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The Hapsidopareiidae is a group of “microsaurs” characterized by a substantial reduction of several elements in the cheek region that results in a prominent, enlarged temporal emargination. The clade comprises two markedly similar taxa from the early Permian of Oklahoma,  Hapsidopareion lepton  and  Llistrofus pricei , which have been suggested to be synonymous by past workers.  Llistrofus  was previously known solely from the holotype found near Richards Spur, which consists of a dorsoventrally compressed skull in which the internal structures are difficult to characterize. Here, we present data from two new specimens of  Llistrofus . This includes data collected through the use of neutron tomography, which revealed important new details of the palate and the neurocranium. Important questions within “Microsauria” related to the evolutionary transformations that likely occurred as part of the acquisition of the highly modified recumbirostran morphology for a fossorial ecology justify detailed reexamination of less well-studied taxa, such as  Llistrofus . Although this study eliminates all but one of the previous features that differentiated  Llistrofus  and  Hapsidopareion , the new data and redescription identify new features that justify the maintained separation of the two hapsidopareiids.  Llistrofus  possesses some of the adaptations for a fossorial lifestyle that have been identified in recumbirostrans but with a lesser degree of modification (e.g., reduced neurocranial ossification and mandibular modification). Incorporating the new data for  Llistrofus  into an existing phylogenetic matrix maintains the Hapsidopareiidae’s ( Llistrofus  +  Hapsidopareion ) position as the sister group to Recumbirostra. Given its phylogenetic position, we contextualize  Llistrofus  within the broader “microsaur” framework. Specifically, we propose that  Llistrofus  may have been fossorial but was probably incapable of active burrowing in the fashion of recumbirostrans, which had more consolidated and reinforced skulls.  Llistrofus  may represent an earlier stage in the step-wise acquisition of the derived recumbirostran morphology and paleoecology, furthering our understanding of the evolutionary history of “microsaurs.”

New material of the ‘microsaur’Llistrofusfrom the cave deposits of Richards Spur, Oklahoma and the paleoecology of the Hapsidopareiidae