CYANOBACTERIA IN JURASSIC GEOTHERMAL DEPOSITS FROM PATAGONIA, ARGENTINA

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

mendoza

Congreso; 4th International Paleontological Congress; 2014

International Palaeontological Association

This papers describes structurally preserved in-situ, in-life-position coccoid and filamentous cyanobacteria from the only known Mesozoic fossiliferous geothermal deposits. Fossils are silicified within cherts from the best known geothermal localities of the La Matilde Formation in the Deseado Massif, Patagonia, Argentina. Coccoid cyanobacteria are morphologically variable and represented by individual spherical or nearly spherical cells arranged in up to three perpendicular planes in sheathed or naked, loose, irregular forms or cylindrical to cubical colonies of varying numbers of units. These fossils are similar in morphology and habit to extant benthic colonial Chroococcales, such as Myxosarcina, Cyanosarcina and Gloeocapsopsis. Other cyanobacteria in the fossil deposit consist of uniseriate filaments of different lengths and composed of polygonal or nearly polygonal units bounded by a sheath. Some of these display somewhat enlarged and melanized intercalary cells, that may represent heterocysts (N-fixing cells) as in Anabaena and Aphanizomenon of the extant order Nostocales, whereas the remaining filaments or trichomes resemble members of the order Oscilatoriales, such as Lyngbia. Additional filaments are arranged in dense groups forming semi-spherical to spherical epibiotic colonies on a variety of substrates including degraded plant stems. Associated with filaments are thin-walled, spherical to more irregularly shaped propagules, possibly representing nitrogen-fixing structures, such as heterocysts. Based on habit, morphology and anatomy this fossil resembles members of the Rivulariaceae within the Nostocales. The fossil record of cyanobacteria extends as far back as the Early Precambrian, but their presence in younger, non-marine settings is poorly documented. The current record from Jurassic geothermal settings in Patagonia helps fill a gap in the geological record for the cyanobacteria, a key group in the development and functioning of terrestrial ecosystems. This record provides a reference point for the evolution of different cyanobacteria taxa and new insights about their structural diversity and roles in non-marine, Mesozoic ecosystems.