Pteridinium (Pflug, 1970)

Elliott, David A., Vickers-Rich, Patricia, Trusler, Peter & Hall, Mike, 2011, New evidence on the taphonomic context of the Ediacaran Pteridinium, Acta Palaeontologica Polonica 56 (3), pp. 641-650 : 642-644

Pteridinium taphonomy

Pteridinium fossils are preserved within a well−sorted quartzite containing mica flakes visible in hand specimen, and occur in both negative and positive relief. Beds of Pteridinium are present in at least three widely separated localities at Farm Aar. These fossiliferous beds can extend over several metres. The discontinuous nature of the outcrop prevents mapping in entirety. Pteridinium fossils lie parallel to the bedding but exhibit no preferred orientation.

At one locality, a Pteridinium −rich bed is underlain by a scour−and−fill structure ( Fig. 3 ), indicating that Pteridinium deposition occurred as part of a high−energy mass flow event. At two other localities, extensive dish structures are present, the largest outcrop covering an area of at least 60 square metres. The dish structures can be stacked up to 26 layers deep, forming a distinctive facies composed entirely of well−sorted quartzite ( Fig. 4 ). Two specimens of Pteridinium are present within this facies, as well as rare features that resemble aspects of Pteridinium anatomy ( Fig. 4B ). At one of the two localities where they are observed, the dish structures are present less than a metre stratigraphically below a bed of Pteridinium . The structures superficially resemble linguoid ripples (see Wynn et al. 2002: fig. 5b), but they lack a convex ripple crest. It is possible that they represent a form of load−casting, but this must be considered unlikely on the basis of the uniformity between underlying and overlying sediment and the thinness of overlying beds. The structures are preserved concave−upward, and the possibility that the facies has been overturned can be discounted, as at one of the two sites these structures are present above a sequence of sediments of the Kliphoek Member with distinct cross−bedding and a clear younging direction. This fact rules out phenomena such as gas doming ( Gerdes et al. 1993) and hummocks in the sediment, and obscures comparisons with those cyanobacterial mats that display a convex−upward “domal” morphology ( Scheiber 1999). Apart from the inclusion of at least two fossil Pteridinium , there are no immediately identifiable biologically−controlled features. The surfaces do not doi:10.4202/app.2010.0060

resemble previously identified sedimentary structures associated with microbial mats (see Scheiber 2004 for an overview), and thus cannot be regarded as microbially induced sedimentary structures ( Noffke et al. 2001; Noffke 2009), leaving dish structures as the most robust interpretation.

Figures

Fig. 3. Bed of Pteridinium fossils under− lain by a scour−and−fill structure. Lamina− tions are visible in the underlying sedi− ment, cross−cut by the material containing Pteridinium fossils. From the top of the Lower Kliphoek on Farm Aar (see Figs. 1 and 2).

Fig. 4. A. Side view of a section bearing dish structures. The top of the bed is toward the top of the photograph. B. Pteridinium fossil embedded within dish structures, indicating that Pteridinium fossils formed a component of consolidating sediment, underlying rapidly deposited beds. Both from the top of the Lower Kliphoek on Farm Aar (see Figs. 1 and 2).