EDIACARAN TRACE FOSSILS: “TUBEY” OR NOT TO BE

It has long been recognized that there was an increase in the size, complexity and diversity of trace fossils between about 555–535 Ma. Previous tabulations of ichnotaxa in time blocks spanning the Ediacaran to the Ordovician have reported as many as 36 ichnogenera for the Ediacaran, with an increase to 66 ichnogenera for pre-trilobite Lower Cambrian. Among these Ediacaran trace fossils were forms that suggested moderately complex behavior, such as guided meander traces. Also some subsequent studies have suggested a great diversity of Ediacaran trace fossils including arthropod-type scratch marks. The actual diversity and complexity of Ediacaran trace fossils have, however, recently come under scrutiny as alternative interpretations for several of these Ediacaran trace fossils have emerged (e.g., Gehling et al., 2000; Jensen, 2003; Seilacher et al., 2003). It has become increasingly clear that Ediacaran strata are particularly rich in problematic structures, microbial mat structures and the three-dimensional preservation of non-mineralized tubular organisms and that these often have been mistaken for trace fossils. While the presence of widespread mats resulted in trace fossil taphonomy largely distinct from the Phanerozoic, there is no reason to believe that functional criteria used for identifying Phanerozoic trace fossils cannot be applied to Ediacaran structures. Detailed functional and taphonomic re-examination of previously described trace fossils from the Ediacaran strata of the Flinders Ranges and environs show that nearly all of the structures previously interpreted to be trace fossils are not and that Type B of Glaessner (1969), a horizontal trace fossil occurring in both negative hypo- and epirelief remains the only structure convincingly made by an animal moving through sediment. A better understanding of the diversity of Ediacaran trace fossils is not merely an exercise in numbers, but has direct impact on questions of animal evolution. It can be argued that trace fossils provide a good indicator for the appearance of macroscopic bilaterian animals and that conditions for the preservation also of shallow tier trace fossils were particularly favorable in the pre-Phanerozoic.