Evolution of the Middle and Inner Ears of Mammaliaforms: The Approach to Mammals

Abstract

Transformations of ear structures in the evolution of early mammals can be studied with the fossils of mammaliaforms. The middle ear is fully attached to the mandibles in mammaliaforms; however, in Mesozoic eutriconodont and spalacotherioid mammals, it is only connected to the mandible by an ossified Meckel’s cartilage, with the ectotympanic and malleus already displaced from the mandible. Recent morphogenetic studies have shown that the developmental potential for ossification of Meckel’s element is conserved in extant mammals. New fossils further revealed that this pattern actually evolved in mammaliaform phylogeny and that disconnection of the ear from the mandible occurred independently in monotremes, in therians, and in multituberculate mammals. The inner ear of mammaliaforms is derived in having a single petrosal bone enclosing the entire inner ear and a promontorium for an elongate cochlear canal. Mammaliaforms and most Mesozoic mammals had ancestral features of a simple cochlear canal with a single cochlear nerve foramen but no interior bony laminae nor did they have a bony canal for the cochlear ganglion. The sieve-like foramina for cochlear nerve fibers to enter the cochlear canal evolved independently three times in Mesozoic mammals. Cochlear canal curvature is homoplastic among mammaliaform groups, and a curvature beyond 270° only evolved in cladotherians, accompanied by Rosenthal’s canal for the cochlear ganglion. The homoplasies of ear structures in early mammalian evolution, although seemingly complex, are consistent with the new understanding of a labile morphogenesis of mammalian ears under a complex developmental genetic network.