The ossified Meckel's cartilage and internal groove in Mesozoic mammaliaforms: implications to origin of the definitive mammalian middle ear

Abstract

The ossified Meckel's cartilage is described in detail from three adult individuals of two triconodont mammals, Repenomamus and Gobiconodon, which have been discovered in the Lower Cretaceous of Liaoning, China. A possible ossified Meckel's cartilage has also been recognized in the Early Cretaceous symmetrodont Zhangheotherium from Liaoning. The rod-like ossified Meckel's cartilage in Repenomamus bridges the dentary and the ear region of the cranium. Its shape and position are similar to those of Meckel's cartilage in prenatal and in some postnatal extant mammals. The ossified Meckel's cartilage may have functioned as an attachment site for the medial pterygoid muscle. These specimens provide direct evidence for the function of the internal groove which is commonly present in the dentary of early mammals and their relatives. The evidence weakens the hypothesis of multiple origins for the definitive mammalian middle ear. It supports the assumption that a persistent or ossified Meckel's cartilage has been present in adults of the common ancestor of mammals. The new evidence of Repenomamus does not support the model in which brain expansion and negative allometry of the auditory chain are primarily responsible for the detachment of ear ossicles in mammalian ontogeny and evolution. An alternative hypothesis is proposed which does not require brain expansion as the initial factor for the detachment of ear ossicles during mammalian evolution. (C) 2003 The Linnean Society of London.