The discovery and description of an extremophile microcosmos in a Cretaceous paleogeothermal field

Abstract

We report a new Fossil-Lagerstätte at the Cretaceous hydrothermal field ofSanagasta, La Rioja, northwestern Argentina, which opens a window into thepaleobiology of an exceptionally preserved fossil ecosystem. Active geothermalsystems harbor a diverse microbial life that thrive in high temperature, uniquemineral composition and fluctuating pH. These environments are dominated by ameso- and hyperthermophilic community, including bacteria and archaea, thatproliferate around the hot springs and geysers. In these environments, fossilization occurred rapidly by abiotic factors and through biomineralization—chiefly biosilicification. This process is mediated by thermophilic microorganisms, mainly cyanobacteria, allowing for the exceptional preservation of the biota. Here,we describe an ~80 million-year-old fossil extremophile biocoenosis encrustedwithin microbialites from the Sanagasta paleohydrothermal field. The core of themicrobialites was explored using petrography, Scanning Electron Microscopy(SEM) and Energy-Dispersive Spectroscopy (EDS) analyses in SEM. These microbialites exhibit remarkable three-dimensional preservation of microorganisms through extensive biosilicification, directly mediated by the extremophile communities. The taxonomic range of the sinterized assemblageencompasses the three domains: a noteworthy array of thermophilic ‘prokaryotes’(chiefly bacteria and archaea), along with a rich diversity of eukaryotic ‘protists’microorganisms (i.e., amoebozoans, myxomicetes, red algae, diatoms, andalveolates), green algae, plant remains, fungi and a variety of arthropod remains, all preserved in notable microanatomical details. The Sanagasta GeoPark containsone of the most diverse and exquisitely preserved records of microfossils known to date, illuminating a complex inland ecosystem within the first documented Cretaceous terrestrial hydrothermal environment.