When, during Devonian, vertebrates occupied terrestrial habitats, they were not adapted to hearing airborne sound. Since they were usually not agile and had a sprawling body posture, their body was in close contact to the ground. Accordingly, it has been suggested that early synapsids detected substrate sound by means of their mandible. In contrast, in species with a more erect body posture and increased agility the contact between mandible and substrate has been lost. Tympanic hearing and new mechanisms of sound localization therefore became important for survival. We have investigated skulls of therapsids by means of neutron tomography at the ANTARES facility of FRM II at Garching so as to analyze the putative earliest tympanic middle ears in synapsids. Based on the tomographic data we have generated three-dimensional models of the skulls, created mechanical models of the putative hearing mechanism, and analyzed their acoustic properties. Our findings indicate that some early synapsids were already able to hear low-frequency airborne sound through their mandible. Furthermore, we suggest that therapsids possessed a continuous air-filled connection between both middle ears and were able to localize sound sources by means of internally coupled ears (ICE).