Salopella caespitosa Tims and Chambers, 1984

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 : 196-198

Salopella caespitosa Tims and Chambers, 1984 ( fig 3A–I (holotype) and fig. 4A–E )

Material: NMV P50016 (holotype) and NMV P235941 both with part and counterpart .

The circumscription of Salopella caespitosa was based on a single specimen with its counterpart from the Wilson Creek Shale Formation on Frenchmans Spur Track approximately 10 km west of Matlock in central Victoria. No new data have been added on this taxon since its inception. The discovery of a new specimen from the base of the Humevale Formation on Ghin Ghin Road near Yea preserved in semi-relief contributes information on its dimensions, gross morphology and temporal range.

Description. The new specimen’s length is 135 mm and consists of two synchronous dichotomies with longer than wide sporangia terminating each axis. The naked slender axes are 0.8–1.8 mm wide, with the axial width decreasing equally after each dichotomy. The first dichotomy occurs c. 80 mm from the base of the fossil and c. 31 mm from the second dichotomy ( fig. 4A at lower arrow). The subtending axes to the sporangia are 0.9–1.0 mm wide and 22–25 mm high with some slight variation in the width along all four axes and a slight narrower evident beneath sporangia three and four ( fig. 4B , the subtending axis to sporangium four pinches slightly about 1 mm below the sporangium). The sporangia are fusiform 1.5–2.0 mm wide and 4.8–6.0 mm high with the subtending axis slightly widening into the base of each sporangium. The widest part of the sporangia occurs in the lowest quarter length, followed by a gentle tapering to rounded sporangial tips. A sporangial wall is evident primarily but is not confined to the basal parts of the sporangia measuring 0.2–0.3 mm wide and is continuous with the subtending axes. Sporangia two, three on the part and four on the counterpart possess what appears to be an infilled oval presumed sporogenous area, which are no greater than 1.2 mm wide and 1.5 mm high. Sporangium four on the counterpart shows a fine linear cavity 0.15 mm wide entering the base of the sporogenous area and sporangium three on the part shows similar but larger opening beneath the sporogenous area, and beneath this is a fine central axis 0.1 mm wide; beneath sporangium two on the subtending axis, this axis is 0.2 mm wide.Sporangium two (part) upper half possesses longitudinally oblique striations, 0.06–0.6 mm apart.

Remarks. Comparison to holotype. The new specimen measures 50 mm longer than the holotype ( fig. 3 ), and its axial width is broadly in range with the holotype, which measures 1.1–1.9 mm wide according to Tims and Chambers (1984). What appears to be the widening of the parent axis in the specimen ( fig. 4A ) is where the first dichotomy occurs, with the two daughter axes aligned for 26 mm before diverging for the remaining 5 mm before the second dichotomy. From the holotype, the best examples of branching to compare to the specimen were found on the part specimen (right-hand side) with isotomous dichotomies subtending sporangia one–seven ( fig. 3A , fig. 6D ). Proximally on the axes ultimately terminate in sporangia one–seven show the axes emanating from a horizontal axis. This horizontal axis may represent part of a rhizomatous system or a distorted axis. Tims and Chambers (1984) appear to have considered this a dichotomy because no other part of the fossil would give three dichotomies. However, it is equivocal as to whether it is a distorted axis with a dichotomy. Nonetheless, the axes of the new specimen examined are noticeably less flexuous than the holotype; for example, on the holotype axes leading to sporangia eight and nine showing changes in orientation of about 45° in conjunction with the daughter axes of dichotomies occurring at 10° and 45°. These differences possibly represent differences in both current strength during deposition and the level of degradation of each of the plants. There is a noticeable variation in the distance between the ultimate branching points and sporangia in the holotype, from c. 6 mm as seen in sporangium 12, to c. 26 mm for sporangium seven, and up to at least 57 mm for sporangium eight, where the presumed branching point is not visible. The axes decreased after each dichotomy, as noted by Tims and Chambers (1984), but with the specimen examined herein there is a much greater decrease in axial width after the first dichotomy. Examination of the holotype did reveal a slight narrowing in the subtending axes ( fig. 3D–F ) of most of the sporangia ( Tims and Chambers 1984: pl. 34, fig. 3 ); this was observed beneath two of the sporangia on the specimen, but in both cases may be artefacts of preservation, and more specimens are required to confirm this character.

The size range of the sporangia in the holotype, 3.2–6.0 mm high and 1.3–2.0 mm wide ( Tims and Chambers, 1984), is similar to the specimen examined herein, and both have a comparable average height to width ratio of 2.6 ( Morris and Edwards, 2014: table 3) for the holotype and 3.2 herein. The holotype shows slight variation in the location of the widest point of the sporangia, occurring either in the lowest quarter ( fig. 4F ), like with NMV P 235941 ( fig. 4C ) or midway ( fig. 3D ). The minor difference is possibly accounted for by the different types of preservation. The specimen NMV P 23591 shows these sporangial bodies in semi-relief and indicate they were not flattened bodies. Furthermore, examination of sporangia of the specimen revealed an oval presumed sporogenous region in the basal part of the sporangia, which is also present on sporangium 13 of the holotype ( fig. 3H ). Tims and Chambers (1984: pl. 33, figs 3–4 ) noted that on sporangium 13 of the holotype it may either be a double sporangium or dissociation of the sporangial wall giving the appearance of a double sporangium. The evidence of a sporogenous area basally and relatively thick sporangial walls from the specimen examined herein ( fig. 4B ) suggests either a crushed sporangium or stage of senescence pertaining to dehiscence. Furthermore, some of the sporangia in the holotype have a darkened presumed sporogenous area completely filling insides of the sporangia (fig. D– F), which may have resulted from the breakdown of the wall holding the sporogenous area prior to fossilisation, as the sporogenous area occurs as a basal oval body on all sporangia of the specimen examined.

Tims and Chambers (1984: pl. 33, fig. 3 ) found central lines of 0.2 mm diameter throughout the specimen, one of which was beneath sporangium 13. On the specimen NMV P 235941, the central line was found to connect to base of the sporogenous region ( fig. 4B, C ), which Tims and Chambers (1984) had postulated. Tims and Chambers (1984) noted some axes of the holotype to dichotomise (c. 6.0 mm) relatively close to the base of the sporangia ( fig. 3C ); however, this was not the case with NMV P 235941.

The presence of longitudinal oblique striations on the upper half of sporangium two may reflect cell orientation and possible twisting as a means of dehiscence. Obliquely orientated cells in the sporangia have been recorded for some S. allenii and absent for S. marcensis ( Fanning et al., 1992; Morris and Edwards, 2014), and throughout the entire sporangia and subtending axes of Tortilicaulis ( Edwards et al., 1994) . There was no indication of twisting found for the subtending axes of the specimens’ sporangia, suggesting it may only affect the upper part of the sporangia; however, this is conjectural. As noted by Morris and Edwards (2014: 42), sporangia containing spores are required for unequivocal identification and assessment of relationships. While the specimen described is three-dimensionally preserved, we believe erecting a new taxon to segregate it from the form genus would be premature and we should wait for more data. The presence of this specimen at the base of the Humevale Formation potentially suggests a late Silurian age ( Rickards, 2000). However, as outlined in the stratigraphy section, we suggest a Lower Devonian age range for specimens found at Ghin Ghin Road until unequivocal evidence of age becomes available.

Figures

Figure 3. Salopella caespitosa (holotype) from Wilson Creek Shale on Frenchmans Spur track, 10 km west of Matlock. A, B, part (NMV P202987.1) and counterpart (NMV P202987.2), respectively; numbering follows Tims and Chambers (1984, pl. 33, fig. 1). Counterpart images are reversed to be in the same orientation as the part specimen. A, on right-hand side of part specimen, double isotomous dichotomies lead to sporangia one–seven. Note, at the arrows, there is perpendicular branching that is suggestive of a rhizomatous system. C, sporangium 12; at the arrow there is another axis that is terminated in a sporangium that is partially visible. Specimen NMV P202987.1. D–F, Evidence of a slight constriction beneath sporangia at arrows. D, sporangia eight and nine (on the left). Widest part of each sporangium occurring approximately midway along their length. Note, rephotographed; originally published in Tims and Chambers (1984: pl. 34, fig. 3). Specimen NMV P202987.1. E, sporangium 22 is c. 4.06 mm long and 1.4 mm wide. The axis decreases from 1.2 mm proximally to 0.5 mm just beneath the sporangium. Specimen = NMV P202987.2. F, sporangia four (right) and five, both sporangia are slightly wider in the lower quarter of each sporangia. Specimen = NMV P202987.2. G, sporangium 22, appears to be two immature sporangia juxtaposed. At arrow, the apex (rounded) of the smaller fusiform sporangium is apparent. Specimen NMV P202987.1. H, sporangium two, lower arrow at walls surrounding presumed oval sporogeneous area, which reaches approximately halfway the length of the sporangium to the upper arrow. Specimen NMV P202987.1. I, sporangium 13, arrows at walls surrounding sporangeneous area and upper arrow showing extent of oval sporogenous body. The walls does not recombine apically like in sporangium two, suggesting it may have been crushed, or hidden, beneath the matrix. Rephotographed; originally figured by Tims and Chambers (1984: pl. 33, fig. 3). Specimen NMV P202987.2.

Figure 4. Salopella caespitosa NMV P235941 from Devil’s Elbow on Ghin Ghin Road, northwest of Yea. A, overview of whole specimen. NMV P235941.1. B, arrow (Vt) at vascular trace entering base of oval presumed sporogenous body of sporangium four, and the subtending axis to sporangium four pinches slightly about 1 mm below the sporangium. Lower arrow at region where subtending axis is continuous with sporangial wall and upper arrow shows extent of sporangial body of sporangium three. Note, for both sporangia, the distal parts appear to be hidden in the matrix. NMV P235941.2. C, lower arrow (Vt) at vascular trace entering base of oval sporogenous body, with upper arrow at distal extent of sporogenous body on sporangium two on the counterpart. Note, no longitudinally oblique striations are evident in upper half of sporangium but are evident on the part specimen. NMV P235941.2. D, sporangium two at arrow longitudinal oblique striations on the upper half of the sporangium. NMV P235941.1. E, close-up of parent axis with longitudinal striations. NMV P235941.1.

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.