Evolutionary Development and Homoplasy of the Middle Ear in Early Mammal Evolution

摘要

To understand the origins of complex structures and evolutionary novelties is a central quest in evolutionary studies. A key innovation in the early evolution of mammals is in the middle ear, a significant adaptation to sensitive hearing. The Definitive Mammalian Middle Ear (DMME), a major apomorphy of modern mammals, is defined by the loss of the embryonic Meckel's cartilage that connects the middle ear to the mandible in ear-ly ontogeny. The resorption of this cartilage in the adult separates the middle ear from the mandible, and this separation enhances the auditory function of the ear and the feeding function of the jaws. On the broadest scale of 310 million years of evolution of mammals from pre-mammalian synapsid ancestors, transformation of the middle ear and its precursory structures ap-pears to be incremental and orderly. However, recent discoveries of new fossils have demonstrated major evolutionary convergences and reversals, also known as homoplasy, in the middle ear structure among Mesozoic mammal clades. Some homoplasies of the middle ear structures in the Mesozoic mammals can now be correlated with heterochrony, or timing changes in ontogeny. Other homo-plasies show the same phenotypic characteristics of gene patterning in the embryogenesis of extant mammals. Recent discoveries in new fossils and in developmental genetics have added to a growing body of evidence that the seemingly labile evolution of the middle ears in Mesozoic mammals was influenced, in a major way, by developmental heterochrony, and gene patterning. Changes in devel-opmental genes and ontogenetic timing are a driving mechanism for the evolution of complex structures with major biological functions, as seen in the early evolution of mammalian ears.