CYANOBACTERIA IN JURASSIC GEOTHERMAL DEPOSITS FROM PATAGONIA, ARGENTINA

GARCIA MASSINI J.L.; KRINGS, M.; GUIDO, D.; CAMPBELL K.

Conferencia; 10th European Paleobotany and Palynology Conference; 2018

Structurally preserved, filamentous cyanobacteria occur in Middle-Upper Jurassic siliceous cherts from the La Matilde Formation(Bahia Laura Group) in the Deseado Massif, Santa Cruz, Patagonia,Argentina. Specimens consist of colonies of densely spaced, heteropolar,uniseriate trichomes (> 300 μm long) that occur firmlyattached to different substrates, commonly on partially degradedland plant parts. Based on the size and morphology of the cyanobacteriaand habit characteristics, the fossils closely resemblecertain extant members in the family Rivulariaceae (Oscillatoriales),such as Gloeotrichia, Homeothrix and Rivularia. The presenceof Rivulariaceae in the La Matilde Fm geothermal deposits is suggestiveof calcareous aquatic settings with periodically variableconcentrations of available phosphate and fluctuating anoxia onlake floors, much like the settings of some lacustrine-associatedmodern hot-spring environments where representatives of thisfamily of cyanobacteria also are common. Affiliated with many ofthe cyanobacterial colonies are spheroidal to ovoid vesicles, up to45 μm in diameter, which occur singly or in small clusters (coenobia?),and typically are located near the bases of the cyanobacterialfilaments. The vesicles resemble certain present-day lower fungi,algae, and cyanobacteria, but their systematic affinities remain unresolved.Nevertheless, their co-occurrence with the filamentouscyanobacteria suggests that the cyanobacterial overgrowths fromthe La Matilde Fm were not monospecific, but rather consisted ofseveral different organisms that perhaps even interacted with eachother. For example, the fossil association is reminiscent of associationsbetween present-day Rivulariaceae and certain other cyanobacteriain modern hot-spring environments, which are effectivein warding off cyanobacterivorous microinvertebrates.