Trepostome bryozoans, with their thick calcitic skeletons, formed the largest number of symbiotic associations with endobionts in the Phanerozoic. Such associations were also formed by cystoporates, fenestrates, cyclostomes and cheilostomes. Bryozoans formed most of their symbiotic associations with endobiotic cnidarians, and markedly fewer with endobiotic worms and endobiotic lophophorates. The majority of Ordovician endobionts colonized borings in living bryozoans, or bored themselves into living hosts, during the Ordovician Bioerosion Revolution, which created new niches for the evolution of symbiotic relationships. The bryozoans likely became more selective and less symbiont tolerant over the time. Assumed mutualistic endobionts were more common than likely parasites in Phanerozoic bryozoans. The decrease in diversity of parasitic associations and the increase in the number of mutualistic associations from the Ordovician to Devonian can be explained by the evolution of possible bryozoan defense mechanisms likely in the form of chemical secretions. Paleozoic endobiont faunas were more diverse than their Mesozoic and Cenozoic counterparts because of endobiont-friendly Paleozoic trepostomes, and because of the peak in diversity of bryozoans with massive colonies in the early and middle Paleozoic.
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