Taungurungia garrattii, McSweeney & Shimeta & Buckeridge, 2022

McSweeney, Fearghus R., Shimeta, Jeff & Buckeridge, John St J. S., 2022, Taungurungia gen. nov., from the Lower Devonian of Yea, central Victoria, Australia, Memoirs of Museum Victoria 81, pp. 43-53 : 45-48

Taungurungia garrattii sp. nov.

Description (based on one specimen part and counterpart): The specimen consists of six parent axes that are 2.4–7.5 mm wide and taper gently acropetally; three of these axes are poorly preserved. The axes are oxidised and golden to yellow in coloration, with a vascular trace evident in parts of the axes, most notably proximally measuring 1.6 mm wide. Three of the larger axes terminate in a single sessile large ovate sporangium (fig. 3a). One is poorly preserved with only part of its apical region preserved (fig. 3b). The largest sporangium is 7.5 mm wide and 15.5 mm long and is ovate, reaching its maximum width 6.4 mm up from its base (figs 3, 4a, b). The sporangium does not possess a stalk, appears sessile and is partially embedded in the axis with a curved junction. Another large sporangium occurs low on the specimen (figs 3, 5) and is 14.7 mm long and 5.6 mm wide and is missing part of its cast basally. This sporangium sits with part of its basal region embedded into its subtending axis, again with a curved junction. The distal region of this sporangium is more elongate. These two sporangia have two fractures running transversely at an oblique angle. There is no evidence of a bounded region along these fractures to indicate they are related to dehiscence. Additionally, there is no evidence of a marginal rim preserved on any of the sporangia.

Emergences are 0.6–1.5 mm wide and 1.3–3.4 mm long, varying in morphology according to length (fig. 6) and more occur on the largest axis than the other axes. The smaller emergences are deltoid, and the larger emergences are elongate and perpendicular to the parent axes. One elongate emergence extends perpendicular to the parent axis for about 0.3 mm before re-orientating at about 45° to the axis (fig. 6d). One of the deltoid emergences has a fine vascular trace 0.07 mm wide along its length (fig. 6b).

Fusiform bodies (figs 7, 8) up to 0.5 mm wide and 1 mm long occur on the axes, most occurring distally but in two instances are found to occur in discrete areas of no more than 8 mm long and 2 mm wide on the sides of the main parent axes (fig. 8a, b). Dégagement of the distal region of one of the daughter axes revealed fine fusiform bodies with short stalks (fig. 7a, b). Additionally, similar fusiform bodies were found to occur on the narrow lateral axes that emanate from the main parent axes at almost right angles (fig. 7e), and on the counterpart, faint impressions of numerous fusiform bodies were found on the large central terminal sporangium (fig. 4c). Dégagement above the main large terminal sporangium revealed more of these fusiform bodies extending beyond the large sporangium (figs 4d, 9). There is no connection between the small fusiform bodies and the large terminal sporangium.

Three types of branching are evident. The first is where a parent axis branches anisotomously and is only seen once, occurring 33 mm from its apex producing a daughter axis 2.9 mm wide that tapers to 1.5 mm wide (figs 3, 7a). The daughter axis and distal region of the parent axis both appear lax. The second type of branching occurs along the sides of the parent axes and consists of narrow lateral axes usually branching perpendicular from the parent axis. These narrow lateral axes are about 0.2 mm wide and up to 2–3 mm long and are sparsely distributed on the parent axes; some occur just beneath the large terminal sporangium of the central axis (fig. 7e). None of these narrow lateral axes branch, and they possess two to three fusiform bodies interpreted as dormant buds or sterile sporangia, one of which is always terminal. One large example of a lateral axis occurs proximally off one of the parent axes and is 15 mm long, varying in width from 0.3–1.1 mm, curving orthotropically with a terminal fusiform body 0.24 mm wide and 0.53 mm long (fig. 7c, d). A third type of branching occurs basally on the central axis with an oblique axial extension forming an approximate K-branch (fig. 3a), with some smaller protuberances, possibly remnants of fusiform bodies (fig. 8c), close to each other. The acroscopic part of this vegetation is similar in morphology to the distal regions of the two large terminal sporangia.A portion of axis was extracted from a poorly exposed axis (fig. 3) within the matrix, revealing longitudinal structures possibly indicative of the cell’s original orientation (fig. 10).

Comments: The location of sporangia has been used by many workers ( Edwards et al., 1989; Gensel 1992; Kenrick and Edwards, 1988; Niklas and Banks, 1990) to differentiate zosterophylls into either the Gosslingiales or Zosterophyllales. Zosterophyllales have both lateral and terminal sporangia. However, the presence of emergences suggests affinities with the Gosslingiales; but Gosslingiales lacks terminal sporangia ( Hao and Xue, 2013). Additionally, the sporangia of Taungurangia garrattii are not reinform or globose as in zosterophylls, which precludes assignation to Zosterophyllum ( Edwards et al., 2016; Gensel, 1992: 455). Instead, the terminal sporangia are noticeably elongate and lack a thickened zone bounding a dehiscence line as seen in most zosterophylls ( Hao and Xue, 2013). The apparent limited branching and single terminal elongate sporangium suggests rhyniophytes affinities, but while branching appears limited, numerous slender axes emanate from the parent axes. Based on the primary characteristics – notably the presence of emergences, K- or H-branching, conspicuous large terminal elongate sporangium and noticeably sparse branching – we provisionally place Taungurangia garrattii in the Class incertae sedis.

We believe the small fusiform bodies may represent buds or sterile vestigial sporangia (figs 7, 8). This is a similar conundrum to the original consideration of what were originally believed to be sporangia along the axes of Sawdonia ornata (Dawson) Hueber but were later found to be buds/ arrested apices ( Gensel and Berry, 2016: 619; Hueber, 1992: fig. 3). We found no fusiform bodies of intermediary size with the large terminal sporangia; instead, they are all broadly the same size. Furthermore, most of the fusiform bodies have narrow subtending axes that clearly differ from the large sporangia, which are sessile. If these fusiform bodies were to grow to a similar size as the terminal sporangia, they would likely have caused the plant to become unstable because they occur on narrow lateral axes and at the end of daughter axes.

The oblique region of extended vegetation (?K- or H-branching) seen basally is indicative of creeping vegetation, such as that seen with Discalis longistipia Hao (1989: 159) and other zosterophylls ( Walton, 1964). Branching frequency and pattern from the emergences could not be determined. The lax appearance of the daughter axes (fig. 7a) could be interpreted as being almost recurved, a similar pattern of growth as circinate axes ( Lyon and Edwards, 1991: 327) associated with indeterminate growth ( Niklas and Banks, 1990). However, we consider this lax appearance to be due to partial wilting because there is no evidence of recurved growth on the smaller axes, which would be expected for this characteristic.