One of the major differences between mammals and reptiles is in our ears.
We have three small bones that transmit sound to the inner ear. Technically called "auditory ossicles", they are the malleus (hammer), incus (anvil), and stapes (stirrup).
Reptiles have just one bone in their ears, the stapes. The additional ossicles in our ears act as a lever to amplify the compressions and rarefactions of the air. The challenge for evolution is to explain where these additional mammalian bones came from.
What reptiles lack in the ear they more than make up for in their jaw. Our lower jaw is made of just one bone but reptiles have a whole array of them. Since evolution frequently adapts existing features and puts them to new uses, could it be that jaw bones of our reptile-like ancestors have been modified to do the job of hearing instead of chewing?
Evidence from comparative anatomy, embryology and paleontology all combine to prove that this is the case.
There are many transitional fossils that illustrate this fascinating aspect of evolution. In this post I want to highlight three of them.
Dimetrodon lived between 290 and 270 million years ago. It belongs to a group of species known as the synapsids which are reptile-like but more closely related to mammals than modern reptiles. It was already extinct 40 million years before the dinosaurs first appeared.
You can see from the picture that it had an impressive collection of bones in its lower jaw. The articular bone hinged against the quadrate bone of the skull and both abutted against the stapes transmitting sounds from the jaw to the inner ear. The dentary bone which holds the simple teeth is not involved in the joint.
As we come forward in time the fossil record shows all the non-dentary bones reducing in size until they become just tiny splints in the inside back of the jaw. At the same time the dentary bone gets bigger until it becomes the entire jaw. As synapsids became more active in chewing with their differentiated teeth they needed a stronger bone. One bone is better than many bones stitched together.
Diarthrognathus provides us with an exceptional example of this stage of evolution. It lived around 200 million years ago. Its name literally means "two jaw joint". Its dentary bone has expanded upward and made contact with the squamosal bone of the skull to form a new jaw joint. However the old articular/quadrate joint is still fully intact.
We could not ask for a more perfect example of a transitional form.
Moving further along the timeline the fossils show that the old articular/quadrate joint becomes so diminished that it no longer serves that purpose and the dentary/squamosal joint takes over completely. So what became of the quadrate and articular bones? They continue to reduce, become detached from the jaw and move into the middle ear to become the incus and malleus respectively.
In 2007 Luo Zhexi and his colleagues described a fossil called Yanoconodon from the Yixian Formation in China. It is a true mammal that lived around 122 million years ago and has been labelled as "The Rosetta Stone" of mammalian ear evolution. The amazing thing about this creature is that it has the tiny auditory ossicles like any modern mammal but they are still fused to the lower jaw.
Supporting the evidence for ear evolution is the interesting fact that in the human embryo the incus and malleus bones first appear in the jaw and move up to the middle ear during development. Even after birth the little bones may retain a few lingering filaments of so-called Meckel’s cartilage.
Dr Karen Sears investigated the embryology of opossums and discovered that the process is slowed in the case of that species. They are born with the incus and malleus still part of the jaw and looking a lot like the arrangement of a reptile jaw. As they grow up the bones migrate the middle ear as in all other mammals.
This story that is told so eloquently by the fossil record as well as in the embryonic development of modern species is an amazing example of evolution of a major transition.
When creationist demand a fossil of a creature with "half an ear" we can actually provide them in abundance.
