Herein, we report the presence of a plant paleocommunity dominated by ferns of the family Osmundaceae embedded in siliceous chert blocks from the only known Mesozoic, fossiliferous geothermal deposits, from the La Matilde Formation (Middle-Upper Jurassic), in the Deseado Massif, southern Patagonia, Argentina. A total of 13 blocks sampled in an area of approximately 250 m2, which includes multiple structurally preserved, monotypic, in-situ, rhizomatous stems of Osmundaceae, embedded within their original swampy substrate are documented. Additional Osmundaceae and other ferns, and conifers are present in chert levels adjacent to the previous blocks. This mimics similar monotypic associations of Osmundaceae in high-altitude lagoons in northeastern Argentina in the middle of the Paraná Forest, suggesting that the fern assemblage found occupied similar open spaces in the Jurassic, becoming preserved in analogous configurations. In addition, observation of the preserved fern tissues and surrounding substrate revealed a variety of interacting microorganisms, which are comparable to those that make up the microbiological communities inhabiting modern osmundaceous environments. This finding reveals a case of ecological stasis, where members of the same fern family separated by more than 150 million years formed similar exclusive groupings of individuals in microecosystems of comparable structure and general characteristics.