Skeletobiosis on favositid corals: a case study from the Middle Devonian of the Mader Basin, Morocco

Tabulate corals of the genus Favosites Lamarck, 1816 from the Middle Devonian of Madène el Mrakib (eastern Anti-Atlas, Morocco) were qualitatively and quantitatively studied with respect to their encrusting and boring organisms (skeletobionts). The assemblage, comprising 18 taxa, is numerically dominated by bryozoans, microconchid tubeworms, and auloporid tabulates. Although less diverse, the recognised Favosites-hosted skeletobiont fauna contains taxa described previously from co-occurring brachiopods. As evidenced by the lower mean abundance and density of the skeletobionts, in contrast to the brachiopod hosts, the favositid corals were, however, not preferred substrates for colonisation. Although the skeletobionts occur on both the upper and lower sides of the host colonies, the majority of colonisers thrived on the latter. Such a colonisation pattern may indicate that the favositids were colonised first on the surfaces devoid of the hosts’ soft tissue. The upper sides, in turn, were largely covered by polyps, so these areas might have been either colonised post mortem, or the larvae settled on those parts of the living hosts that were devoid of soft tissue. The lack of any skeletobiont group present exclusively on the lower sides indicates that none of the abundant taxa were obligate cryptobionts. The favositids lack any traces after parasitic endobionts, such as, e.g., Chaetosalpinx Sokolov, 1948 and allied cecidotaxa, which may either point to the general absence of such endobionts in the habitat, limited survival of their larvae, or an efficient immune system of the hosts, preventing their settlement