Trace fossil preservation and the early evolution of animals

The trace fossil record is an important element in discussions of the timing of appearance of bilaterian animals. A conservative approach does not extend this record beyond about 560-555 Ma. Crucial to the utility of trace fossils in detecting early benthic activity is the preservational potential of traces made close to the sediment-water interface. Our studies on the earliest Cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the Phanerozoic. This can be attributed both directly and indirectly to the low levels of sediment mixing. The low levels of sediment mixing meant that thin event beds were preserved. The shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment-water interface, increasing the likelihood of recording shallow tier trace fossils in muddy sediments. Preservation of surficial trace fossils in this type of setting remains problematic but the above factors suggest that also these can be expected to have left a reasonable record. Overall, the trace fossil record can be expected to provide a sound record of the onset of bilaterian benthic activity. The lack of convincing trace fossils significantly before the Cambrian supports models of late appearance of macroscopic benthic bilaterians. (c) 2004 Elsevier B.V. All rights reserved.