RESISTANCE TO AND REPAIR OF SHELL BREAKAGE INDUCED BY DUROPHAGES IN LATE ORDOVICIAN BRACHIOPODS

Repaired shell breakage in Late Ordovician brachiopods from the Cincinnatian Series in the tri-state area of Indiana-Kentucky-Ohio [USA] may be described in increasing order of severity as scalloped, divoted, cleft and embayed. Concavo-convex brachiopod taxa display disproportionately higher frequencies of shell repair assigned to each category, whereas inflated, biconvex, plicate, sulcate taxa display disproportionately lower frequencies of shell repair. Certain plicate biconvex taxa lack examples of cleft and embayed valves. Plano-convex and dorsi-biconvex, costate taxa showed intermediate frequencies of shell repair, but lack representatives of embayed valves. Selective pressure for evolution of morphologic characters resistant to shell breakage may have favored phyletic trends of increasing size, geniculation and progressive development of a commissural ridge around the lophophore platform of the interior of the concave brachial valve of Leptaena and Rafinesquina. Size-frequency distributions for repaired and undamaged valves provide equivocal evidence of a size refuge from predator-induced shell breakage in Rafinesquina. Among the contemporaneous, potentially durophagous predators, nautiloids probably inflicted the sublethal injuries sustained by the brachiopods. The incriminating evidence includes a fragment of a crushing element inbedded in a valve of Rafinesquina that bears a very striking resemblance to calcified rhyncholites of Mesozoic to Recent nautiloids.