A new Jurassic chert deposit with exceptionally preserved plants and microorganisms from Patagonia, Argentina

GARCIA MASSINI J.L.; ESCAPA I.E.; GUIDO D.M.; CHANNING A.

Mendoza

Congreso; 4th International Paleontological Congress; 2014

International Paleontological Association

Jurassic geothermally-related chert deposits in the Deseado Massif, Patagonia, Argentina, have been known for some time, but its associated biota has only been recently revealed in a limited number of localities. Here we report on the discovery of a large complex of well exposed Middle-Upper Jurassic epithermal siliceous deposits represented by organic-rich cherts preserved in distal paleoenvironments within the geothermal system from La Bajada. These deposits represent a series of several distinct chert-bearing localities including different stages of community development set within a volcanic setting and contain an exceptionally well preserved multitude of in-situ and transported silicified microorganisms and plants from middle latitudes in western Gondwana. This provides information about unknown aspects of the diversity, interactions and associations of plants and microorganisms in middle Mesozoic terrestrial ecosystems at a critical time of evolution of life on Earth. The microorganism assemblage contains vegetative and sexual structures of fungi, fungus-like organisms, such as oomycetes, algae and other protists, cyannobacteria, as well as numerous additional remains of unknown affinity. Microorganisms are preserved isolated in the chert matrix or directly associated with plants and other organic remains in mutualistic, parasitic and saprotrophic engagements. Also present are a large number of coprolites and arthropod and crustacean parts, which, along with the named microorganisms provide further evidence about trophic relationships in the ecosystem. The plant assemblage includes disarticulated as well as organically connected conifer, cycad and fern and sphenophyte organs of variable preservation, preserved in-situ, in-life position or transported. In the assemblage also are numerous palynomorphs, including fern sporangia and saccate and monocolpate pollen consistent with the plant taxa identified based on larger remains. Paleoecological inferences from fossils present are consistent with the development of freshwater settings variably influenced by geothermal activity. Overall, this record represents a unique window into the evolution and diversification of ancient plant and microorganism floras during the Mesozoic. Most importantly it provides base line evidence to start building up our understanding of the composition, trophic interactions and, ultimately, the structure and dynamics of middle Mesozoic terrestrial ecosystems.