Parasitic 3D-preserved fossil organisms from the Ordovician of Sweden

Parasites are ubiquitous and an important part of the modern biodiversity, representing at least half of all extant organisms. Many of them play an important role in biological interactions, like in food webs, and in the co-evolution between host and parasite. Parasitism evolved convergently many times in Metazoa, and we can find parasitic forms in all larger lineages of animals. They demonstrate a huge diversity in morphology, niche occupation, or life cycle. Also in Euarthropoda, parasites appeared several times, with better and with less known forms, such as ticks, parasitic wasps, fleas, barnacles, or tongue worms. All of them evolved convergently to a parasitic mode of life. Despite this important diversity in parasitic organisms, they are still overlooked, and many things remain to be understood, especially concerning their evolution. Fossil specimens are often neglected, and several studies focus on observations of living species. In addition, fossil parasites tend to have a lower chance of preservation, because many of them possess small and soft bodies, or they infect internal organs (soft tissue) of their hosts. In Euarthropoda, the fossil record is better than in other lineages due to their cuticle composed of chitin that shows a rather high preservation potential. An exceptional fossil assemblage from the Ordovician of Sweden delivered organisms in beautiful three-dimensional Orsten-type preservation, representing unusual parasitic organisms. Those fossils yield important information about the early specialisation of parasitism during the Palaeozoic.