Parasitism as a Long‐Lasting Interaction—First Evidence From Paleozoic Corals

The peak of reef development in the middle Paleozoic (Silurian‐Devonian) resulted in a dense network of interactions between corals and their symbionts. Due to their skeletonization, fossil corals and sponges preserved past interspecific relationships very effectively. Macrosymbionts of typical Paleozoic reef builders—corals and stromatoporoid sponges were traditionally interpreted as their commensals or parasites, despite their unclear systematic affinities. While the interpretations of parasitism were mostly based on alterations of the host's skeleton, one of the important features of parasitism, its long duration, remained unevidenced so far. Here we report on a Middle Devonian (approx. 395 Ma) alveolitid coral (Anthozoa: Tabulata), Mariusilites sp. (from Ardennes, Belgium), hosting numerous extracellular metazoan endosymbionts (Torquaysalpinx sp.) and displaying growth banding. The host (coral) growth banding allows an estimate of its growth rate as 3–4 mm per year, and as a result, the duration of the interaction appeared to be at least more than a year. The long duration of the interaction, together with the host's skeletal modification, suggests that these endosymbionts were parasites. This is the first case where the duration (longevity) of the parasitism can be determined in the hosting Paleozoic bioconstructing organisms.