Microbial origin of the Ordovician stromatoporoid-like organism Zondarella from the Argentine Precordillera and the post- Cambrian persistence of stromatolite microbialite reefs

Abstract

A laminated, dome-shaped structure (with stromatolitic features) forming reefs in the Ordovician of western Argentina was previously described as a probable primitive form of stromatoporoid named as Zondarella. New collected material allowed to realize that the laminae are constituted by well-preserved filaments, assigned to the cyanobacterium Acuasiphonoria Liu et al., 2016, which forms the lamination seen in “Zondarella” alternating filament density and orientation. According to this, the stromatoporoid status of the growth form named “Zondarella” is no longer sustainable, and is considered to be a skeletal stromatolite. Stromatolites occur widely in Furongian and Early Ordovician rocks, slightly predating the Ordovician cyanobacterial calcification episode, indicated by a many-fold increase in global calcified cyanobacteria diversity during the late Middle to Upper Ordovician. The Acuasiphonoria stromatolites of the mid- to upper San Juan Formation are earliest Dapingian, occurring together with renalcids and Allonema boundstone. Reefs of the Argentine Precordillera prove the persistence of stromatolite reefs without a conspicuous metazoan framework into the early Middle Ordovician. The lower Dapingian reefs bearing calcified cyanobacterial stromatolites of the San Juan Formation are morphologically and dimensionally comparable to lower Cambrian reefs, sharing similar paleoecological features such as community structure and composition; they also record an increase in calcimicrobial diversity and appeared directly above the Basal Dapingian Negative Isotopic Carbon Excursion. This suggests that a replication of similar environmental or oceanographic states, might have triggered the return of comparable ecosystems.