Applications of fossil taxonomy in palaeoenvironmental reconstruction: a case study of ostracod identification and diversity in Permian-Triassic boundary microbialites

The value of taxonomy as a tool in palaeoenvironmental analysis depends on accuracy of determination of relevant taxa; in cases where taxa present unresolved problems of distinction (identification uncertainty and overlap), difficulties may exist in their application in facies studies. A prime example is found in ostracods of the Permian-Triassic boundary interval, considered here in sequences from south China. Low-latitude shallow-marine carbonate facies in the immediate aftermath of the end-Permian extinction (EPE) have common widespread microbialite biostromes containing abundant shelly fossils including ostracod assemblages not found elsewhere, stimulating the earlier idea that the microbialite was a refuge from stressors of extinction. These assemblages are dominated by the Family Bairdiidae that are mostly smooth-shelled ostracods notoriously difficult to resolve into sub-familial taxonomic units. Studies of ostracod taxonomy require a careful approach of integrating cornerstone aspects of their biology such as ontogenetic development and sexual dimorphism to disentangle taxonomically discrete groups. These significant difficulties of taxonomic resolution have a knock-on impact on application of the faunas in facies analysis; several studies remain open to interpretation because of these issues. Resolution of ostracod taxonomy is critical to the refuge hypothesis, because ostracods (mostly as complete closed carapaces, including juveniles and adults) accumulated in the microbialite; thus shell morphology is critical to analysis of ostracods in the microbialites so understanding the taxonomy is vital. The microbialites comprise two main facies where ostracods are abundant: (1) layers consisting of microbial components and intervening micrite; (2) uncommon shell-rich lenses of packstone-grainstone fabric between microbial layers. The refuge hypothesis is considered unlikely by some authors, who instead interpret the microbialites as a taphonomic window for imported ostracod shell preservation. However, post-extinction microbialite sheets are extensive on shallow marine carbonate platforms in Tethys and show little evidence of physical damage. We deduce that, instead of a taphonomic window, the well-preserved ostracod assemblages lived on the microbialite, and that the 'refuge hypothesis' remains viable. The concepts discussed in this study may be applicable to other fossil groups where taxonomic problems are acute.