Influence of valve geometry, ornamentation, and microstructure on fractures in Late Ordovician brachiopods

Among Late Ordovician brachiopods from southeastern Indiana. strophomenids display a ratio of 4:1 parabolic to linear repaired fractures in contrast to the 1:2 ratio found for orthids and rhynchonellids. Additionally, only strophomenids display repaired elliptical fractures. The weakest parts of strophomenid and orthid-rhynchonellid shells are the regions of the adductor muscle scars and the sulcus, respectively. Fractured biplanate shells of strophomenids are commonly cleaved anteriorly to posteriorly, whereas fractures are localized on the anterior of strongly curved to geniculate conspecific specimens. Rugae on leptaenids, thickened anterior margins of the brachial valve of rafinesquinids, and the dense distribution of pseudopunctate in all strophomenids, functioned to localize anteriorly the (un)repaired linear and parabolic fractures. No sublethal fracture occurs on any biconvex shell where the height is greater than 14 mm, despite the fact that numerous specimens of certain species attained shell heights of 20 mm or more, an observation suggesting the upper limit in the gape of the crushing elements of the predator. Crushing experiments on valve 'models' reveal that the inflated equibiconvex, plicate shape of the shells of Plarysfrophia is the strongest. However, the architecture of the concave strophomenid valves is relatively stronger than the corresponding valves of many orthids and rhynchonellids when normalized for valve thickness.