Restudy of Proexenocrinus inyoensisStrimple and McGinnis, 1972, shows that this earliest-known rhodocrinitid diplobathrid camerate crinoid (late Floian, Early Ordovician) expresses the only known record of ambulacral floor plates within pinnules. These pinnule floor plates are remarkably conserved plesiomorphic expressions, with anatomy similar to floor plates of some of the earliest pentaradiate echinoderms, although on a smaller scale. Proexenocrinus floor plates provide direct skeletal evidence that the general resemblance of blastozoan (eocrinoid, diploporan, rhombiferan) brachioles and crinoid pinnules is the product of homoplasy. Proexenocrinus posterior cup morphology is interpreted to include an anitaxis, a distinctive posterior interray morphology.