Is Australian Ediacaran NeoProterozoic Fossil Genus Aspidella Really a Holdfast Trace Fossil?

Taphonomy and Morphology of the Ediacara Form Genus Aspidella

Authors:

Tarhan et al

Abstract:

Aspidella, the disk-like Ediacaran form genus, is a common and globally-distributed member of the Ediacara Biota. In South Australia, it occurs prolifically (n greater than 1000) in locally dense assemblages on the bases of sandstone beds in the eponymous Ediacara Member of the Rawnsley Quartzite. Association with stalks, fronds and textured organic surfaces (TOS) has led to the interpretation of Aspidella as the holdfast of a frondose, Charniodiscus-like organism which lived with its holdfast secured within or under a sandy microbial mat and its stalk and frond protruding above the substrate and into the water column. As the dominant component of four fossiliferous beds and a minor component of most others, Aspidella exemplifies the bed-scale faunal heterogeneity characteristic of Ediacara fossil assemblages. Aspidella itself, moreover, is characterized by strong morphological variability, including variation in the presence or absence of particular distinct morphological features, variable preservation as internal and external molds, variable relief and a broad size range. However, the distribution of morphological characters is unrelated to either Aspidella specimen size or bed assemblage composition. Herein we demonstrate that this morphological diversity is an expression of neither ontogenetic nor species-level anatomical differences, but rather is the product of differential taphonomy related to variation in local substrate-related sedimentological and biogenic factors, particularly the presence or absence of TOS composed of the eukaryotic tubular organism Funisia.