HIGH MORPHOLOGICAL DIVERSITY OF ORGANIC-WALLED MICROFOSSILS FROM THE MID-NEOPROTEROZOIC ALINYA FORMATION, OFFICER BASIN, AUSTRALIA

Broad observations of Proterozoic eukaryotic diversity suggest major diversifications occurred in the early to mid-Neoproterozoic. However, the reliability of eukaryotic diversity trends has been questioned due to taxonomic problems inherent in dealing with the main fossil group of the time, the enigmatic, organic-walled microfossils known as acritarchs. These taxonomic difficulties include splitting due to taphonomic or ontogenetic variation and lumping due to morphological simplicity (or apparent simplicity) of forms. In some cases—as with the smooth-walled leiosphaerids—little information can be gleaned from external morphology and we are left with form-taxa that cannot inform our understanding of eukaryotic diversity. Additionally, the more complex acritarchs of this time tend to be nearly opaque and bear fine-scale ornamentation difficult to resolve with transmitted light microscopy.

Here we present a high-magnification SEM study of an acritarch assemblage from the mid-Neoproterozoic Alinya Formation, Officer Basin, Australia that has revealed a previously unappreciated level of morphological diversity and has allowed recognition of taphonomic variation in individual species. This assemblage contains as many at six new species and fossils typical of the mid-Neoproterozoic such as Simia annulare and Pterospermopsimorpha insolita as well as Valeria lophostriata, an acritarch often found in older Proterozoic units.

Detailed morphological study by SEM, a widely available but often under-utilized technique, has not only revealed micro- and nano-scale ornamentation of these fossils, but has also allowed constraint of taxonomic inflation resulting from taphonomic variability. Variations in preservation among and within individual specimens allowed reconstruction of taphonomic spectra and recognition of previously unidentifiable specimens. The richness of the Alinya Formation supports reconstructions of high eukaryotic diversity during the mid-Neoproterozoic and the discoveries made in this study advocate for routine use of SEM in primary paleontological studies of Proterozoic acritarchs both for revealing fine-scale detail and for understanding the variable faces of that taxon.