Diverse nature of ubiquitous microborings in Cenomanian corals (Saxonian Cretaceous Basin, Germany)

Abstract

Abstract Upper Cenomanian rocky shore conglomerates exposed in the abandoned Ratssteinbruch quarry in Dresden (Saxonian Cretaceous Basin, Eastern Germany) contain numerous small coral colonies. The skeletons are commonly encrusted with thin ferruginous microbial crusts. Skeletal elements, especially radial elements (septa), contain abundant microborings filled with iron oxyhydroxides. Natural casts of microborings were studied under SEM. Two categories of microborings (2–12\xa0μm in diameter) were distinguished in respect of their time of production. Type 1 microborings occur in the inner part of the colonies and are typically distributed more or less along the septa in the direction of the coral growth. This type is represented by Ichnoreticulina elegans (most common traces; produced by chlorophyte green alga), Scolecia filosa (traces of cyanobacteria), and much more rarely by Conchocelichnus seilacheri (traces of red algae). They were produced during coral life (in\xa0vivo), and provide insight into the very poorly recognised skeleton microbiome of fossil corals. Chlorophyte alga Ostreobium quekettii – the most common microendolith in the skeletons of living modern corals – produces I.\xa0elegans, which dominates the Type 1 microborings. Type 2 microborings include I.\xa0elegans, S.\xa0filosa, Scolecia serrata (made by bacteria) and undetermined microborings. They occur directly below the microbial crusts coating the entire colony, or below thin ferruginous films coating the surfaces of skeletal elements. Microborings are distributed randomly or are more or less perpendicular to skeleton surfaces, demonstrating that Type 2 microborings were evidently made by microendoliths after coral death (post-mortem), when skeletal elements were exposed.