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

Using polarized light microscopy, the large, triangular or cylindrical second brachial plate of the Petalocrinidae is demonstrated to be a compound brachial 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, fused brachials form a second brachial that assumed the same crystallographic orientation, but in Spirocrinus, multidirectional extinctions preserve some of the original multiplate arrangement.