Worm burrows

After other seemingly complex trace fossils have been identified as either xenophyophores or pseudofossils, there still remains a fair number of distinctive 'worm' burrows (Figure 10.6). For example, Nenoxites trails are found on top surfaces with an apparently pelleted wall, while Palaeophycus commonly is preserved as ahypichnial furrow. The positive epirelief of Aulichnites (Figure 10.6) indicates active backfilling, but the median furrow and angular bending would better fit a short, possibly molluscan trace maker. Specimens of'Aulozoon' (Figure 10.7), looking like a highly compressed sand sausage, likely are the backfilled burrows of flatworms (but see Droser et al. (2005)).

Although made by infaunal animals, these burrows penetrated no more than a few millimetres below the sediment-water interface. They can be interpreted as the works of undermat miners (Seilacher, 1999), which fed on the decaying lower zone of the biomat. Deeper and more extensive sediment mixing (bioturbation) started only after the Cambrian substrate revolution (Bottjer et al., 2000), when many animals responded to the menace of macropredation by becoming infaunal.

Trace fossils also convey another message. As one knows from later examples, biomats provide an ideal substrate for tracks of arthropods, because their pointed leg tips (mineralized or not) penetrated the mat as easily as did radula teeth. Nevertheless, no such tracks have been found in the Precambrian.

While larger arthropods were evidently absent, other trace fossils (Bergaueria, Figure 10.6) document the presence of actinia-like coelenterates. Unlike the resting traces of vendobionts (Figure 10.2), they show concentric scratch patterns, which formed when the bottom parts of the hydraulic polyps actively burrowed for anchoring the body and withdrew it into the sediment when disturbed.

In conclusion, stem groups of various modern animal phyla were certainly present in the benthic communities of Ediacaran times and increasingly so towards the end of the period. Yet they remained rare and small compared to the associated vendobionts and xenophyophores. The earliest animals neither evolved sophisticated search programs, nor ravaged the unprotected giant protists. Overall,

EDIACARAN SNAPSHOT ON BIOMAT SOLE, S. AUSTRALIA

Fig. 10.7. Inverse relief, sharp contours, and the tendency of the trails to hug each other with little overcrossing suggest that the uniformly sized individuals of Phyllozoon are preserved in situ (but see Gehling et al. (2005), p. 52) below the biomat, while the ghost impressions of Dickinsonia are probably pressed through from above the mat. In contrast, the flat sand sausages ('Aulozoon') cross over each other but respect Phyllozoon. (After Seilacher et al. (2003); see Gehling et al. (2005a), fig. 5 for a drawing of a complete slab.)

EDIACARAN SNAPSHOT ON BIOMAT SOLE, S. AUSTRALIA

Fig. 10.7. Inverse relief, sharp contours, and the tendency of the trails to hug each other with little overcrossing suggest that the uniformly sized individuals of Phyllozoon are preserved in situ (but see Gehling et al. (2005), p. 52) below the biomat, while the ghost impressions of Dickinsonia are probably pressed through from above the mat. In contrast, the flat sand sausages ('Aulozoon') cross over each other but respect Phyllozoon. (After Seilacher et al. (2003); see Gehling et al. (2005a), fig. 5 for a drawing of a complete slab.)

the peaceful 'Garden of Ediacara' (McMenamin, 1986) appears to have been dominated by prokaryote biomats and giant protozoans. Even though the taxonomic composition of the biota may have changed with time (Narbonne, 2005), the general situation remained the same throughout Ediacaran times and in various environments. What made this strange world survive through tens of million years, in spite of the presence of metazoan animals?

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