DOES THE CAMBRIAN-ORDOVICIAN “MICROBIALITE RESURGENCE” REFLECT DISASTER TAXA OR SUCCESSFUL LOCALIZED MICROBIAL ECOSYSTEMS?

It has been noted that following the decline in both abundance and diversity of microbialites at the end of the Proterozoic, archaeocyath reefs dominate the Early Cambrian and then there is a pronounced “Microbialite Resurgence” of abundance that spans the Middle Cambrian through Early Ordovician. The increase in diversity and abundance of sessile benthic invertebrates, chiefly poriferans, in the Middle Ordovician is seen as the displacing agent of the microbialites and they have been relegated to minor roles within shallow marine ecosystems ever since. The commonly cited reason for this pattern invokes the microbialites as a sort of ‘disaster taxa,’ filling an empty niche between two distinct poriferan episodes.

However, this model fails to take into account some important points recognizable in the fossil record. First, the model does not explain the proliferation of calcified microbes and the spread of thrombolites in a protracted interval spanning the Neoproterozoic through Late Cambrian. Second, the ‘resurgence’ interval was certainly not devoid of reef-building poriferans (or anthozoans, echinoderms, or brachiopods for that matter). Third, the microbialite distribution pattern spanning the Neoproterozoic-Middle Cambrian does not match one consistent for disaster taxa as there are several synecological relationships between microbialites and the various invertebrates they are supposedly superseding. At a minimum, the patterns are more complex then a simple model might explain.

As an alternative, I propose that the Neoproterozoic-Early Ordovician patterns of microbialite abundance, distribution and diversity instead suggest the success of new forms in the shallow marine ecosystems. In some cases, it can be argued that there was a return to Proterozoic conditions already suited for stromatolites; in other cases, the fossil record reflects optimum conditions for new forms of thrombolites and dendrolites. Synecologic studies of poriferan-microbial buildups (Early Cambrian archaeocyath and Late Cambrian anthaspidellid) help to explain the ecosystem distribution and provide arguments for plasticity among microbialite-constucting biota as being the root cause for their success.