Laflamme et al.—Three-dimensional preservation of Arborea
329
Figure 4. Primary branching morphology in Arborea from the Ediacara Member of the Pound Subgroup. Branches arranged in increasing quality of preservation, from poorest-preserved branches to highest-quality preservation with secondary and possibly tertiary branching exposed. (1) Specimen P34499 displaying parallel rectangular primary branches with possible secondary branching structures perpendicular to the primary branches. (2) Specimen P13801b with peg-like margins to the primary branches that pass beyond the margin of the stalk, suggesting the anchoring site is internal to the stalk and preserved as a composite mold. (3) Specimen P14307 with primary branches composed of peg-like structures. (4) Specimen P40775 displaying an arched primary branch with distinct secondary branching emerging from the base of the branch and overlying the base of the subsequent adjacent branch. (5) Specimen P40776 with well- defined cylindrical primary branch stalks and rounded to globular secondary branching. Secondary branching begins in the middle of the primary branch, attached to the primary branching stalks. (6) Specimen P40786, with higher-relief primary branches and with evidence of an organic sheet behind the branches, attaching them together and preventing significant branch movement. All scale bars=1 cm.
the folding or rolling-up of the soft-bodied primary branches at their distal tips.
Comparisons.—Arborea is most similar to Charniodiscus in construction, with both genera sharing an ovate petalodium attached to a stem and anchored to the substrate by a circular to bulbous holdfast. Arborea can be distinguished from Charnio- discus concentricus by the bifoliate petalodium and the smaller number of primary branches that are more uniform in size and shape.
Reconstruction of Arborea
A detailed reconstruction of Arborea is possible by comparing the best-preserved two-dimensional molds of Arborea with the newly discovered three-dimensional specimens. The most important change proposed by the reevaluation of Arborea concerns the morphology of the petalodium. The morphology of the stalk varies from a smooth, featureless rectangle to a
zigzagging central axis, with numerous examples forming a transition between end members (Fig. 3). Cylindrical stalks are
believed to represent instances in which the stalk was quickly filled with sediment thus resisting compaction or, alternatively, was buried rapidly and therefore did not undergo significant decay prior to burial, which would have compromised the strength of the integument. This morphotype typically lacks the preservation of the anchoring points of the primary branch stalks, presumably due to the difficulty in preserving composite molds when the stalk is sand-filled. The sinuous stalk is believed to represent the compaction of the stalk and the resulting com- posite molding of the alternating primary branch stalks as they overprint the central stalk (Fig. 3.5). The primary branches also vary greatly in shape due to
different degrees of preservation. For example, the gradation from rectangular primary branches that tend to lack secondary branches (Fig. 4.1) into sigmoidal branches with well-defined secondary branches (Fig. 4.4–4.6) can be explained through taphonomic variation. Cylindrical branches are typically
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