Boreskov

2.3. Cellular thalli (or colonies?) Eosaccaromyces ramosus built of filament‐likestructures each consisting of 1–2 series of elliptical cells.3. Mycelium‐forming organisms with elliptical sporangia (usually preserved as hollowstructures) connected to the filamental mycelium by short relatively dark holdfasts.4. Organisms forming a plasmodium‐type thallus.Fossils of first three groups possess morphological features similar to mycelium ofmodern eukaryotes that belong to fungi‐like protists or true fungi. Sporangium morphologyof Caudosphaera expansa resembles that of true fungi, but the absence of cellular septa inthe hypha does not allow making direct comparisons. The second group of the studiedorganisms comprises morphologically similar fossils (two‐dimensional netlike structures)suggesting a similar life style and feeding strategies. This type of morphology is optimal forextracting dispersed organic matter and distinctive of saprotrophic organisms. Coenocyticmycelium (fossils of groups 2.1 and 2.2) occurs in Zygomycota and other groups of fungi‐likeprotists, but the overall morphology does not have direct counterparts among modernheterotrophic eukaryotes. It is possible that these organisms, including Caudosphaeraexpansa, were representatives of stem groups (not crown groups) of heterotrophiceukaryotes. Morphology of the third group of fossils resembles oogonium of representativesof genus Saprolegniales of the class Oomycota, a large group of aquatic heterotrophic fungilikeprotists. The third group, therefore, could be referred to as possible oomycetes. Thefourth group includes fossils that are characterized by absence of visible cell walls (thallibecome thinner along the edges) and significant morphological variation while maintainingthe same body plan. These features are indicative of plasmodium stage of modernheterotrophic protists Myxomycetes. All described fossils were obviously eukaryotic,because their morphological elements exceed the size of hypha in Actinobacteria, the onlygroup of prokaryotic organisms that form mycelium.123