Two new species of primitive cladid crinoids, Costalocrinus ibericus and C. thymos, from the Lower Devonian (Emsian) of northwest Spain, provide important information on the evolution of the Costalocrinus-Barycrinus lineage during the Devonian and Mississippian. Costalocrinus ibericus n. sp. is morphologically similar to Barycrinus rhombiferus (Owen and Shumard, 1852) from the Mississippian, except for the characters of the anal X plate and degree of calcification of the anal sac. These two species are separated by approximately 55 m.y. and provide evidence of slow morphologic change, or phenotypic bradytely, in this lineage. Such slow morphologic change is consistent with the previous interpretation of Barycrinus as an ecologic generalist. Very few other genera of Paleozoic crinoids evolved at such a slow rate. Costalocrinus thymos n. sp. is more typical of other species of Costalocrinus from the Middle Devonian and Lower Mississippian. Phylogenetic analysis suggests the ancestral lineage of the Mississippian genus Barycrinus evolved from Costalocrinus near the base of its radiation in the Early Devonian (Emsian).