Oct. 24-26, 2009, Yingjie Overseas Exchange Center, Peking University, Beijing, China中文版

Development And Evolution Of The Mammalian Ears

Zhe-Xi Luo

Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA

Abstract

The advanced middle and inner ears are the major distinguishing features of mammals from non-mammalian vertebrates. Mammalian ears are an evolutionary adaptation in hearing function, making it possible for mammals to achieve early diversification. The middle ear bones in the pre-mammalian synapsids are attached to the mandible. By contrast, the middle ear in living mammals is connected to the mandible by the embryonic Meckel’s cartilage in early ontogeny. But this cartilage is re-adsorbed and lost in adult; the middle ear bones are separated from the mandible. The mammalian inner ear has a coiled cochlear canal with elaborated bony structures to more sensitive hearing, in contrast to non-mammalian vertebrates (including the pre-mammalian synapsids) that lack such structures.

Did mammalian middle and inner ears evolve in a singular evolutionary innovation? Or did it evolve multiple times in the early mammalian history? New mammal fossils of the Mesozoic show that the separation of the middle ear from the mandible are homoplastic, and evolved separately in unrelated eutriconodont and symmetrodont lineages, or that connection of the middle ear occurred as reversal to the primitive premammalian condition multiple times. The coiling of the inner ear cochlea also occurred separately in living monotremes and living therians, but not in the extinct lineages phylogenetically nested in between monotremes and living therians.

Recent genetic studies demonstrated that the separation of middle ear ossicles from mandible during ontogeny is patterned by a network of genes and their signaling pathways. In mutant mice with the knockout of these genes and signaling pathways, the middle ear bones failed to separate from the mandible, as seen in normal development. The phenotype of the mutant mice shows unambiguous resemblance to the ear–mandible connection as in the Mesozoic eutriconodont and symmetrodont mammals. Ontogenetic timing changes (heterochrony), and changes in genes and signaling network for the middle ear development of living mammals, appear to be a major mechanism for the homoplasies in the middle ear evolution in Mesozoic mammals. It is now known that several genes and signaling pathways have influenced the development of cochlear neural structure, which, in turn, induces the formation of the coiled bony cochlear canal. The induction of the bony cochlear canal by neural structure during the ontogeny of modern mammals is reflected by the evolutionary sequence of neural characters, such as the cochlear innervation and its spiral ganglion in the Mesozoic fossil relatives of living marsupials and placentals. Development impacted transformation of the mammalian ear characters in phylogeny, and evolutionary development represents a major mechanism for the homoplasies in the early mammal evolution.


Zhe-Xi Luo

Curator of Vertebrate Paleontology, Associate Director of Science Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA

Biography

Dr. Zhe-Xi Luo (“Luo”) is a curator of vertebrate paleontology and the associate director of Carnegie Museum of Natural History. He is also an adjunct professor at University of Pittsburgh. Born in Beijing and grew up in Sichuan, Luo received his BS in geology and paleontology at Nanjing University, Ph.D. in paleontology from UC Berkeley in 1989 and postdoctoral training in zoology at Harvard.

Luo’s research focused on fossil vertebrates of the Mesozoic. He is interested in the evolution of key mammalian biological adaptations, phylogeny of their major lineages, and their ecological diversification. He also studied the evolution of whales. In his fieldwork to search for dinosaurs and fossil mammals, he worked in many parts of US and China. He has extensive collaboration with Chinese colleagues on the studies of early mammals, including the world’s earliest-known placentals and marsupials.

At Carnegie Museum, he was the curator for the 1998 exhibit on “China’s Feathered Dinosaurs.” He is a member of the museum team that built Carnegie’s permanent exhibit “Dinosaurs In Their Time.” Dr. Zhe-Xi Luo is a recipient of the CAREER award from the National Science Foundation (USA), and the Humboldt Research Award for Senior US Scientists from the Alexander von Humboldt Foundation (Germany).

Reference: Luo, Z.-X. 2007. Transformation and diversification in the early mammalian evolution. NATURE 450: 1011-1019.