (fig. 10). The spike measures 21 mm high and 10 mm wide, with at least nine sporangia arranged helically, becoming more compact distally. The fertile naked axis measures 1.5–3.75 mm wide, narrowing distally before terminating in sporangium nine. Sporangia in face view (1.2–3.0 mm wide and 3.9–4.3 mm long) occur in the distal part of the spike and are circular to oblong. Proximally, sporangia one to three are in lateral view, being longitudinally elliptical and infolded. Sporangia are inserted at an angle of ∼ 20°. Proximally on the spike there is an elliptical (lateral view) junction at the point of attachment (face view) on the abaxial valve between the stalk and the sporangium. The stalks are decurrent, narrowing slightly before widening distally at the base of the sporangium. The sporangia possess a dehiscence zone along their entire distal margins, with a border 0.1–0.2 mm wide gradually tapering proximally towards the stalk attachment.

Remarks. There is a notable disparity in size between the much larger proximal sporangia (S1–5) and distal sporangia (S6–9), suggesting the spike may be immature. Sporangia one–three are in a lateral position and are infolded, a common feature seen especially in laterally placed sporangia of Zosterophyllum . It suggests that the sporangia may have originally had relatively flat bodies, as proposed by Lang (1927) when explaining this feature in Zosterophyllum myretonianum . The points of attachment of the stalk to the sporangia are visible on sporangia one and three ( Figs 11a, c ), with the stalks widening into the base of the sporangia. The attachment is elliptical, and it is possible that the raised regions defining the elliptical region, which appear partially raised, may reflect the splitting of the attachment at the base of the sporangium, possibly feeding each of the valve. However, the preservation is too poor and this remains equivocal. The attachment occurs on the adaxial side of the valve, but the demarcations between both valves are faint or absent on the sporangia. Most of the distal sporangia show the stalk centred beneath the sporangium, with a gradual widening of the stalk into the valve. Sporangium seven, which is in face-view, appears to show the widening of the distal part of the stalk on the lower part of a valve, producing a sub-circular zone of attachment. Sporangium five, while poorly preserved, is in side view and greatly compressed laterally with the projecting edge of the border visible in the apex, and a fine stalk is present in the basal region. The adaxial valve only appears marginally darker than the surrounding matrix and is convex, while the upper valve is flatter and delimited by its iron-oxide colouration. Sporangia four and six also appear to show a slight separation at the distal-most region of the valves. This may be a result of compression as the sporangia are small relative to the proximal sporangia, suggesting they were not fully mature.

The dehiscence line can be seen along the distal margins of the proximal sporangia, visible down to the stalk attachment (fig. 11c) but is only visible on some of the distal sporangia. There is a dehiscence zone beside the dehiscence line no wider than 0.2 mm and is clearest on the proximal sporangia.

The proximal part of the fertile axis expands into an ovalshaped body (fig. 11d) that measures 3.74 mm wide and 5.80 mm high. The point where the axis starts increasing in diameter was taken as the start of this structure because there is no other way to differentiate it from the axis. The oval body is similar to the corm-like structure found basally on Horneophyton lignieri Barghoorn and Darrah (= Hornea lignieri Kidston and Lang, 1920 ). However, because no anatomy is preserved, its nature remains equivocal and may be a quirk of preservation. A small, fine, faint linear structure (fig. 11d) appears to emanate from the oval body, but remains equivocal because sampling did not reveal any organic remains and high magnification did not reveal any morphological characters.

The specimen fits into the class Zosterophyllopsida based on the presence of naked axes, cauline sporangia made up of two valves that dehisce along their distal margins, and vascularized stalks ( Croft and Lang, 1942). The Zosterophyllopsida includes two orders, Zosterophyllales and Gosslingiales, and because this specimen possessed a terminal sporangium, it has been assigned to order Zosterophyllales . Numerous characteristics are notably absent, such as H- or K-branching and circinate vernation. Because the sporangia were not in rows, the specimen was excluded from the subgenus Platyzosterophyllum and tentatively placed into the subgenus Zosterophyllum . However, it was difficult to determine the true morphological outline of the sporangia, with some of the distal sporangia appearing slightly longer than wider. Furthermore, the absence of sporangia proximally in face view added to the uncertainty of whether the sporangia were slightly vertically elongate. Distally, the sporangia are poorly preserved with no unequivocal demarcation of both the valves and junctions between the valves and their subtending stalks. For the subgenus Zosterophyllum , the sporangia are reinform, fan-shaped and isovalved, and are excluded if anisovalvate or vertically longer than wide according to Edwards et al. (2016) and Edwards and Li (2018 a). Additionally, the presence of an oval region at the proximal end of the fertile axis added further doubt to its true assignation, and so the specimen was placed into cf. Zosterophyllum sp.

cf. Zosterophyllum sp. B differs from known Zosterophyllum spp. from Victoria ( Z. australianum and Z. ramosum ), despite the limited characters available for comparison. Zosterophyllum australianum possess horizontally elliptical sporangia with large, thickened margins (0.4–1.1 mm wide) and sporangia far larger than cf. Zosterophyllum sp. B ., reaching up to 8.0 mm wide and 5.0 mm high ( Hao and Xue, 2013; Lang and Cookson, 1930). Additionally, the stalks of Z. australianum are inserted at an angle of ~90°, in contrast to ∼ 20°˚ in cf. Zosterophyllum sp. B . Zosterophyllum ramosum , like Z. australianum , possess much larger sporangia, reaching up to 6.0 mm wide and 5.5 mm high, and are circular to reinform in shape ( Hao and Wang, 2000).

In comparison with other zosterophylls with vertically elongate sporangia outside the subgenus Zosterophyllum , cf. Zosterophyllum sp. B . appears distinct, despite the paucity of available characters with which to compare. For example, Guangnania cuneata Wang and Hao, 2002 , possess anisovalvate upright sporangia, but differ from cf. Zosterophyllum sp. in the spike not being compact, the sporangia being much longer than wide (3.9–6.2 mm high and 1.5–1.9 mm wide) and not possessing a terminal sporangium. Yunia dichotoma Hao and Beck, 1991 , from the Zhichang section of the Posongchong Formation, differs from cf. Zosterophyllum sp. B in the morphology of the sporangia, which are elongate–elliptical to ovoid, but the sporangia did not form spikes ( Hao and Beck, 1991). Additionally, the axes of Y. dichotoma possess small spines ( Hao and Beck, 1991). Huia recurvata Geng, 1985 , of the Posongchong Formation, Yunnan, China, produced sporangia that were ovate to ovoid 2D to 3D ( Hao and Xue, 2013: 70). Huia recurvata differs in having adaxially reflected closely arranged sporangia as opposed to the crowded arrangement seen in cf. Zosterophyllum sp. B ., with sporangia 3–5 mm wide and 6–10 mm high ( Hao and Xue, 2013).

Primarily, because the specimen lacks unequivocal evidence as to the characteristics of the sporangia, such as whether the sporangia are longer than wide, it has been placed in cf. Zosterophyllum sp. B .

Figures

Figure 10. cf. Zosterophyllum sp. B, from the Humevale Siltstone, Ghin Ghin Road, Yea: a, part (NMV P NMV P256742.1); b, counterpart (NMV P NMV P256742.2) respectively; c, line drawing, with S1–9 (sporangia one to nine). Note: Part image reversed to be in the same orientation as the counterpart. Note. Arrow in part, points to sporangium 4, which is obscured in the counterpart.

Figure 11. cf. Zosterophyllum sp. B. NMV P256742.1: a, c, (line drawing) sporangium 3 (counterpart) in-folded, with stalk attachment widening out in basal region of sporangium. Poorly defined dehiscence zone (zone) and line; b, sporangium one (counterpart) with border visible; d, basal oval structure (part), with what appears to be a much smaller spike emanating from it (at arrow), and a poorly preserved axis beneath the oval structure.