Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont

• Published in: Nature (London) Vol. 472; no. 7342; pp. 181 - 185
• Main Authors: Meng, Jin, Wang, Yuanqing, Li, Chuankui
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 14.04.2011; Nature Publishing Group
• Subjects: 631/181/414; Animals; Biological Evolution; Biota; Cartilage - anatomy & histology; China; Cretaceous; Ear Ossicles - anatomy & histology; Ear Ossicles - physiology; Ear, Middle - anatomy & histology; Ear, Middle - physiology; Ears & hearing; Earth sciences; Earth, ocean, space; Evolution; Evolutionary biology; Exact sciences and technology; Extinction, Biological; Fossils; History, Ancient; Humanities and Social Sciences; Jaw - anatomy & histology; Mammals; Mammals - anatomy & histology; Mammals - classification; Middle ear; multidisciplinary; Paleontology; Phylogeny; Physiological aspects; Science; Science (multidisciplinary); Studies; Tympanic Membrane - anatomy & histology; Vertebrate paleontology; Vertebrates; Liaoning China; Far East; Asia; China; Mesozoic; skulls; Chordata; morphology; anatomy; Cretaceous; Vertebrata; biologic evolution; Mammalia; Lower Cretaceous; phylogeny; Phanerozoic; Tetrapoda
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature09921

The transference of post-dentary jaw elements to the cranium of mammals as auditory ossicles is one of the central topics in evolutionary biology of vertebrates. Homologies of these bones among jawed vertebrates have long been demonstrated by developmental studies; but fossils illuminating this critical transference are sparse and often ambiguous. Here we report the first unambiguous ectotympanic (angular), malleus (articular and prearticular) and incus (quadrate) of an Early Cretaceous eutriconodont mammal from the Jehol Biota, Liaoning, China. The ectotympanic and malleus have lost their direct contact with the dentary bone but still connect the ossified Meckel’s cartilage (OMC); we hypothesize that the OMC serves as a stabilizing mechanism bridging the dentary and the detached ossicles during mammalian evolution. This transitional mammalian middle ear narrows the morphological gap between the mandibular middle ear in basal mammaliaforms and the definitive mammalian middle ear (DMME) of extant mammals; it reveals complex changes contributing to the detachment of ear ossicles during mammalian evolution. The road from jaw to ear The lower jaw of reptiles is made up of several bones. In mammals, however, it consists of just one, the tooth-bearing dentary. Most of the rest have become the ossicles that transmit sound through the middle ear. This transformation is an iconic example of evolutionary change, but direct fossil evidence of the transition has been hard to find. That's why a fossil described by Jin et al . is so important. It is from a triconodont (a type of extinct mammal) from the Cretaceous period in China. In it, the lower-jaw elements have started to resemble middle-ear ossicles, but are still joined to the lower jaw by a sliver of ossified cartilage. This element, Meckel's cartilage, is an important part of the inner surface of the lower jaw; the new fossil shows that it was a vital piece in the evolutionary jigsaw that led to the formation of the mammalian middle ear.