A new captorhinid reptile is described on the basis of materials originally assigned to Captorhinikos parvus Olson, 1954, as well as newly discovered and more complete materials from the same locality. All the specimens are from the lower Permian Hennessey Formation in central Oklahoma. Newly described specimens include a suite of nearly complete skulls allowing confident and thorough description and reconstruction of the skull. The new Oklahoman captorhinid is distinguished from all other members of the family by the unique combination of being the smallest known mature member of the family, the lack of the supratemporal, and possession of multiple maxillary and dentary tooth rows despite its very small size. A new genus, Rhodotheratus, is named to include Captorhinikos parvus, as the new combination, Rhodotheratus parvus (Olson, 1954).

A phylogenetic analysis places Rhodotheratus parvus within more basal members of the eureptilian family Captorhinidae. This placement suggests that multiple maxillary and dentary tooth rows must have developed independently at least three times within the family. The interpretation of Rhodotheratus as a small, but well-ossified and mature, adult member of the family demonstrates the first significant example of miniaturization in the family and demonstrates further that the family is a dramatic example of diversification within basal Amniota.