HALF-BORINGS AND MISSING ENCRUSTERS ON BRACHIOPODS IN THE UPPER ORDOVICIAN: IMPLICATIONS FOR THE PALEOECOLOGICAL ANALYSIS OF SCLEROBIONTS

Brachiopod shells in the Upper Ordovician have been the subject of dozens of paleoecological studies assessing the sclerobiont (hard substrate-dwelling) communities commonly found encrusting them. A basic assumption of this work has been that skeletal calcitic encrusters such as bryozoans, echinoderms and cornulitids remain on the host surface unless they have been abraded away, a process which leaves obvious results on the brachiopod shell. We have found in the Cincinnatian Series of the midwestern United States, though, a type of boring in brachiopod shells and encrusting bryozoan zoaria which questions this assumption of encruster permanence. Strophomenid brachiopod shells in particular often have concave epichnial tubular half-borings incised in their exterior surfaces which are either Trypanites or Paleosabella. The same type of half-borings are also found on the undersides of detached encrusting bryozoans as concave hypichnia. The borings must have been excavated at the boundary between the exterior surface of the brachiopod shell and the attaching undersurface of the encrusting bryozoan, cutting equally into both. The bryozoan later detached from the brachiopod shell, revealing the length of the borings in each substrate. The exterior surface of the brachiopod shell and the attaching surface of the bryozoan show little if any signs of abrasion. If not for the half-borings, the brachiopod shell would appear to have never been inhabited by an encruster. The encrusters may have detached because there was a periostracum present which prevented full cementation. We can thus no longer assume that a pristine brachiopod shell in the Cincinnatian never hosted a calcitic encruster, nor that we have all the generations in a multiply-encrusted shell. The detached bryozoans themselves, though, can give us direct information about the pioneering encrusters, both soft-bodied (through bioimmurations) and skeletal.