The fossil record of animal symbiosis as a component of marine ecosystems dates back to at least the Early Cambrian. The embedment of animals inside the growing skeletons of other animals produces trace fossils known as bioclaustrations, which provide direct evidence of animal symbiosis. Bioclaustrations primarily record commensal relationships and demonstrate strong host preference among most symbiont–host pairings. Late Ordovician and Middle Devonian pulses in the diversity and abundance of bioclaustrations coincide with general increases in marine animal diversity, as well as other adaptive strategies that serve to partition habitats.