June 21 , 2016

The earliest known mammalian stapes with unequivocal morphology from an Early Cretaceous eutriconodontan, Chaoyangodens lii, is described. The stapes is roughly rectangular in dorsal view with parallel crura and equal width on both the distal and proximal ends. The stapes has a large stapedial foramen and a prominent process that is interpreted as for insertion of the stapedius muscle (PISM). The PISM is homologized with the proximal end of the interhyal, instead of the dorsal process of the sauropsid extrastapes; the latter is fused to the crista parotica of the otic capsule as the tympanohyal in mammals. The morphology of the stapes is probably characteristic of the transitional mammalian middle ear and supports the notion that the rod-like, perforated stapes with a broad end-on contact with the incus is a primitive stapes condition for mammals. During the evolution of mammalian middle ear, the distal end of the stapes and the PISM reduced the size, along with reduction of other ear ossicles. Because the stapedius muscle is absent in monotremes, the columelliform-imperforate stapes of monotremes probably lacks the homologue of the PISM. Key words: Stapes, eutriconodontan, mammal, Cretaceous, middle ear, evolution.

Meng, J. and Hou, S.-L. (2016)
Earliest known mammalian stapes from an Early Cretaceous eutriconodontan mammal and implications for transformation of mammalian middle ear.
Palaeontologia Polonica 67: 181–196.
doi: 10.4202/pp.2016.67_181