Résumé
Prokaryotes were the first organisms to colonize Earth, but little evidence of their existence has been found in the fossil record. Recent studies of amber, a fossil resin from gymnosperms or angiosperms, have revealed a number of rarely fossilized microorganisms. Several amber-bearing localities of Mid-Cretaceous age in southwestern France (Charentes and Aude regions) led to the discovery of a rich and diverse biota of resin-preserved microorganisms. These amber microcoenoses are dominated by sheathed prokaryotic filaments similar to those of the cyanobacterium Palaeocolteronema cenomanensis Breton and Tostain (2005) and to those of the bacterium Leptotrichites resinatus Schmidt 2005. These sheathed filaments appear as peripheral cortexes around some pieces of amber from the Charentes outcrops and as peripheral dark areas on amber from the Aude locality. Macroscopic and microscopic features, as well as measurements of phycocyanin concentrations from the filaments, made it possible to identify two different taxa. The sheathed filaments from Charentes correspond to P. cenomanensis. They were growing in freshwater ponds when amber trapped them. Those of the Aude outcrop represent L. resinatus. The latter were probably trapped in less humid environments than were P. cenomanensis filaments.