332


Journal of Paleontology 92(3):323–335


Figure 7. SAM P35704a, b from Nilpena Farm: (1) Part (P; facing downward) and counterpart (CP; facing upward) of SAM P35704a, b. (2–4) Secondary branches stemming from primary branching stalk and oriented in the opposite direction (directed proximally toward the base of the petaloid; arrows) from typical orientation. This suggests that the secondary branches are free and, under specific taphonomic settings, can come to rest in the opposite direction. Diameter of Australian $1 coin=2.5 cm. Stalk, stem, and disc not preserved. Scale bars=1 cm or 1cm increments.


specimens displaying secondary elements (Jenkins and Gehling, 1978; Laflamme et al., 2004). Detailed study of numerous three- dimensional specimens preserving the secondary branches may offer an additional explanation. As demonstrated in Figure 6, the primary branches are sausage-shaped and lack secondary branching. However, when the most distal branch is broken away (Fig. 6.2), a series of secondary branches presumably lying under the overlying primary branch are exposed. Secondary branches can be seen in their entirety as downwardly directed and teardrop-shaped structures (Fig. 6.2–6.4). As secondary branches are basally anchored to the primary branch stalk, which forms a midline in the center of the primary branch,


the distal ends of secondary branches come to rest overtopping subsequent primary branches (Fig. 5.1–5.2), similar to over- lapping shingles on a house. It is proposed that primary bran- ches lacking secondary branches represent the dorsal side of the petalodium, and that secondary branch preservation is depen- dent on which side of the frond is cast. A second specimen preserved as both part and counterpart showcases the feature- less, presumed dorsal side of the petalodium in addition to the secondary branches. In Figure 7, the proximal portions of the primary branches lack any morphological detail until a break in the distal portion of the rock (arrow in Fig. 7.1 CP) reveals the ventral side of the primary branch, complete with well-defined


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