Laflamme et al.—Three-dimensional preservation of Arborea


327


Figure 2. Circular Arborea holdfasts attached to cylindrical stem. (1) Specimen P14186 with large central boss and several radial striations most likely representing soft-tissue folding during compression. (2) Specimen P40332 with deflated disc similar in size to the stem. Scale bars=1 cm.


branches. Probable secondary branches preserved on isolated branches. Stem and holdfast not preserved.


Remarks.—Circular discs closely associated with stems and fronds (Fig. 2) are interpreted as rooting holdfasts. In relatively low-energy settings, frond specimens are typically attached to an anchoring disc (Laflamme et al., 2004). However, the higher- energy setting of Ediacara, South Australia, results in the over- whelming majority of circular holdfasts being disassociated from their respective fronds, having been presumably plucked away from the sediment, leaving distinct sedimentary structures in their place (Tarhan et al., 2010). Furthermore, several instances of surface expressions of collapsed craters with stalks likely represent deflated or collapsed holdfasts (Tarhan et al., 2010). Holdfasts were bulbous in shape, resulting in a promi- nent surface expression in addition to a significant portion of the holdfast that lay beneath (or within) the microbial mat, pre- ferentially biasing its preservation over the complete frond (Laflamme et al., 2011). To date, no known trace fossils have been found directly associated with the discs (although see Liu et al., 2010; Menon et al., 2013), so whether the frond was capable of burrowing with its rooting holdfast is difficult to speculate. The stem in Arborea (Figs. 1, 2.2) is typically flattened and


rectangular in most specimens, although it is interpreted as being originally cylindrical in shape. The stem was most likely composed of a structurally rigid material that restricted compaction during fossilization (Laflamme et al., 2004). In South Australia, frond stems are uncommon but represent the basal extension of the stalk (Fig. 2.1). The stem continues into the petalodium to form the stalk


(Fig. 1). The stalk varies in shape, from strongly parallel-sided (Fig. 3.1) to an alternating, sinuous (zigzagging) central axis (Fig. 3.2–3.4). The stalk is typically preserved in positive relief, suggesting it may be constructed of similar material as the stem. The variation in stalk morphology has been suggested as a means of taxonomically differentiating Arborea species (Jenkins and Gehling, 1978), although reevaluation of speci- mens from South Australia reveals a seamless transition between specimens representing a parallel-sided cylindrical


stalk and those with a sinuous, alternating stalk (Fig. 3.5). In specimens with a parallel-sided stalk (Fig. 3.1), the primary branches appear to terminate at the stalk boundary, whereas in specimens with a sinuous stalk, the primary branches go beyond the boundary of the central stalk and meet at the central axis (Fig. 3.2). In these cases, the outer margin of the central stalk appears sinuous in shape due to the alternating branching pattern along the central axis. It is possible that the cylindrical and sinuous stalks represent the dorsal and ventral aspects of the fronds, respectively. This hypothesis would imply that the anchoring points of the primary branches, whether they be found on the ventral or dorsal side, are only preserved when the respective side of the frond is preserved. This may be the case in some instances; however, specimen P40444 (Fig. 3.4) show- cases an alternative interpretation. This specimen shows a transition between end members: a cylindrical (Fig. 3.4A) to a sinuous (Fig. 3.4C) stem. Wepropose that the difference in stalk shape could represent a taphonomic artefact resulting from compression of the cylindrical stem and the composite molding of the alternating primary branching stalks. The primary branches are attached to the central stalk,


while the smaller secondary branches stem perpendicularly from the primary branch stalk (Figs. 1, 4). Reported tertiary branches are typically poorly preserved and likely represent a taphonomic artefact resulting from wrinkling or folding of soft tissue. Presumed well-preserved examples of secondary branch- ing lack any evidence of tertiary branching, and instead suggest a smooth, teardrop shape for the secondary branches (Figs. 5–7). Primary branches in Arborea are strongly parallel along their entire length, suggesting that they are attached to each other or to a dorsal sheet (Fig. 4.6) or strongly anchored to the stalk proximally, and distally connected to an outer rim (Fig. 1.1; although see Fig. 8 and discussion in the following). The primary branches vary in shape, from cylindrical (Fig. 4.1) to sinuous with strongly tapering proximal and distal terminations (Fig 4.6).


Secondary branches vary in shape, from straight to slightly


curved structures emanating upward (rarely downward) from the primary branches (Figs. 5–7). In rare instances, the secondary branches are preserved as teardrops with the tapering


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