Extremely well-preserved specimens of the species Rennensismilia complanata and Aulosmilia cuneiformis occur in Santonian (Upper Cretaceous) strata of the Lower Gosau beds, near Gosau, Austria. Two of these, here reported, have aragonitic skeletal mineralogy and skeletal structures that are typical for their families, and, in addition, show distribution of trace elements (Sr and Mg above all) that confirm the biogenic origin of structures observed. R. complanata also has proteinaceous matrix preserved within its skeleton, which, seen here for the first time in electron micrographs of living or fossil corals, forms sheaths of organic matrix surrounding bundles of skeletal crystallites. Matrix is most abundant along the axial plane of septa, which also is the first-formed part of each septum. Although A. cuneiformis lacks observable organic matrix materials, its skeletal structure and its distribution and amount of trace elements are analogous to that seen in R. complanata and also in modern corals.