Salopella laidae, McSweeney & Shimeta & Buckeridge, 2021

McSweeney, Fearghus R., Shimeta, Jeff & Buckeridge, John St J. S., 2021, Early land plants from the Lower Devonian of central Victoria, Australia, including a new species of Salopella, Memoirs of Museum Victoria 80, pp. 193-205 : 201-203

Salopella laidae sp. nov. ( fig. 5A, B , fig. 6C )

Description. Specimen preserved as a faint white compression on blue-grey claystone with six of the eight sporangia preserved. There is no clear sporogenous area within the sporangia. However, sporangia five, seven and eight ( fig. 6C ) on the part ( fig. 5A ) possess darkened creamy-grey regions extending up two-thirds of their lengths and possibly represent a sporogeneous region. No internal anatomy or spores were recovered when Tims and Chambers (1984) examined this specimen. The sporangia are slightly wider than the widths of their subtending axes and do not taper distally but come to a rounded apical region. There are two synchronous dichotomies clearly visible, with the axial width decreasing only after each dichotomy. Folding of tissue appearing as a longitudinal lineation along the edges of the axes and sporangia are visible; no evidence of dehiscence mechanism is present ( fig. 5A ).

Comments and comparison with other species. The diagnosis and description of this taxon is based on one specimen (part and counterpart). While we are mindful of issues in naming a new species on one specimen, we have done so because of the shape of the sporangia and compact branching architecture that set it apart from other rhyniophytoids (Table 1). The use of the paraphyletic genus Salopella ( Edwards et al., 1994) is to emphasise its gross morphology and to follow previous workers ( Tims and Chambers, 1984) in identifying Victorian flora. However, we note that work done by Edwards et al. (1994) on in situ Salopella spores demonstrated significant diversity, that can be difficult to ascertain because of convergence of some sporangial shapes ( Edwards et al. 2001: 11), making assertions on palaeogeographic distribution and spread unwise without palynological evidence. Tims and Chambers (1984) originally assigned this specimen to S. australis , but Salopella laidae sp. nov. differs to S. australis in a number of its characters ( fig. 6 – comparative line drawings of Australian Salopella ). Salopella laidae sp. nov. possesses sporangia that are clearly delineated from their subtending axes, differing noticeably from coeval S. australis ( fig. 2A–E ) where the sporangia are as wide as their subtending axes. Furthermore, the sporangia in S. laidae sp. nov. do not taper from half-way to one-third from their apices as in S. australis ; instead, they hold their sporangial widths before terminating with rounded apices. While the sporangial apices appear obtuse, this cannot be assumed to have been the case in life, as Morris and Edwards (2014: 40) found when examining S. allenii that compressed specimens possessed rounded apices of varying degrees, except that one specimen of S. allenii that was preserved in 3D had a pointed apex. The parent axes leading to the ultimate dichotomy in S. australis are long, erect and aligned parallel to each other ( fig. 2A ), being easily distinguished from S. laidae , where the axes leading to the ultimate dichotomy are at an angle to the vertical. Furthermore, S. australis only possesses one dichotomy, while S. laidae possesses two, or possibly three, as suggested in Tims and Chambers (1984: text-fig. 2C). However, it should be noted the proximal axes of S. laidae sp. nov. are orientated away from each other, suggesting they may have come from a rhizomatous axis instead of a dichotomising one.

One of the parent axes of S. laidae sp. nov. is incomplete, but likely possessed four sporangia ( fig. 6C ). This is based on the relative dimensions, orientation and positions of both parent axes to each other, and that four sporangia terminate the daughter axes of the other parent axis.

The sporangia of Salopella caespitosa are ovate to fusiform and up to 6.0 mm high and 2.0 mm wide ( Tims and Chambers, 1984), which is approximately half as high than S. laidae . Furthermore, the axes of S. caespitosa are more gracile than those of S. laidae , as the latter reached a maximum width of 1.9 mm proximally and 1 mm beneath the sporangia, whereas for S. laidae axes were at least 3.2 mm proximally and 1.3 mm beneath the sporangia. In comparison to other Salopella globally, the sporangia of S. marcensis and S. xinjiangensis are much smaller than S. laidae , reaching a maximum length of 3.4 mm and 3.5 mm long, respectively, and both much narrower ( Morris and Edwards, 2014: table 3). Moreover, the sporangia of S. xinjiangensis are tongue-shaped ( Cai et al., 1993; Dou and Sun, 1983), but S. marcensis sporangia are similar in gross morphology to S. laidae , being elongate and tapered at their apices according to Fanning et al. (1992). Salopella marcensis are much smaller plant than S. laidae , reaching 6.38 mm high, while S. laidae reached at least 55 mm high ( Fanning et al., 1992: 174). While disarticulation for S. marcensis during transport seems likely, the isotomously branched axes bearing the sporangia for S. marcensis are much smaller than for S. laidae , ranging 0.05–0.5 mm wide ( Fanning et al., 1992).

In Fanning et al. (1992: fig. 35), there is a silhouette of S. marcensis that shows a sporangium occurring on a lateral axis below a further bifurcation, indicating in this case at least that sporangia did not necessarily occur at approximately the same level, as is the case with S. laidae sp. nov. Finally, the type species of Salopella , S. allenii possessed fusiform or spear-shaped sporangia ( Edwards and Richardson, 1974; Morris and Edwards, 2014) – significantly different to those of S. laidae sp. nov.

Figures

Figure 2. Salopella australis displaying parallel parent axes dichotomising into much shorter daughter axes that are terminated with elongate sporangia, delineated with a constriction just above dark sporogeneous region. All from Wilson Creek Shale on Frenchmans Spur track, 10 km west of Matlock.A, erect parent axes parallel to each other, dichotomising into two elongate sporangia from Wilson Creek Shale. Re-photographed here; originally published in Tims and Chambers (1984: pl. 32, fig. 4) and Tims (1980: fig. 4.1.9). Specimen NMV P50014. B, forked dichotomy terminated by sporangia from Wilson Creek Shale. And to the right hand side of the forked axis is another long axis, which based on its orientation may also be part of the same plant. Constriction at arrow, lower arrow at dichotomy and double arrow at two aligned axes. Specimen NMV P33219. C, close-up of fructification in A, sporangia barely extend beyond the confines of their subtending axes, with slight constriction present above sporogeneous region (at arrow). Specimen NMV P50014. D, E, holotype, part and counterpart. On part, constriction at arrow in sporangium. On counterpart, both parent axes are parallel to each other (at dotted arrow). Re-photographed here, originally published in Tims and Chambers (1984: pl. 32, figs. 1, 2). Specimens NMV P50008.1 and NMV P50008.2, respectively. F, Gen. et sp. indet. – short daughter axes terminated in elongate sporangia. The cortex may be absent from subtending axes, with only the central line visible. The lack of cortex prevents assigning to S. australis as width of subtending axis to sporangial width is required. Originally photographed by Tims (1980: fig. 4.1.13). Specimen NMV P50010.2. G, S. australis, with two short daughter axes, with constriction at arrow of the sporangium, which is the same width as its subtending axis. Specimen NMV P202886.

Figure 5. Salopella laidae sp. nov. (holotype) NMV P50011.1 and NMV P50011.2, part and counterpart, respectively, with counterpart reversed to be in the same orientation as part specimen. From location 4 (Brackley’s cutting) on Limestone Road, Yea. A, double isotomous dichotomy visible, terminated with eight elongate sporangia, five visible. At lower arrow central line and at F, folding of tissue. On sporangium seven?sporogenous region highlighted. Rephotographed; originally figured by Tims and Chambers, 1984: pl. 32, fig. 3 and text-fig. 2C. B, arrow at daughter axis missing on part present.

Figure 6. Line-drawings of Victorian early land plants with longer than wider sporangia. A, Salopella australis from Wilson Creek Shale on Frenchmans Spur track, near Matlock. Specimen NMV P50014.B, Gen. et sp. indet. from Limestone Road, Yea. Originally placed in S. australis but branching architecture clearly differs. Specimen NMV P157323. C, Salopella laidae sp. nov. from Limestone Road, Yea. NMV P50011. D, Salopella caespitosa (holotype) from Wilson Creek Shale on Frenchmans Spur track, near Matlock. Specimen NMV P202987. E, Salopella caespitosa from Ghin Ghin Road, Yea. Specimen NMV P235941.