title: Ear ossicle morphology of the Jurassic euharamiyidan Arboroharamiya and evolution of mammalian middle ear.
abstract: The middle ear bones of Mesozoic mammals are rarely preserved as fossils and the morphology of these ossicles in the earliest mammals remains poorly known. Here, we report the stapes and incus of the euharamiyidan Arboroharamiya from the lower Upper Jurassic (∼160 Ma) of northern China, which represent the earliest known mammalian middle ear ossicles. Both bones are miniscule in relation to those in non-mammalian cynodonts. The skull length/stapedial footplate diameter ratio is estimated as 51.74 and the stapes length as the percentage of the skull length is 4%; both numbers fall into the stapes size ranges of mammals. The stapes is "rod-like" and has a large stapedial foramen. It is unique among mammaliaforms in having a distinct posterior process that is interpreted as for insertion of the stapedius muscle and homologized to the ossified proximal (stapedial) end of the interhyal, on which the stapedius muscle attached. The incus differs from the quadrate of non-mammalian cynodonts such as morganucodontids in having small size and a slim short process. Along with lack of the postdentary trough and Meckelian groove on the medial surface of the dentary, the ossicles suggest development of the definitive mammalian middle ear (DMME) in Arboroharamiya. Among various higher-level phylogenetic hypotheses of mammals, the one we preferred places "haramiyidans" within Mammalia. Given this phylogeny, development of the DMME took place once in the allotherian clade containing euharamiyidans and multituberculates, probably independent to those of monotremes and therians. Thus, the DMME has evolved at least three times independently in mammals. Alternative hypothesis that placed "haramiyidans" outside of Mammalia would require independent acquisition of the DMME in multituberculates and euharamiyidans as well as parallel evolution of numerous derived similarities in the dentition, occlusion pattern, mandibles, cranium, and postcranium between the two groups and between "haramiyidans" and other mammals. J. Morphol. 279:441-457, 2018. © 2016 Wiley Periodicals, Inc.
authors.affiliation: Division of Paleontology, American Museum of Natural History, New York City, New York.|Key Laboratory of Vertebrate Evolution and Human Origin of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China.
authors.forename: Jin
authors.initials: J
authors.lastname: Meng
authors.affiliation: Key Laboratory of Vertebrate Evolution and Human Origin of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China.|Department of Biology, Indiana University of Pennsylvania, Indiana, Pennsylvania.
authors.forename: Shundong
authors.initials: S
authors.lastname: Bi
authors.affiliation: Shandong Tianyu Museum of Nature, Pingyi, Shandong, China.|Institute of Geology and Paleontology, Linyi University, Linyi, Shandong, China.
authors.forename: Xiaoting
authors.initials: X
authors.lastname: Zheng
authors.affiliation: Shandong Tianyu Museum of Nature, Pingyi, Shandong, China.|Institute of Geology and Paleontology, Linyi University, Linyi, Shandong, China.
authors.forename: Xiaoli
authors.initials: X
authors.lastname: Wang
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002681:China; D004429:Ear Ossicles / Q000033:anatomy & histology*; D005580:Fossils; D007188:Incus / Q000033:anatomy & histology; D008322:Mammals / Q000033:anatomy & histology*; D008334:Mandible / Q000033:anatomy & histology; D010802:Phylogeny; D013199:Stapes / Q000033:anatomy & histology; D013997:Time Factors
keywords: Meckelian groove; homology; incus; postdentary trough; stapes