A Critical Look at the Ediacaran Trace Fossil Record

Abstract

Chapter 05- It has long been known that there was an increase in the size, complexity and diversity of trace fossils around 555–535 Ma (e.g., Seilacher, 1956; Alpert, 1977; Crimes, 1987; Narbonne et al., 1987). Crimes (1987, 1992, 1994) published important overviews of Ediacaran and Cambrian trace fossils, including tabulations of ichnotaxa in time blocks spanning the Ediacaran to the Ordovician. These showed a respectable diversity of Ediacaran trace fossils as measured in number of ichnotaxa, with one list (Crimes, 1994, table 4.2) giving 36 ichnogenera for the Ediacaran, with an increase to 66 ichnogenera for pre-trilobite lower Cambrian. Among these Ediacaran trace fossils were forms that suggest moderately complex behavior, such as guided meander traces. Also, some subsequent studies (e.g., Jenkins, 1995) 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 (Gehling et al., 2000; Jensen, 2003; Seilacher et al., 2003). It has become increasingly clear that Ediacaran strata are particularly rich in problematic structures and that these often have been mistaken for trace fossils (see Seilacher et al., 2005; Droser et al., 2005). It may not be entirely inappropriate to compare the changing view on Ediacaran trace fossils to the shift in the interpretation of Ediacaran body fossils over the last several decades. In the case of trace fossils, however, the problem is not in organismic affinities, but in recognizing what is a genuine trace fossil and what is not. 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 and for understanding of Ediacaran communities. It can be argued that trace fossils provide a good indicator for the appearance of macroscopic bilaterian animals (e.g., Valentine, 1994; Budd and Jensen 2000), and that conditions for the preservation of shallow tier trace fossils were particularly favorable in the pre-Phanerozoic (Droser et al., 2002a,b). The record of pre-Ediacaran trace fossils is at present problematic for reasons concisely summarized by Conway Morris (2003, p. 506). Even if some of these pre-Ediacaran trace fossils turn out to be genuine, their relation to Ediacaran trace fossils or to the radiation of animals is doubtful (cf., Rasmussen et al., 2002). A better understanding of the Ediacaran trace fossil record is therefore of great interest because the complexity of trace fossils may tell us something about the complexity of their producers. For example, Ediacaran imprints of arthropod-type limbs, if accepted, would provide anchoring points for likely crown-group bilaterians. An improved understanding of Ediacaran trace fossil diversity is also necessary in order to explore possible roles of trace fossils in the establishment of Ediacaran Period subdivisions. The purpose of this paper is to provide a brief critical overview of the Ediacaran trace fossil record in order to establish a more solid foundation for assessing changing patterns in the diversity and complexity of trace fossils at the end of the Ediacaran.