Problems In The Interpretation Of Ediacaran Trace Fossils

General principles on which to distinguish trace fossils from body fossils and structures of inorganic origin have been discussed by Ekdale et al. (1988). Jensen et al. (2005a) and, in particular, Droser et al. (2005), discuss the identification and preservation of Ediacaran trace fossils. For example, signs of sediment displacement, such as raised levees, provide convincing evidence for a trace fossil origin. Sediment-filled shrinkage cracks (Fig. 1A) are the inorganic structures most commonly reported as trace fossils. However, a more serious problem is that making the distinction between Edicaran body fossils and trace fossils is surprisingly difficult. We believe that expectations from the Phanerozoic, where a trace fossil-like bedding-plane structure generally is a trace fossil has played a part in causing an inflation of Ediacaran ichnotaxa. Ediacaran siliciclastic sediments appear to have had conditions particularly favorable to the preservation of non-mineralized organisms as casts and molds, including well-known forms such as Aspidella and Dickinsonia (Gehling, 1999). A contributing factor was the extensive development of biomats, which led to the formation of protective husks of mineral precipitation, and also protected the carcasses from physical disturbance (Gehling, 1999). The absence of deep and intensive bioturbation (e.g., Droser et al., 1999) also meant that such structures were not destroyed when preserved close to the sediment-water interface. There were Ediacaran organisms with a morphology that, when preserved as casts and molds in siliciclastic sediments, are easily mistaken for trace fossils. It is important to note that these were as much part of the Ediacaran biotas as emblematic forms like Dickinsonia (see Droser et al, 2006). The majority of these organisms are poorly understood and remain little studied, but in terms of gross morphology two types are particularly important for the study of Ediacaran trace fossils.

Ediacaran Images

Figure 1. Trace fossil-like Ediacaran body fossils and inorganic structures. (A) Irregular sand-filled shrinkage cracks on sole of thin sandstone bed. Nama Group, Namibia. (B, C) Different styles of preservation of palaeopaschichnids (?algae or protists) resulting in similarity to meander trace fossils or rows of fecal pellets, Ust Pinegia Formation, White Sea area, northern Russia. (D) Branching tubular fossil with transverse grooves. Fossils of this type have been interpreted as fecal rows. Ediacara Member, South Australia. (E) Tubular organism (probably a sabelliditid) preserved as dark films. Note angular termination near center of image, and smooth curvature as well as folds with reduced tube width. Ibor group, central Spain. (F) Looping shallow furrows interpreted as trace fossil by Vidal et al. (1994), but probably representing a tubular organism. Light from lower right. Domo Extreme├▒o group, central Spain. (G) Looping filaments preserved as dark film. Note uniform width of film. Ibor Group, central Spain, terminal Ediacaran or earliest Cambrian. Scale bars represent 10 mm in A; 2 mm in B and C; 20 mm in D; 5 mm in E, F and G.

Figure 1. Trace fossil-like Ediacaran body fossils and inorganic structures. (A) Irregular sand-filled shrinkage cracks on sole of thin sandstone bed. Nama Group, Namibia. (B, C) Different styles of preservation of palaeopaschichnids (?algae or protists) resulting in similarity to meander trace fossils or rows of fecal pellets, Ust Pinegia Formation, White Sea area, northern Russia. (D) Branching tubular fossil with transverse grooves. Fossils of this type have been interpreted as fecal rows. Ediacara Member, South Australia. (E) Tubular organism (probably a sabelliditid) preserved as dark films. Note angular termination near center of image, and smooth curvature as well as folds with reduced tube width. Ibor group, central Spain. (F) Looping shallow furrows interpreted as trace fossil by Vidal et al. (1994), but probably representing a tubular organism. Light from lower right. Domo Extreme├▒o group, central Spain. (G) Looping filaments preserved as dark film. Note uniform width of film. Ibor Group, central Spain, terminal Ediacaran or earliest Cambrian. Scale bars represent 10 mm in A; 2 mm in B and C; 20 mm in D; 5 mm in E, F and G.

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