Life in a living substrate: Modular endosymbionts of bryozoan hosts from the Devonian of Spain

A new bioclaustration structure is described from the Emsian (Lower Devonian) Aguion Formation of Asturias, NW Spain. It is the result of intergrowth of two bryozoan species, Leioclema elegans and Loxophragma cf. leptum, hosting an unknown soft-bodied modular symbiont. These structures point to the development of host-specific symbiotic associations in which both partners grew simultaneously. The association is tentatively assessed as commensalism; no evidence suggesting parasitism or mutualism has been found. A revision of the existing literature on Devonian bryozoans reveals that tubular structures similar to those described herein have been observed in different regions of the world. Only a few trepostome genera developed this kind of association during the Devonian, and Leioclema seems to have been a suitable partner in intergrowths with a range of soft-bodied and skeletonized organisms. It is suggested that the association described herein was relatively common during the Devonian and that molecular factors such as chemical mediation may have played a role in conducting the development of these specific intergrowths.