During the course of evolution, the mammalian cranio-mandibular secondary joint—formed by the dentary condyle and the squamosal glenoid fossa, which replaced the reptilian articular–quadrate joint—represents an innovative structure in vertebrate evolution. By CT-scanning two classic fossils, Chinese researchers found previously unknown jaw joints and proposed a clear, four-step sequence showing how chewing and hearing functions were gradually split between jaw and ear.