Evolutionary paleoecology of macroscopic symbiotic endobionts in Phanerozoic corals

Symbiotic associations are key interactions in benthic ecosystems; they drive evolutionary changes that influence the complexity of life. Out of the major groups of Phanerozoic corals, scleractinians, established the highest number of symbiotic associations, followed by tabulate and rugose corals. Corals were most frequently associated with "worms", followed by arthropods, mollusks, lophophorates, and other corals. Parasitic relationships were more common than commensal or mutualistic ones. Symbiotic skeletal organisms colonized Phanerozoic corals more often than soft-bodied organisms were recorded as bioclaustrations. Among these symbionts, suspension feeders were the dominant group, though micro-predators, detritivores and carnivores were also present. The faunal composition of endobiotic coral symbionts closely mirrors Sepkoski's evolutionary faunas. Paleozoic corals were primarily associated with species from the Paleozoic evolutionary fauna, while Mesozoic to Cenozoic corals were inhabited by members of the modern fauna. The most intriguing aspect is that there was no significant escalation in coral symbiosis levels during the Paleozoic. While the increase in predation intensity throughout the Phanerozoic is well-documented, a similar pattern in symbiotic relationships is not observed. Both tabulate and rugose corals maintained relatively stable, group-specific levels of symbiosis throughout the Paleozoic. The apparent lack of data from the Permian may be due to study and sampling biases. Similarly, scleractinians show no significant increase in symbiosis from the Jurassic to the Paleogene. These findings suggest that the evolution of symbiotic relationships may have been less competitive compared to predation, which involves continuous escalation between predators and prey. It also implies that certain organism groups may quickly reach an optimal level of symbiosis, beyond which further symbiotic relationships offer no additional evolutionary advantage compared to non-symbiotic taxa.