Sponges are categorized into three main groups: Hexactinellida, Demospongiae, and Calcarea. The Hexactinellida further divides into Hexactinosa, Lychniscosa, and Lyssacinosa, with the first two featuring rigid skeletons, while the latter consists of sponges with loose spicules in their soft tissues. Lyssacinosa are widely distributed in oceans but are challenging to study due to their deep-sea habitats, resulting in limited knowledge about their lifestyles. Fossilized Lyssacinosa are even rarer, as their microscleres, crucial for identification, are often lost in the fossil record. This work employs megascleres, which maintain the sponge's shape, to describe a Lyssacinosan sponge fauna from the Upper Cretaceous period in Bornholm, Denmark. It introduces three new genera and eighteen new species, comparing them with Paleozoic Lyssacinosa and contemporary species in terms of diversity, geographic and bathymetric distribution, and characteristics like shape and wall thickness. The research also explores the sedimentology and faunal composition of the Arnager Limestone (circa 88 million years ago), providing a comprehensive view of the habitat conditions and nature in which these Lyssacinosa thrived.
