Disarticulated shells of Late Ordovician brachiopods: inferences on strength of hinge and valve architecture

Abstract

More than five thousand specimens of nine species of articulate brachiopods from the Waynesville-Liberty Formation (Late Ordovician) of southeastern Indiana were analyzed for the percentage of specimens disarticulated and the ratio of pedicle to brachial valves. Among biconvex species, the rhynchonellid Hiscobeccus capax displayed the highest ratio (2.66) of articulated to disarticulated specimens, an observation attributed to its cyrtomatodont hinge. The six orthid species from the same assemblages have much higher frequencies of disarticulated valves and deltidiodont hinges, which did not necessitate breakage of the teeth or sockets in order to disassociate the valves. Platystrophia clarksvillensis has the highest percentage of articulated specimens among the orthids, a fact attributed to the hydrodynamic stability of this species and the interlocking of the valves by the plicae around the commissure. The largest of the three concavo-convex taxa, Rafinesquina ponderosa, has the highest ratio of articulated to disarticulated specimens (3.03) and requires the fastest current velocities to transport the shells based on flume experiments.Additionally, the deltidiodont hinge of R. ponderosa is more complex than those of Strophomena planumbona and Leptaena richmondensis, which might have increased the amount of jostling necessary to disarticulate the valves. The infrequency of brachial valves of H. capax, Hebertella occidentalis, Plaesiomys subquadrata, S. planumbona, and R. ponderosa and the pedicle valves of L. richmondensis probably reflects the comparative mechanical weaknesses of these valves given the high frequency of fragments in the biosparites, which resemble tempestites. Pedicle valves of orthids and rhynchonellids are up to 10 times thicker posteriorly relative to corresponding points on the opposite valve. Brachial valves of L. richmondensis are more than four times thicker anteriorly relative to the pedicle valve. The occurrence of posterior remnants of the fragmented brachial valve still hinged to the unfractured pedicle valve support the idea that the thinner valve is mechanically much weaker.