Fusion or hypertrophy?: The unusual arms of the Petalocrinidae (Ordovician-Devonian; Crinoidea)

The large, triangular or cylindrical second brachial plate of the Petalocrinidae was formed through fusion of brachial plates along the distal margin of the growing arms. Based on the number of ambulacral bifurcations, brachials from the primibrachitaxis through at least the quintibrachitaxis may have been fused to form this large plate. In Petalocrinus, all calcite of fused second brachials assume the same crystallographic orientation, but in Spirocrinus more than one crystal comprises the second brachial plate.