EDIACARAN ICHNOLOGY AND THE DAWN OF ANIMALS

Ichnology is providing critical information for our understanding of Ediacaran paleobiology. In the light of recent research, it is important to evaluate if the Seilacherian view of the dawn of animal in the shadow of large creatures of unknown affinities is still supported by the ichnologic record. The fact that alternative interpretations have been suggested for almost every type of Ediacaran structure may provide ground for skepticism regarding the existence of Ediacaran animal trace fossils, at least before the appearance of treptichnids. However, a review of the Ediacaran record suggests that there is still robust evidence to make the case of the first animal trace fossils by the Ediacaran. First, the direct association of Kimberella and Kimberichnus represents a rare case of a trace fossil in connection with its producer, providing clear evidence of a mat-scratching strategy by a bilaterian, irrespective of the precise phylogenetic affinity of Kimberella. Second, the presence of continuous sinuous trails with well-developed levees is hard to reconcile with a body-fossil origin (i.e. a carbonaceous filament), a protozoan trail or a microbially induced sedimentary structure. Third, the preservational styles of the structures associated to microbial mats as both positive or negative reliefs on the same surface is more consistent with a trace-fossil origin. Fourth, the view that all the simple horizontal structures identified as trails and burrows in Ediacaran rocks are not trace fossils is hard to support. Identical structures are present throughout the whole Phanerozoic and have been observed in modern sediments as well. Fifth, Fortunian deposits contain a similar suite of trace fossils preserved on matgrounds, with the addition of arthropod trackways and resting traces, suggesting the persistence of the Ediacaran taphonomic window into the Phanerozoic. In any case, Ediacaran sediments displayed almost invariably no bioturbation (BI 0) to very locally sparse bioturbation (BI 1), and very limited infaunal tiering. Critical evaluation of the available information places the earliest evidence of complex behavior in the treptichnids recorded by the end of the Ediacaran, certainly representing the prelude of the dramatic increase in complexity evidenced by the Cambrian diversification event.