Middle ear structures in the Permian Glanosuchus sp. (Therocephalia, Therapsida), based on thin sections

Transverse sections of the skull of the Permian therocephalian Glanosuchus sp. were studied with regard to the structures of the middle ear region. It is generally accepted that most of the skeletal elements of the mammalian middle ear are derived from the postdentary bones of the lower jaw. During synapsid evolution there is a gradual transition from a primitive amniote condition to derived mammalian condition; the latter is characterized by the decoupling of the remaining middle ear elements (angular, prearticular, articular) from the dentary, which forms a secondary jaw articulation with the squamosal. Morganucodon from the Triassic‐Jurassic boundary represents an evolutionary stage, where both jaw articulations are present in a coaxial position and where the primary joint is a Pready a fully effective sound transmitter. Therocephalians are considered to be a good representation of the transitory state of this evolutionary process; this may be especially true for primitive taxa such as the lycosuchid Glanosuchus , whose anatomy may represent the “groundplan” (ancestral morphotype) of Lower to Middle Permian eutheriodonts. We studied a complete sectional series of a young specimen of Glanosuchus sp. prepared using the grind‐and peel‐technique. This showed that the reflected lamina of Glanosuchus is in major parts an extremely thin bony plate, which is best interpreted as a sound‐receiving element overlying an air‐filled recessus of the pharynx. In this specimen, the vestibular foramen and the stapes are preserved in situ; it is likely, however, that both structures were framed by cartilage which fixed the anular ligament. Both the stapes and the quadrate process of the pterygoid are in direct contact with the quadrate. Comparison of the area of the reflected lamina and the vestibular foramen shows that impedance matching was still very ineffective in Therocephalia when compared to extant mammals.