Abstract
One of the most amazing transitions and innovations during the evolution of mammals was the formation of a novel jaw joint and the incorporation of the original jaw joint into the middle ear to create the unique mammalian three bone/ossicle ear. In this review, we look at the key steps that led to this change and other unusual features of the middle ear and how developmental biology has been providing an understanding of the mechanisms involved. This starts with an overview of the tympanic (air-filled) middle ear, and how the ear drum (tympanic membrane) and the cavity itself form during development in amniotes. This is followed by an investigation of how the ear is connected to the pharynx and the relationship of the ear to the bony bulla in which it sits. Finally, the novel mammalian jaw joint and versatile dentary bone will be discussed with respect to evolution of the mammalian middle ear.This article is part of the themed issue ‘Evo-devo in the genomics era, and the origins of morphological diversity’.
Highlights
One of the most amazing transitions and innovations during the evolution of mammals was the formation of a novel jaw joint and the incorporation of the original jaw joint into the middle ear to create the unique mammalian three bone/ossicle ear
We look at the key steps that led to this change and other unusual features of the middle ear and how developmental biology has been providing an understanding of the mechanisms involved
This starts with an overview of the tympanic middle ear, and how the ear drum and the cavity itself form during development in amniotes
Summary
In modern amniotes (birds, reptiles and mammals), the middle ear comprises an air-filled space known as a tympanic ear. A similar loss of lower jaw structures and mirror image duplication of upper jaw elements is observed when endothelin signalling is inhibited in the chick; strikingly in this case the ear canal is not lost but is expanded, with the formation of a duplicated tympanic membrane associated with a duplicated columella [22]. It is very clear from an analysis of the development of the external ear canal that the process of forming a canal is very different in the chick and mouse. It would be interesting to assess whether the non-mammalian external ear canal forms in a region distinct from the first pharyngeal cleft
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