Reconstruction of a diverse soil microbiocoenosis from French Cretaceous amber
Résumé
The Albian-Cenomanian ambers from Charente-Maritime in south-western France contain, amongst numerous arthropods, also diverse assemblages of microorganisms. The study of these ambers therefore allows insights into the macroscopic and microscopic levels of Cretaceous sylvan biocoenoses. In the course of a current French-German project, the microinclusions of selected highly fossiliferous amber pieces were investigated with respect to their syninclusions of arthropods and plants. The results presented herein are based on the analysis of only two amber pieces, one from the Late Albian of Archingeay/Les-Nouillers and one from the Early Cenomanian of Cadeuil. These pieces belong to the so called litter amber because they contain arthropods of the sylvan litter inhabiting fauna which indicates that the resin solidified at the forest floor, not at the tree bark. Decomposers were the most abundant ecological group in the litter amber and they are represented by three taxa of actinomyctes various fungi and nematodes. Also rod-shaped bacteria and sheathed bacteria are frequently enclosed. Among the fungi, the most remarkable species is a carnivorous fungus which possessed trapping rings and yeast stages. This fungus probably represents an extinct lineage of predatory fungi. In the Cretaceous litter and soil biocoenosis, it was a saprophyte and a specialized consumer of small metazoans and possibly also protozoans. It occupied this ecological niche prior to or together with carnivorous fungi of modern lineages. Cyanobacteria and green algae of the genus Enallax and a Myrmecia-like fossil represent producers in the microbiocoenosis of the forest floor. Protozoans such as testate amoebae and small metazoans occurred as consumers. A larva of the daphniid genus Scapholeberis is particularly remarkable. Modern representatives of this crustacean inhabit aquatic microhabitats wet mosses and the neuston of small ponds and feed on various microbes such as bacteria, algae and protozoans. Since the enclosed soil-microorganisms were embedded in the places where they lived, they can be considered as members of a single microbiocoenoses. They settled in limnetic-terrestrial microhabitats in a humid coastal forest and represent different trophic levels and ecological groups of a Cretaceous microbial food web. Archaic representatives which are not known from extant ecosystems could be found among microbes which are morphologically assignable to extant genera. We thank Heinrich Dörfelt (Halle/Saale) and Wilfried Schönborn (Jena) for discussion on the taxonomy of several inclusions.