COMPARING SCLEROBIONT COMMUNITIES BETWEEN THREE ORDOVICIAN BRACHIOPOD HOST SPECIES

Sclerobionts are organisms that encrust or bore into a hard substrate (biotic or abiotic). In the fossil record the skeletonized and trace components of sclerobiont communities on individual hosts are preserved in situ, and are a valuable resource for studying community ecology. Devonian and Carboniferous sclerobiont communities are often studied, but those from the Ordovician are less well known; this study analyzes the sclerobiont communities on three common host brachiopod species (Rafinesquina alternata (n=28), Vinlandostrophia sp. (n=35), and Hebertella occidentalis (n=31)) from the Ordovician (Caradocian) Fairview Formation, USA.

We identified sclerobionts and used a Polar Ordination, with Beals’ endpoints and Sorensen distance, to visualize differences between the communities on each host species. We calculated diversity indices for the communities on each host and compared them. Using Bookstein shape coordinates we plotted the position of each sclerobiont to visualize encrustation patterns.

The ordination showed strong overlap among host communities on the first axis (45% of the total variation). Trepostome bryozoans dominated the communities; cornulitid worm tubes and boring traces were also common. Rarer sclerobiont taxa drove most differences between the communities on each host species, notably Corynotrypa which was common on R. alternata, uncommon on H. occidentalis, and absent on V. sp. Community evenness was similar between all hosts, but species richness was lower for V. sp. than for the other hosts.

The sclerobiont communities encrusting R. alternata showed no pattern of distribution, but were distinct between the ventral and dorsal valves; it is likely that at least the dorsal valve was encrusted after the host’s death. On H. occidentalis, sclerobionts clustered near to the hinge line, also suggesting post mortem encrustation. The sclerobionts on V.sp clustered to the sides, avoiding the central fold/sulcus, possibly because they were utilizing the lateral inhalant current created by V. sp’s. lophophore.

Only a few sclerobiont taxa showed preferences for certain hosts; most sclerobionts were able to thrive on every host, despite differing host morphologies. More case studies are needed before these patterns can be generalizable on broader scales.