Bacinella, a discrete type of Mesozoic calcimicrobial structure

Abstract

The controversy surrounding Bacinella irregularis Radoičić, 1959, and Lithocodium aggregatum Elliott, 1956, has involved both sedimentologists and paleontologists. Arguments to exclude them from the Codiaceae are reviewed. The same arguments may be advanced to exclude their ascription to most other organisms. The only hypothesis resisting all elements of disproof is that of a consortium involving microbial communities. Accordingly, these structures are treated here as biosedimentary structures, namely bacinella structures, and not as taxa. One of the features specific to the organisms that built these structures is their strong corroding ability, as shown by the examples of fossils partly or fully cannibalized. This represents one state of corrosion beyond surficial etching and borings. Besides their capacity to corrode calcareous substrates, these microbial organisms formed nodules or oncoids and even built biostromes, which, depending on their stage of development, may have formed soft, firm or even hard substrates. However, to our knowledge, microbial communities contributing to bacinella structures never built up any bioherm. To complete this summary review, a building-block model that coherently takes into account most architectural variations is presented in the form of a figure sketch. The endolithic foraminifer Troglotella incrustans, which is commonly found associated with bacinella structures, is not an encruster or a borer. Lastly, contrary to certain erroneous hypotheses, no major bacinella episode has been ever found coeval with any significant Oceanic Anoxic Event.