Influence of substrate size and morphology on skeletobiont assemblages: a case study from the Middle Devonian brachiopods of Morocco

Assemblages formed by skeletobionts, which encrust and bore into shells of other organisms, represent a promising system for tracking changes in intra- and interspecific interactions, and primary productivity over evolutionary timescales. However, identification of processes structuring these assemblages across large spatial and temporal scales requires an understanding of potential confounding local factors such as surface area and topography of the skeletal substrates available for colonization, which inevitably limit settlement and survival of skeletobionts. Using a large collection of Middle Devonian brachiopods derived from the same facies in the Mader Basin of Morocco, we evaluate the effect of host size and shell ornamentation on the key metrics used to describe skeletobiont assemblages. We found that both skeletobiont abundance and richness tend to increase with the shell surface area, but the correlation is relatively weak and significant only in the two largest taxa, Schizophoria and Glyptogypa. However, the importance of shell ornamentation becomes evident only when the variation in host size is considered: small-sized, but coarsely ribbed taxa, such as Quiringites and Kransia, host the highest skeletobiont densities, suggesting that they were preferred substrate for colonization. Moreover, differences in skeletobiont richness between the hosts disappear when data are standardized to the same surface area. Our results demonstrate that even though surface area alone may be a poor predictor of the number of individuals and taxa found on a given shell, variation in size among the hosts can still confound interpretations of skeletobiont specificity for particular shell morphologies and bias comparisons of skeletobiont assemblages.