Pseudophlegethontia turnbullorum, new genus and species, is the third distinct aïstopod known from the Pennsylvanian of Mazon Creek, Illinois. It is morphologically intermediate between the derived phlegethontiids and the more basal, but equally unusual, “ophiderpetontids,” making it crucial to understanding the anatomy of both groups and inferring possible developmental mechanisms leading to the phlegethontiid condition. Like “ophiderpetontids,” Pseudophlegethontia possesses a relatively short body (140 vertebrae), “k-shaped” ribs extending far back on the vertebral column, distinct skull-roof bones, and a temporal opening without a lower temporal bar. However, it has a pointed snout, quadrate condyles well anterior to the occiput, an epipterygoid that rises high on the lateral surface of the braincase, thin gastralia, and no dorsal osteoderms, all of which are phlegethontiid characters. Uniquely, Pseudophlegethontia has an extremely anteroposteriorly elongate quadrate, and an extended postparietal region. Preliminary phylogenetic analysis places it as sister taxon to Phlegethontia, with Oestocephalus, Ophiderpeton, and Lethiscus placed successively basally.